TSConfig Reference

Specifies an allowlist of files to include in the program. An error occurs if any of the files can’t be found.

{
  "compilerOptions": {},
  "files": [
    "core.ts",
    "sys.ts",
    "types.ts",
    "scanner.ts",
    "parser.ts",
    "utilities.ts",
    "binder.ts",
    "checker.ts",
    "tsc.ts"
  ]
}

This is useful when you only have a small number of files and don’t need to use a glob to reference many files. If you need that then use include.

The value of extends is a string which contains a path to another configuration file to inherit from. The path may use Node.js style resolution.

The configuration from the base file are loaded first, then overridden by those in the inheriting config file. All relative paths found in the configuration file will be resolved relative to the configuration file they originated in.

It’s worth noting that files, include and exclude from the inheriting config file overwrite those from the base config file, and that circularity between configuration files is not allowed.

Currently, the only top-level property that is excluded from inheritance is references.

Example

configs/base.json:

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

tsconfig.json:

{
  "extends": "./configs/base",
  "files": ["main.ts", "supplemental.ts"]
}

tsconfig.nostrictnull.json:

{
  "extends": "./tsconfig",
  "compilerOptions": {
    "strictNullChecks": false
  }
}

Properties with relative paths found in the configuration file, which aren’t excluded from inheritance, will be resolved relative to the configuration file they originated in.

Specifies an array of filenames or patterns to include in the program. These filenames are resolved relative to the directory containing the tsconfig.json file.

{
  "include": ["src/**/*", "tests/**/*"]
}

Which would include:

.
├── scripts                ⨯
│   ├── lint.ts            ⨯
│   ├── update_deps.ts     ⨯
│   └── utils.ts           ⨯
├── src                    ✓
│   ├── client             ✓
│   │    ├── index.ts      ✓
│   │    └── utils.ts      ✓
│   ├── server             ✓
│   │    └── index.ts      ✓
├── tests                  ✓
│   ├── app.test.ts        ✓
│   ├── utils.ts           ✓
│   └── tests.d.ts         ✓
├── package.json
├── tsconfig.json
└── yarn.lock

include and exclude support wildcard characters to make glob patterns:

• * matches zero or more characters (excluding directory separators)
• ? matches any one character (excluding directory separators)
• **/ matches any directory nested to any level

If a glob pattern doesn’t include a file extension, then only files with supported extensions are included (e.g. .ts, .tsx, and .d.ts by default, with .js and .jsx if allowJs is set to true).

Specifies an array of filenames or patterns that should be skipped when resolving include.

Important: exclude only changes which files are included as a result of the include setting. A file specified by exclude can still become part of your codebase due to an import statement in your code, a types inclusion, a /// <reference directive, or being specified in the files list.

It is not a mechanism that prevents a file from being included in the codebase - it simply changes what the include setting finds.

Project references are a way to structure your TypeScript programs into smaller pieces. Using Project References can greatly improve build and editor interaction times, enforce logical separation between components, and organize your code in new and improved ways.

You can read more about how references works in the Project References section of the handbook

When:

• undefined (default) provide suggestions as warnings to editors
• true unreachable code is ignored
• false raises compiler errors about unreachable code

These warnings are only about code which is provably unreachable due to the use of JavaScript syntax, for example:

function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

With "allowUnreachableCode": false:

function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

This does not affect errors on the basis of code which appears to be unreachable due to type analysis.

When:

• undefined (default) provide suggestions as warnings to editors
• true unused labels are ignored
• false raises compiler errors about unused labels

Labels are very rare in JavaScript and typically indicate an attempt to write an object literal:

function verifyAge(age: number) {
  // Forgot 'return' statement
  if (age > 18) {
    verified: true;
  }
}

Ensures that your files are parsed in the ECMAScript strict mode, and emit “use strict” for each source file.

ECMAScript strict mode was introduced in ES5 and provides behavior tweaks to the runtime of the JavaScript engine to improve performance, and makes a set of errors throw instead of silently ignoring them.

With exactOptionalPropertyTypes enabled, TypeScript applies stricter rules around how it handles properties on type or interfaces which have a ? prefix.

For example, this interface declares that there is a property which can be one of two strings: ‘dark’ or ‘light’ or it should not be in the object.

interface UserDefaults {
  // The absence of a value represents 'system'
  colorThemeOverride?: "dark" | "light";
}

Without this flag enabled, there are three values which you can set colorThemeOverride to be: “dark”, “light” and undefined.

Setting the value to undefined will allow most JavaScript runtime checks for the existence to fail, which is effectively falsy. However, this isn’t quite accurate colorThemeOverride: undefined is not the same as colorThemeOverride not being defined. For example "colorThemeOverride" in settings would have different behavior with undefined as the key compared to not being defined.

exactOptionalPropertyTypes makes TypeScript truly enforce the definition provided as an optional property:

const settings = getUserSettings();
settings.colorThemeOverride = "dark";
settings.colorThemeOverride = "light";
 
// But not:
settings.colorThemeOverride = undefined;

Report errors for fallthrough cases in switch statements. Ensures that any non-empty case inside a switch statement includes either break or return. This means you won’t accidentally ship a case fallthrough bug.

const a: number = 6;
 
switch (a) {
  case 0:
    console.log("even");
  case 1:
    console.log("odd");
    break;
}

In some cases where no type annotations are present, TypeScript will fall back to a type of any for a variable when it cannot infer the type.

This can cause some errors to be missed, for example:

function fn(s) {
  // No error?
  console.log(s.subtr(3));
}
fn(42);

Turning on noImplicitAny however TypeScript will issue an error whenever it would have inferred any:

function fn(s) {
  console.log(s.subtr(3));
}

When working with classes which use inheritance, it’s possible for a sub-class to get “out of sync” with the functions it overloads when they are renamed in the base class.

For example, imagine you are modeling a music album syncing system:

class Album {
  download() {
    // Default behavior
  }
}
 
class SharedAlbum extends Album {
  download() {
    // Override to get info from many sources
  }
}

Then when you add support for machine-learning generated playlists, you refactor the Album class to have a ‘setup’ function instead:

class Album {
  setup() {
    // Default behavior
  }
}
 
class MLAlbum extends Album {
  setup() {
    // Override to get info from algorithm
  }
}
 
class SharedAlbum extends Album {
  download() {
    // Override to get info from many sources
  }
}

In this case, TypeScript has provided no warning that download on SharedAlbum expected to override a function in the base class.

Using noImplicitOverride you can ensure that the sub-classes never go out of sync, by ensuring that functions which override include the keyword override.

The following example has noImplicitOverride enabled, and you can see the error received when override is missing:

class Album {
  setup() {}
}
 
class MLAlbum extends Album {
  override setup() {}
}
 
class SharedAlbum extends Album {
  setup() {}
}

When enabled, TypeScript will check all code paths in a function to ensure they return a value.

function lookupHeadphonesManufacturer(color: "blue" | "black"): string {
  if (color === "blue") {
    return "beats";
  } else {
    "bose";
  }
}

Raise error on ‘this’ expressions with an implied ‘any’ type.

For example, the class below returns a function which tries to access this.width and this.height – but the context for this inside the function inside getAreaFunction is not the instance of the Rectangle.

class Rectangle {
  width: number;
  height: number;
 
  constructor(width: number, height: number) {
    this.width = width;
    this.height = height;
  }
 
  getAreaFunction() {
    return function () {
      return this.width * this.height;
    };
  }
}

This setting ensures consistency between accessing a field via the “dot” (obj.key) syntax, and “indexed” (obj["key"]) and the way which the property is declared in the type.

Without this flag, TypeScript will allow you to use the dot syntax to access fields which are not defined:

interface GameSettings {
  // Known up-front properties
  speed: "fast" | "medium" | "slow";
  quality: "high" | "low";
 
  // Assume anything unknown to the interface
  // is a string.
  [key: string]: string;
}
 
const settings = getSettings();
settings.speed;
settings.quality;
 
// Unknown key accessors are allowed on
// this object, and are `string`
settings.username;

Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.

const settings = getSettings();
settings.speed;
settings.quality;
 
// This would need to be settings["username"];
settings.username;

The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists.

TypeScript has a way to describe objects which have unknown keys but known values on an object, via index signatures.

interface EnvironmentVars {
  NAME: string;
  OS: string;
 
  // Unknown properties are covered by this index signature.
  [propName: string]: string;
}
 
declare const env: EnvironmentVars;
 
// Declared as existing
const sysName = env.NAME;
const os = env.OS;
 
// Not declared, but because of the index
// signature, then it is considered a string
const nodeEnv = env.NODE_ENV;

Turning on noUncheckedIndexedAccess will add undefined to any un-declared field in the type.

declare const env: EnvironmentVars;
 
// Declared as existing
const sysName = env.NAME;
const os = env.OS;
 
// Not declared, but because of the index
// signature, then it is considered a string
const nodeEnv = env.NODE_ENV;

Report errors on unused local variables.

const createKeyboard = (modelID: number) => {
  const defaultModelID = 23;
  return { type: "keyboard", modelID };
};

Report errors on unused parameters in functions.

const createDefaultKeyboard = (modelID: number) => {
  const defaultModelID = 23;
  return { type: "keyboard", modelID: defaultModelID };
};

The strict flag enables a wide range of type checking behavior that results in stronger guarantees of program correctness. Turning this on is equivalent to enabling all of the strict mode family options, which are outlined below. You can then turn off individual strict mode family checks as needed.

Future versions of TypeScript may introduce additional stricter checking under this flag, so upgrades of TypeScript might result in new type errors in your program. When appropriate and possible, a corresponding flag will be added to disable that behavior.

When set, TypeScript will check that the built-in methods of functions call, bind, and apply are invoked with correct argument for the underlying function:

// With strictBindCallApply on
function fn(x: string) {
  return parseInt(x);
}
 
const n1 = fn.call(undefined, "10");
 
const n2 = fn.call(undefined, false);

Otherwise, these functions accept any arguments and will return any:

// With strictBindCallApply off
function fn(x: string) {
  return parseInt(x);
}
 
// Note: No error; return type is 'any'
const n = fn.call(undefined, false);

When enabled, this flag causes functions parameters to be checked more correctly.

Here’s a basic example with strictFunctionTypes off:

function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}
 
type StringOrNumberFunc = (ns: string | number) => void;
 
// Unsafe assignment
let func: StringOrNumberFunc = fn;
// Unsafe call - will crash
func(10);

With strictFunctionTypes on, the error is correctly detected:

function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}
 
type StringOrNumberFunc = (ns: string | number) => void;
 
// Unsafe assignment is prevented
let func: StringOrNumberFunc = fn;

During development of this feature, we discovered a large number of inherently unsafe class hierarchies, including some in the DOM. Because of this, the setting only applies to functions written in function syntax, not to those in method syntax:

type Methodish = {
  func(x: string | number): void;
};
 
function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}
 
// Ultimately an unsafe assignment, but not detected
const m: Methodish = {
  func: fn,
};
m.func(10);

When strictNullChecks is false, null and undefined are effectively ignored by the language. This can lead to unexpected errors at runtime.

When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.

For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will:

declare const loggedInUsername: string;
 
const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];
 
const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

Setting strictNullChecks to true will raise an error that you have not made a guarantee that the loggedInUser exists before trying to use it.

declare const loggedInUsername: string;
 
const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];
 
const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

The second example failed because the array’s find function looks a bit like this simplification:

// When strictNullChecks: true
type Array = {
  find(predicate: (value: any, index: number) => boolean): S | undefined;
};

// When strictNullChecks: false the undefined is removed from the type system,
// allowing you to write code which assumes it always found a result
type Array = {
  find(predicate: (value: any, index: number) => boolean): S;
};

When set to true, TypeScript will raise an error when a class property was declared but not set in the constructor.

class UserAccount {
  name: string;
  accountType = "user";
 
  email: string;
  address: string | undefined;
 
  constructor(name: string) {
    this.name = name;
    // Note that this.email is not set
  }
}

In the above case:

• this.name is set specifically.
• this.accountType is set by default.
• this.email is not set and raises an error.
• this.address is declared as potentially undefined which means it does not have to be set.

In TypeScript 4.0, support was added to allow changing the type of the variable in a catch clause from any to unknown. Allowing for code like:

try {
  // ...
} catch (err) {
  // We have to verify err is an
  // error before using it as one.
  if (err instanceof Error) {
    console.log(err.message);
  }
}

This pattern ensures that error handling code becomes more comprehensive because you cannot guarantee that the object being thrown is a Error subclass ahead of time. With the flag useUnknownInCatchVariables enabled, then you do not need the additional syntax (: unknown) nor a linter rule to try enforce this behavior.

When set to true, allowUmdGlobalAccess lets you access UMD exports as globals from inside module files. A module file is a file that has imports and/or exports. Without this flag, using an export from a UMD module requires an import declaration.

An example use case for this flag would be a web project where you know the particular library (like jQuery or Lodash) will always be available at runtime, but you can’t access it with an import.

Lets you set a base directory to resolve non-absolute module names.

You can define a root folder where you can do absolute file resolution. E.g.

baseUrl
├── ex.ts
├── hello
│   └── world.ts
└── tsconfig.json

With "baseUrl": "./" inside this project TypeScript will look for files starting at the same folder as the tsconfig.json.

import { helloWorld } from "hello/world";

console.log(helloWorld);

If you get tired of imports always looking like "../" or "./", or needing to change them as you move files, this is a great way to fix that.

Sets the module system for the program. See the Modules reference page for more information. You very likely want "CommonJS" for node projects.

Changing module affects moduleResolution which also has a reference page.

Here’s some example output for this file:

// @filename: index.ts
import { valueOfPi } from "./constants";
 
export const twoPi = valueOfPi * 2;

CommonJS

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.twoPi = void 0;
const constants_1 = require("./constants");
exports.twoPi = constants_1.valueOfPi * 2;
 

UMD

(function (factory) {
    if (typeof module === "object" && typeof module.exports === "object") {
        var v = factory(require, exports);
        if (v !== undefined) module.exports = v;
    }
    else if (typeof define === "function" && define.amd) {
        define(["require", "exports", "./constants"], factory);
    }
})(function (require, exports) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.twoPi = void 0;
    const constants_1 = require("./constants");
    exports.twoPi = constants_1.valueOfPi * 2;
});
 

AMD

define(["require", "exports", "./constants"], function (require, exports, constants_1) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.twoPi = void 0;
    exports.twoPi = constants_1.valueOfPi * 2;
});
 

System

System.register(["./constants"], function (exports_1, context_1) {
    "use strict";
    var constants_1, twoPi;
    var __moduleName = context_1 && context_1.id;
    return {
        setters: [
            function (constants_1_1) {
                constants_1 = constants_1_1;
            }
        ],
        execute: function () {
            exports_1("twoPi", twoPi = constants_1.valueOfPi * 2);
        }
    };
});
 

ESNext

import { valueOfPi } from "./constants";
export const twoPi = valueOfPi * 2;
 

ES2020

import { valueOfPi } from "./constants";
export const twoPi = valueOfPi * 2;
 

ES2015/ES6

import { valueOfPi } from "./constants";
export const twoPi = valueOfPi * 2;
 

If you are wondering about the difference between ES2015 (aka ES6) and ES2020, ES2020 adds support for dynamic imports, and import.meta.

node12/nodenext

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.twoPi = void 0;
const constants_js_1 = require("./constants.js");
exports.twoPi = constants_js_1.valueOfPi * 2;
 

Introduced in TypeScript 4.5, node12 and nodenext declare support for Node’s ECMAScript Module Support. The emitted JavaScript is the same as ES2020 which is the same import/export syntax in the TypeScript file. You can learn more in the 4.5 release notes.

None

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.twoPi = void 0;
const constants_1 = require("./constants");
exports.twoPi = constants_1.valueOfPi * 2;
 

Specify the module resolution strategy:

• 'node' for Node.js’ CommonJS implementation
• 'node12' or 'nodenext' for Node.js’ ECMAScript Module Support from TypeScript 4.5 onwards
• 'classic' used in TypeScript before the release of 1.6. You probably won’t need to use classic in modern code

There is a handbook reference page on Module Resolution

By default, TypeScript will examine the initial set of files for import and <reference directives and add these resolved files to your program.

If noResolve is set, this process doesn’t happen. However, import statements are still checked to see if they resolve to a valid module, so you’ll need to make sure this is satisfied by some other means.

A series of entries which re-map imports to lookup locations relative to the baseUrl, there is a larger coverage of paths in the handbook.

paths lets you declare how TypeScript should resolve an import in your require/imports.

{
  "compilerOptions": {
    "baseUrl": ".", // this must be specified if "paths" is specified.
    "paths": {
      "jquery": ["node_modules/jquery/dist/jquery"] // this mapping is relative to "baseUrl"
    }
  }
}

This would allow you to be able to write import "jquery", and get all of the correct typing locally.

{
  "compilerOptions": {
    "baseUrl": "src",
    "paths": {
        "app/*": ["app/*"],
        "config/*": ["app/_config/*"],
        "environment/*": ["environments/*"],
        "shared/*": ["app/_shared/*"],
        "helpers/*": ["helpers/*"],
        "tests/*": ["tests/*"]
    },
}

In this case, you can tell the TypeScript file resolver to support a number of custom prefixes to find code. This pattern can be used to avoid long relative paths within your codebase.

Allows importing modules with a ‘.json’ extension, which is a common practice in node projects. This includes generating a type for the import based on the static JSON shape.

TypeScript does not support resolving JSON files by default:

// @filename: settings.json
{
    "repo": "TypeScript",
    "dry": false,
    "debug": false
}
// @filename: index.ts
import settings from "./settings.json";
 
settings.debug === true;
settings.dry === 2;

Enabling the option allows importing JSON, and validating the types in that JSON file.

// @filename: settings.json
{
    "repo": "TypeScript",
    "dry": false,
    "debug": false
}
// @filename: index.ts
import settings from "./settings.json";
 
settings.debug === true;
settings.dry === 2;

Default: The longest common path of all non-declaration input files. If composite is set, the default is instead the directory containing the tsconfig.json file.

When TypeScript compiles files, it keeps the same directory structure in the output directory as exists in the input directory.

For example, let’s say you have some input files:

MyProj
├── tsconfig.json
├── core
│   ├── a.ts
│   ├── b.ts
│   ├── sub
│   │   ├── c.ts
├── types.d.ts

The inferred value for rootDir is the longest common path of all non-declaration input files, which in this case is core/.

If your outDir was dist, TypeScript would write this tree:

MyProj
├── dist
│   ├── a.js
│   ├── b.js
│   ├── sub
│   │   ├── c.js

However, you may have intended for core to be part of the output directory structure. By setting rootDir: "." in tsconfig.json, TypeScript would write this tree:

MyProj
├── dist
│   ├── core
│   │   ├── a.js
│   │   ├── b.js
│   │   ├── sub
│   │   │   ├── c.js

Importantly, rootDir does not affect which files become part of the compilation. It has no interaction with the include, exclude, or files tsconfig.json settings.

Note that TypeScript will never write an output file to a directory outside of outDir, and will never skip emitting a file. For this reason, rootDir also enforces that all files which need to be emitted are underneath the rootDir path.

For example, let’s say you had this tree:

MyProj
├── tsconfig.json
├── core
│   ├── a.ts
│   ├── b.ts
├── helpers.ts

It would be an error to specify rootDir as core and include as * because it creates a file (helpers.ts) that would need to be emitted outside the outDir (i.e. ../helpers.js).

Using rootDirs, you can inform the compiler that there are many “virtual” directories acting as a single root. This allows the compiler to resolve relative module imports within these “virtual” directories, as if they were merged in to one directory.

For example:

 src
 └── views
     └── view1.ts (can import "./template1", "./view2`)
     └── view2.ts (can import "./template1", "./view1`)

 generated
 └── templates
         └── views
             └── template1.ts (can import "./view1", "./view2")

{
  "compilerOptions": {
    "rootDirs": ["src/views", "generated/templates/views"]
  }
}

This does not affect how TypeScript emits JavaScript, it only emulates the assumption that they will be able to work via those relative paths at runtime.

rootDirs can be used to provide a separate “type layer” to files that are not TypeScript or JavaScript by providing a home for generated .d.ts files in another folder. This technique is useful for bundled applications where you use import of files that aren’t necessarily code:

 src
 └── index.ts
 └── css
     └── main.css
     └── navigation.css

 generated
 └── css
     └── main.css.d.ts
     └── navigation.css.d.ts

{
  "compilerOptions": {
    "rootDirs": ["src", "generated"]
  }
}

This technique lets you generate types ahead of time for the non-code source files. Imports then work naturally based off the source file’s location. For example ./src/index.ts can import the file ./src/css/main.css and TypeScript will be aware of the bundler’s behavior for that filetype via the corresponding generated declaration file.

// @filename: index.ts
import { appClass } from "./main.css";

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If typeRoots is specified, only packages under typeRoots will be included. For example:

{
  "compilerOptions": {
    "typeRoots": ["./typings", "./vendor/types"]
  }
}

This config file will include all packages under ./typings and ./vendor/types, and no packages from ./node_modules/@types. All paths are relative to the tsconfig.json.

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If types is specified, only packages listed will be included in the global scope. For instance:

{
  "compilerOptions": {
    "types": ["node", "jest", "express"]
  }
}

This tsconfig.json file will only include ./node_modules/@types/node, ./node_modules/@types/jest and ./node_modules/@types/express. Other packages under node_modules/@types/* will not be included.

What does this affect?

This option does not affect how @types/* are included in your application code, for example if you had the above compilerOptions example with code like:

import * as moment from "moment";

moment().format("MMMM Do YYYY, h:mm:ss a");

The moment import would be fully typed.

When you have this option set, by not including a module in the types array it:

• Will not add globals to your project (e.g process in node, or expect in Jest)
• Will not have exports appear as auto-import recommendations

This feature differs from typeRoots in that it is about specifying only the exact types you want included, whereas typeRoots supports saying you want particular folders.

Generate .d.ts files for every TypeScript or JavaScript file inside your project. These .d.ts files are type definition files which describe the external API of your module. With .d.ts files, tools like TypeScript can provide intellisense and accurate types for un-typed code.

When declaration is set to true, running the compiler with this TypeScript code:

export let helloWorld = "hi";

Will generate an index.js file like this:

export let helloWorld = "hi";
 

With a corresponding helloWorld.d.ts:

export declare let helloWorld: string;
 

When working with .d.ts files for JavaScript files you may want to use emitDeclarationOnly or use outDir to ensure that the JavaScript files are not overwritten.

Offers a way to configure the root directory for where declaration files are emitted.

example
├── index.ts
├── package.json
└── tsconfig.json

with this tsconfig.json:

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./types"
  }
}

Would place the d.ts for the index.ts in a types folder:

example
├── index.js
├── index.ts
├── package.json
├── tsconfig.json
└── types
    └── index.d.ts

Generates a source map for .d.ts files which map back to the original .ts source file. This will allow editors such as VS Code to go to the original .ts file when using features like Go to Definition.

You should strongly consider turning this on if you’re using project references.

Downleveling is TypeScript’s term for transpiling to an older version of JavaScript. This flag is to enable support for a more accurate implementation of how modern JavaScript iterates through new concepts in older JavaScript runtimes.

ECMAScript 6 added several new iteration primitives: the for / of loop (for (el of arr)), Array spread ([a, ...b]), argument spread (fn(...args)), and Symbol.iterator. downlevelIteration allows for these iteration primitives to be used more accurately in ES5 environments if a Symbol.iterator implementation is present.

Example: Effects on for / of

With this TypeScript code:

const str = "Hello!";
for (const s of str) {
  console.log(s);
}

Without downlevelIteration enabled, a for / of loop on any object is downleveled to a traditional for loop:

"use strict";
var str = "Hello!";
for (var _i = 0, str_1 = str; _i < str_1.length; _i++) {
    var s = str_1[_i];
    console.log(s);
}
 

This is often what people expect, but it’s not 100% compliant with ECMAScript iteration protocol. Certain strings, such as emoji (????), have a .length of 2 (or even more!), but should iterate as 1 unit in a for-of loop. See this blog post by Jonathan New for a longer explanation.

When downlevelIteration is enabled, TypeScript will use a helper function that checks for a Symbol.iterator implementation (either native or polyfill). If this implementation is missing, you’ll fall back to index-based iteration.

"use strict";
var __values = (this && this.__values) || function(o) {
    var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;
    if (m) return m.call(o);
    if (o && typeof o.length === "number") return {
        next: function () {
            if (o && i >= o.length) o = void 0;
            return { value: o && o[i++], done: !o };
        }
    };
    throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");
};
var e_1, _a;
var str = "Hello!";
try {
    for (var str_1 = __values(str), str_1_1 = str_1.next(); !str_1_1.done; str_1_1 = str_1.next()) {
        var s = str_1_1.value;
        console.log(s);
    }
}
catch (e_1_1) { e_1 = { error: e_1_1 }; }
finally {
    try {
        if (str_1_1 && !str_1_1.done && (_a = str_1.return)) _a.call(str_1);
    }
    finally { if (e_1) throw e_1.error; }
}
 

You can use tslib via importHelpers to reduce the amount of inline JavaScript too:

"use strict";
var __values = (this && this.__values) || function(o) {
    var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;
    if (m) return m.call(o);
    if (o && typeof o.length === "number") return {
        next: function () {
            if (o && i >= o.length) o = void 0;
            return { value: o && o[i++], done: !o };
        }
    };
    throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");
};
var e_1, _a;
var str = "Hello!";
try {
    for (var str_1 = __values(str), str_1_1 = str_1.next(); !str_1_1.done; str_1_1 = str_1.next()) {
        var s = str_1_1.value;
        console.log(s);
    }
}
catch (e_1_1) { e_1 = { error: e_1_1 }; }
finally {
    try {
        if (str_1_1 && !str_1_1.done && (_a = str_1.return)) _a.call(str_1);
    }
    finally { if (e_1) throw e_1.error; }
}
 

Note: enabling downlevelIteration does not improve compliance if Symbol.iterator is not present in the runtime.

Example: Effects on Array Spreads

This is an array spread:

// Make a new array who elements are 1 followed by the elements of arr2
const arr = [1, ...arr2];

Based on the description, it sounds easy to downlevel to ES5:

// The same, right?
const arr = [1].concat(arr2);

However, this is observably different in certain rare cases. For example, if an array has a “hole” in it, the missing index will create an own property if spreaded, but will not if built using concat:

// Make an array where the '1' element is missing
let missing = [0, , 1];
let spreaded = [...missing];
let concated = [].concat(missing);

// true
"1" in spreaded;
// false
"1" in concated;

Just as with for / of, downlevelIteration will use Symbol.iterator (if present) to more accurately emulate ES 6 behavior.

Controls whether TypeScript will emit a byte order mark (BOM) when writing output files. Some runtime environments require a BOM to correctly interpret a JavaScript files; others require that it is not present. The default value of false is generally best unless you have a reason to change it.

Only emit .d.ts files; do not emit .js files.

This setting is useful in two cases:

• You are using a transpiler other than TypeScript to generate your JavaScript.
• You are using TypeScript to only generate d.ts files for your consumers.

For certain downleveling operations, TypeScript uses some helper code for operations like extending class, spreading arrays or objects, and async operations. By default, these helpers are inserted into files which use them. This can result in code duplication if the same helper is used in many different modules.

If the importHelpers flag is on, these helper functions are instead imported from the tslib module. You will need to ensure that the tslib module is able to be imported at runtime. This only affects modules; global script files will not attempt to import modules.

For example, with this TypeScript:

export function fn(arr: number[]) {
  const arr2 = [1, ...arr];
}

Turning on downlevelIteration and importHelpers is still false:

var __read = (this && this.__read) || function (o, n) {
    var m = typeof Symbol === "function" && o[Symbol.iterator];
    if (!m) return o;
    var i = m.call(o), r, ar = [], e;
    try {
        while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);
    }
    catch (error) { e = { error: error }; }
    finally {
        try {
            if (r && !r.done && (m = i["return"])) m.call(i);
        }
        finally { if (e) throw e.error; }
    }
    return ar;
};
var __spreadArray = (this && this.__spreadArray) || function (to, from, pack) {
    if (pack || arguments.length === 2) for (var i = 0, l = from.length, ar; i < l; i++) {
        if (ar || !(i in from)) {
            if (!ar) ar = Array.prototype.slice.call(from, 0, i);
            ar[i] = from[i];
        }
    }
    return to.concat(ar || Array.prototype.slice.call(from));
};
export function fn(arr) {
    var arr2 = __spreadArray([1], __read(arr), false);
}
 

Then turning on both downlevelIteration and importHelpers:

import { __read, __spreadArray } from "tslib";
export function fn(arr) {
    var arr2 = __spreadArray([1], __read(arr), false);
}
 

You can use noEmitHelpers when you provide your own implementations of these functions.

This flag controls how import works, there are 3 different options:

• remove: The default behavior of dropping import statements which only reference types.

• preserve: Preserves all import statements whose values or types are never used. This can cause imports/side-effects to be preserved.

• error: This preserves all imports (the same as the preserve option), but will error when a value import is only used as a type. This might be useful if you want to ensure no values are being accidentally imported, but still make side-effect imports explicit.

This flag works because you can use import type to explicitly create an import statement which should never be emitted into JavaScript.

When set, instead of writing out a .js.map file to provide source maps, TypeScript will embed the source map content in the .js files. Although this results in larger JS files, it can be convenient in some scenarios. For example, you might want to debug JS files on a webserver that doesn’t allow .map files to be served.

Mutually exclusive with sourceMap.

For example, with this TypeScript:

const helloWorld = "hi";
console.log(helloWorld);

Converts to this JavaScript:

"use strict";
const helloWorld = "hi";
console.log(helloWorld);
 

Then enable building it with inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file.

"use strict";
const helloWorld = "hi";
console.log(helloWorld);
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMifQ==

When set, TypeScript will include the original content of the .ts file as an embedded string in the source map (using the source map’s sourcesContent property). This is often useful in the same cases as inlineSourceMap.

Requires either sourceMap or inlineSourceMap to be set.

For example, with this TypeScript:

const helloWorld = "hi";
console.log(helloWorld);

By default converts to this JavaScript:

"use strict";
const helloWorld = "hi";
console.log(helloWorld);
 

Then enable building it with inlineSources and inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file. Note that the end is different from the example in inlineSourceMap because the source-map now contains the original source code also.

"use strict";
const helloWorld = "hi";
console.log(helloWorld);
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMiLCJzb3VyY2VzQ29udGVudCI6WyJjb25zdCBoZWxsb1dvcmxkID0gXCJoaVwiO1xuY29uc29sZS5sb2coaGVsbG9Xb3JsZCk7Il19

Specify the location where debugger should locate map files instead of generated locations. This string is treated verbatim inside the source-map, for example:

{
  "compilerOptions": {
    "sourceMap": true,
    "mapRoot": "https://my-website.com/debug/sourcemaps/"
  }
}

Would declare that index.js will have sourcemaps at https://my-website.com/debug/sourcemaps/index.js.map.

Specify the end of line sequence to be used when emitting files: ‘CRLF’ (dos) or ‘LF’ (unix).

Do not emit compiler output files like JavaScript source code, source-maps or declarations.

This makes room for another tool like Babel, or swc to handle converting the TypeScript file to a file which can run inside a JavaScript environment.

You can then use TypeScript as a tool for providing editor integration, and as a source code type-checker.

Instead of importing helpers with importHelpers, you can provide implementations in the global scope for the helpers you use and completely turn off emitting of helper functions.

For example, using this async function in ES5 requires a await-like function and generator-like function to run:

const getAPI = async (url: string) => {
  // Get API
  return {};
};

Which creates quite a lot of JavaScript:

"use strict";
var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments || [])).next());
    });
};
var __generator = (this && this.__generator) || function (thisArg, body) {
    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
    function verb(n) { return function (v) { return step([n, v]); }; }
    function step(op) {
        if (f) throw new TypeError("Generator is already executing.");
        while (_) try {
            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
            if (y = 0, t) op = [op[0] & 2, t.value];
            switch (op[0]) {
                case 0: case 1: t = op; break;
                case 4: _.label++; return { value: op[1], done: false };
                case 5: _.label++; y = op[1]; op = [0]; continue;
                case 7: op = _.ops.pop(); _.trys.pop(); continue;
                default:
                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
                    if (t[2]) _.ops.pop();
                    _.trys.pop(); continue;
            }
            op = body.call(thisArg, _);
        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
    }
};
var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {
    return __generator(this, function (_a) {
        // Get API
        return [2 /*return*/, {}];
    });
}); };
 

Which can be switched out with your own globals via this flag:

"use strict";
var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {
    return __generator(this, function (_a) {
        // Get API
        return [2 /*return*/, {}];
    });
}); };
 

Do not emit compiler output files like JavaScript source code, source-maps or declarations if any errors were reported.

This defaults to false, making it easier to work with TypeScript in a watch-like environment where you may want to see results of changes to your code in another environment before making sure all errors are resolved.

If specified, .js (as well as .d.ts, .js.map, etc.) files will be emitted into this directory. The directory structure of the original source files is preserved; see rootDir if the computed root is not what you intended.

If not specified, .js files will be emitted in the same directory as the .ts files they were generated from:

$ tsc

example
├── index.js
└── index.ts

With a tsconfig.json like this:

{
  "compilerOptions": {
    "outDir": "dist"
  }
}

Running tsc with these settings moves the files into the specified dist folder:

$ tsc

example
├── dist
│   └── index.js
├── index.ts
└── tsconfig.json

If specified, all global (non-module) files will be concatenated into the single output file specified.

If module is system or amd, all module files will also be concatenated into this file after all global content.

Note: outFile cannot be used unless module is None, System, or AMD. This option cannot be used to bundle CommonJS or ES6 modules.

Do not erase const enum declarations in generated code. const enums provide a way to reduce the overall memory footprint of your application at runtime by emitting the enum value instead of a reference.

For example with this TypeScript:

const enum Album {
  JimmyEatWorldFutures = 1,
  TubRingZooHypothesis = 2,
  DogFashionDiscoAdultery = 3,
}
 
const selectedAlbum = Album.JimmyEatWorldFutures;
if (selectedAlbum === Album.JimmyEatWorldFutures) {
  console.log("That is a great choice.");
}

The default const enum behavior is to convert any Album.Something to the corresponding number literal, and to remove a reference to the enum from the JavaScript completely.

"use strict";
const selectedAlbum = 1 /* JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}
 

With preserveConstEnums set to true, the enum exists at runtime and the numbers are still emitted.

"use strict";
var Album;
(function (Album) {
    Album[Album["JimmyEatWorldFutures"] = 1] = "JimmyEatWorldFutures";
    Album[Album["TubRingZooHypothesis"] = 2] = "TubRingZooHypothesis";
    Album[Album["DogFashionDiscoAdultery"] = 3] = "DogFashionDiscoAdultery";
})(Album || (Album = {}));
const selectedAlbum = 1 /* JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}
 

This essentially makes such const enums a source-code feature only, with no runtime traces.

There are some cases where TypeScript can’t detect that you’re using an import. For example, take the following code:

import { Animal } from "./animal.js";

eval("console.log(new Animal().isDangerous())");

or code using ‘Compiles to HTML’ languages like Svelte or Vue.

When combined with isolatedModules: imported types must be marked as type-only because compilers that process single files at a time have no way of knowing whether imports are values that appear unused, or a type that must be removed in order to avoid a runtime crash.

For example, in the following code TitleComponent is a function and TitleComponentProps is a type with isolatedModules and preserveValueImports are enabled:

import { TitleComponent, TitleComponentProps } from "./TitleComponent.js";

Which can be fixed by prefixing TitleComponentProps with type to mark it as a type-only import:

import { TitleComponent, type TitleComponentProps } from "./TitleComponent.js";

Strips all comments from TypeScript files when converting into JavaScript. Defaults to false.

For example, this is a TypeScript file which has a JSDoc comment:

/** The translation of 'Hello world' into Portuguese */
export const helloWorldPTBR = "Olá Mundo";

When removeComments is set to true:

export const helloWorldPTBR = "Olá Mundo";
 

Without setting removeComments or having it as false:

/** The translation of 'Hello world' into Portuguese */
export const helloWorldPTBR = "Olá Mundo";
 

This means that your comments will show up in the JavaScript code.

Enables the generation of sourcemap files. These files allow debuggers and other tools to display the original TypeScript source code when actually working with the emitted JavaScript files. Source map files are emitted as .js.map (or .jsx.map) files next to the corresponding .js output file.

The .js files will in turn contain a sourcemap comment to indicate where the files are to external tools, for example:

// helloWorld.ts
export declare const helloWorld = "hi";

Compiling with sourceMap set to true creates the following JavaScript file:

// helloWorld.js
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.helloWorld = "hi";
//# sourceMappingURL=// helloWorld.js.map

And this also generates this json map:

// helloWorld.js.map
{
  "version": 3,
  "file": "ex.js",
  "sourceRoot": "",
  "sources": ["../ex.ts"],
  "names": [],
  "mappings": ";;AAAa,QAAA,UAAU,GAAG,IAAI,CAAA"
}

Specify the location where a debugger should locate TypeScript files instead of relative source locations. This string is treated verbatim inside the source-map where you can use a path or a URL:

{
  "compilerOptions": {
    "sourceMap": true,
    "sourceRoot": "https://my-website.com/debug/source/"
  }
}

Would declare that index.js will have a source file at https://my-website.com/debug/source/index.ts.

Do not emit declarations for code that has an @internal annotation in its JSDoc comment. This is an internal compiler option; use at your own risk, because the compiler does not check that the result is valid. If you are searching for a tool to handle additional levels of visibility within your d.ts files, look at api-extractor.

/**
 * Days available in a week
 * @internal
 */
export const daysInAWeek = 7;
 
/** Calculate how much someone earns in a week */
export function weeklySalary(dayRate: number) {
  return daysInAWeek * dayRate;
}

With the flag set to false (default):

/**
 * Days available in a week
 * @internal
 */
export declare const daysInAWeek = 7;
/** Calculate how much someone earns in a week */
export declare function weeklySalary(dayRate: number): number;
 

With stripInternal set to true the d.ts emitted will be redacted.

/** Calculate how much someone earns in a week */
export declare function weeklySalary(dayRate: number): number;
 

The JavaScript output is still the same.

Allow JavaScript files to be imported inside your project, instead of just .ts and .tsx files. For example, this JS file:

// @filename: card.js
export const defaultCardDeck = "Heart";

When imported into a TypeScript file will raise an error:

// @filename: index.ts
import { defaultCardDeck } from "./card";
 
console.log(defaultCardDeck);

Imports fine with allowJs enabled:

// @filename: index.ts
import { defaultCardDeck } from "./card";
 
console.log(defaultCardDeck);

This flag can be used as a way to incrementally add TypeScript files into JS projects by allowing the .ts and .tsx files to live along-side existing JavaScript files.

Works in tandem with allowJs. When checkJs is enabled then errors are reported in JavaScript files. This is the equivalent of including // @ts-check at the top of all JavaScript files which are included in your project.

For example, this is incorrect JavaScript according to the parseFloat type definition which comes with TypeScript:

// parseFloat only takes a string
module.exports.pi = parseFloat(3.124);

When imported into a TypeScript module:

// @filename: constants.js
module.exports.pi = parseFloat(3.124);
 
// @filename: index.ts
import { pi } from "./constants";
console.log(pi);

You will not get any errors. However, if you turn on checkJs then you will get error messages from the JavaScript file.

// @filename: constants.js
module.exports.pi = parseFloat(3.124);
 
// @filename: index.ts
import { pi } from "./constants";
console.log(pi);

The maximum dependency depth to search under node_modules and load JavaScript files.

This flag is can only be used when allowJs is enabled, and is used if you want to have TypeScript infer types for all of the JavaScript inside your node_modules.

Ideally this should stay at 0 (the default), and d.ts files should be used to explicitly define the shape of modules. However, there are cases where you may want to turn this on at the expense of speed and potential accuracy.

To avoid a possible memory bloat issues when working with very large JavaScript projects, there is an upper limit to the amount of memory TypeScript will allocate. Turning this flag on will remove the limit.

List of language service plugins to run inside the editor.

Language service plugins are a way to provide additional information to a user based on existing TypeScript files. They can enhance existing messages between TypeScript and an editor, or to provide their own error messages.

For example:

• ts-sql-plugin — Adds SQL linting with a template strings SQL builder.
• typescript-styled-plugin — Provides CSS linting inside template strings .
• typescript-eslint-language-service — Provides eslint error messaging and fix-its inside the compiler’s output.
• ts-graphql-plugin — Provides validation and auto-completion inside GraphQL query template strings.

VS Code has the ability for a extension to automatically include language service plugins, and so you may have some running in your editor without needing to define them in your tsconfig.json.

When set to true, allowSyntheticDefaultImports allows you to write an import like:

import React from "react";

instead of:

import * as React from "react";

When the module does not explicitly specify a default export.

For example, without allowSyntheticDefaultImports as true:

// @filename: utilFunctions.js
const getStringLength = (str) => str.length;
 
module.exports = {
  getStringLength,
};
 
// @filename: index.ts
import utils from "./utilFunctions";
 
const count = utils.getStringLength("Check JS");

This code raises an error because there isn’t a default object which you can import. Even though it feels like it should. For convenience, transpilers like Babel will automatically create a default if one isn’t created. Making the module look a bit more like:

// @filename: utilFunctions.js
const getStringLength = (str) => str.length;
const allFunctions = {
  getStringLength,
};

module.exports = allFunctions;
module.exports.default = allFunctions;

This flag does not affect the JavaScript emitted by TypeScript, it only for the type checking. This option brings the behavior of TypeScript in-line with Babel, where extra code is emitted to make using a default export of a module more ergonomic.

By default (with esModuleInterop false or not set) TypeScript treats CommonJS/AMD/UMD modules similar to ES6 modules. In doing this, there are two parts in particular which turned out to be flawed assumptions:

• a namespace import like import * as moment from "moment" acts the same as const moment = require("moment")

• a default import like import moment from "moment" acts the same as const moment = require("moment").default

This mis-match causes these two issues:

• the ES6 modules spec states that a namespace import (import * as x) can only be an object, by having TypeScript treating it the same as = require("x") then TypeScript allowed for the import to be treated as a function and be callable. That’s not valid according to the spec.

• while accurate to the ES6 modules spec, most libraries with CommonJS/AMD/UMD modules didn’t conform as strictly as TypeScript’s implementation.

Turning on esModuleInterop will fix both of these problems in the code transpiled by TypeScript. The first changes the behavior in the compiler, the second is fixed by two new helper functions which provide a shim to ensure compatibility in the emitted JavaScript:

import * as fs from "fs";
import _ from "lodash";

fs.readFileSync("file.txt", "utf8");
_.chunk(["a", "b", "c", "d"], 2);

With esModuleInterop disabled:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const fs = require("fs");
const lodash_1 = require("lodash");
fs.readFileSync("file.txt", "utf8");
lodash_1.default.chunk(["a", "b", "c", "d"], 2);
 

With esModuleInterop set to true:

"use strict";
var __createBinding = (this && this.__createBinding) || (Object.create ? (function(o, m, k, k2) {
    if (k2 === undefined) k2 = k;
    Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
}) : (function(o, m, k, k2) {
    if (k2 === undefined) k2 = k;
    o[k2] = m[k];
}));
var __setModuleDefault = (this && this.__setModuleDefault) || (Object.create ? (function(o, v) {
    Object.defineProperty(o, "default", { enumerable: true, value: v });
}) : function(o, v) {
    o["default"] = v;
});
var __importStar = (this && this.__importStar) || function (mod) {
    if (mod && mod.__esModule) return mod;
    var result = {};
    if (mod != null) for (var k in mod) if (k !== "default" && Object.prototype.hasOwnProperty.call(mod, k)) __createBinding(result, mod, k);
    __setModuleDefault(result, mod);
    return result;
};
var __importDefault = (this && this.__importDefault) || function (mod) {
    return (mod && mod.__esModule) ? mod : { "default": mod };
};
Object.defineProperty(exports, "__esModule", { value: true });
const fs = __importStar(require("fs"));
const lodash_1 = __importDefault(require("lodash"));
fs.readFileSync("file.txt", "utf8");
lodash_1.default.chunk(["a", "b", "c", "d"], 2);
 

Note: The namespace import import * as fs from "fs" only accounts for properties which are owned (basically properties set on the object and not via the prototype chain) on the imported object. If the module you’re importing defines its API using inherited properties, you need to use the default import form (import fs from "fs"), or disable esModuleInterop.

Note: You can make JS emit terser by enabling importHelpers:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const tslib_1 = require("tslib");
const fs = (0, tslib_1.__importStar)(require("fs"));
const lodash_1 = (0, tslib_1.__importDefault)(require("lodash"));
fs.readFileSync("file.txt", "utf8");
lodash_1.default.chunk(["a", "b", "c", "d"], 2);
 

Enabling esModuleInterop will also enable allowSyntheticDefaultImports.

TypeScript follows the case sensitivity rules of the file system it’s running on. This can be problematic if some developers are working in a case-sensitive file system and others aren’t. If a file attempts to import fileManager.ts by specifying ./FileManager.ts the file will be found in a case-insensitive file system, but not on a case-sensitive file system.

When this option is set, TypeScript will issue an error if a program tries to include a file by a casing different from the casing on disk.

While you can use TypeScript to produce JavaScript code from TypeScript code, it’s also common to use other transpilers such as Babel to do this. However, other transpilers only operate on a single file at a time, which means they can’t apply code transforms that depend on understanding the full type system. This restriction also applies to TypeScript’s ts.transpileModule API which is used by some build tools.

These limitations can cause runtime problems with some TypeScript features like const enums and namespaces. Setting the isolatedModules flag tells TypeScript to warn you if you write certain code that can’t be correctly interpreted by a single-file transpilation process.

It does not change the behavior of your code, or otherwise change the behavior of TypeScript’s checking and emitting process.

Some examples of code which does not work when isolatedModules is enabled.

Exports of Non-Value Identifiers

In TypeScript, you can import a type and then subsequently export it:

import { someType, someFunction } from "someModule";
 
someFunction();
 
export { someType, someFunction };

Because there’s no value for someType, the emitted export will not try to export it (this would be a runtime error in JavaScript):

export { someFunction };

Single-file transpilers don’t know whether someType produces a value or not, so it’s an error to export a name that only refers to a type.

Non-Module Files

If isolatedModules is set, all implementation files must be modules (which means it has some form of import/export). An error occurs if any file isn’t a module:

function fn() {}

This restriction doesn’t apply to .d.ts files.

References to const enum members

In TypeScript, when you reference a const enum member, the reference is replaced by its actual value in the emitted JavaScript. Changing this TypeScript:

declare const enum Numbers {
  Zero = 0,
  One = 1,
}
console.log(Numbers.Zero + Numbers.One);

To this JavaScript:

"use strict";
console.log(0 + 1);
 

Without knowledge of the values of these members, other transpilers can’t replace the references to Numbers, which would be a runtime error if left alone (since there are no Numbers object at runtime). Because of this, when isolatedModules is set, it is an error to reference an ambient const enum member.

This is to reflect the same flag in Node.js; which does not resolve the real path of symlinks.

This flag also exhibits the opposite behavior to Webpack’s resolve.symlinks option (i.e. setting TypeScript’s preserveSymlinks to true parallels setting Webpack’s resolve.symlinks to false, and vice-versa).

With this enabled, references to modules and packages (e.g. imports and /// <reference type="..." /> directives) are all resolved relative to the location of the symbolic link file, rather than relative to the path that the symbolic link resolves to.

In prior versions of TypeScript, this controlled what encoding was used when reading text files from disk. Today, TypeScript assumes UTF-8 encoding, but will correctly detect UTF-16 (BE and LE) or UTF-8 BOMs.

This flag changes the keyof type operator to return string instead of string | number when applied to a type with a string index signature.

This flag is used to help people keep this behavior from before TypeScript 2.9’s release.

You shouldn’t need this. By default, when emitting a module file to a non-ES6 target, TypeScript emits a "use strict"; prologue at the top of the file. This setting disables the prologue.

define(["require", "exports"], function (require, exports) {
    exports.__esModule = true;
    exports.fn = void 0;
    function fn() { }
    exports.fn = fn;
});
 

define(["require", "exports"], function (require, exports) {
    "use strict";
    exports.__esModule = true;
    exports.fn = void 0;
    function fn() { }
    exports.fn = fn;
});
 

TypeScript will unify type parameters when comparing two generic functions.

type A = <T, U>(x: T, y: U) => [T, U];
type B = <S>(x: S, y: S) => [S, S];
 
function f(a: A, b: B) {
  b = a; // Ok
  a = b; // Error
}

This flag can be used to remove that check.

Use outFile instead.

The out option computes the final file location in a way that is not predictable or consistent. This option is retained for backward compatibility only and is deprecated.

This disables reporting of excess property errors, such as the one shown in the following example:

type Point = { x: number; y: number };
const p: Point = { x: 1, y: 3, m: 10 };

This flag was added to help people migrate to the stricter checking of new object literals in TypeScript 1.6.

We don’t recommend using this flag in a modern codebase, you can suppress one-off cases where you need it using // @ts-ignore.

Turning suppressImplicitAnyIndexErrors on suppresses reporting the error about implicit anys when indexing into objects, as shown in the following example:

const obj = { x: 10 };
console.log(obj["foo"]);

Using suppressImplicitAnyIndexErrors is quite a drastic approach. It is recommended to use a @ts-ignore comment instead:

const obj = { x: 10 };
// @ts-ignore
console.log(obj["foo"]);

Enables experimental support for emitting type metadata for decorators which works with the module reflect-metadata.

For example, here is the JavaScript

function LogMethod(
  target: any,
  propertyKey: string | symbol,
  descriptor: PropertyDescriptor
) {
  console.log(target);
  console.log(propertyKey);
  console.log(descriptor);
}
 
class Demo {
  @LogMethod
  public foo(bar: number) {
    // do nothing
  }
}
 
const demo = new Demo();

With emitDecoratorMetadata not set to true (default):

"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
function LogMethod(target, propertyKey, descriptor) {
    console.log(target);
    console.log(propertyKey);
    console.log(descriptor);
}
class Demo {
    foo(bar) {
        // do nothing
    }
}
__decorate([
    LogMethod
], Demo.prototype, "foo", null);
const demo = new Demo();
 

With emitDecoratorMetadata set to true:

"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function LogMethod(target, propertyKey, descriptor) {
    console.log(target);
    console.log(propertyKey);
    console.log(descriptor);
}
class Demo {
    foo(bar) {
        // do nothing
    }
}
__decorate([
    LogMethod,
    __metadata("design:type", Function),
    __metadata("design:paramtypes", [Number]),
    __metadata("design:returntype", void 0)
], Demo.prototype, "foo", null);
const demo = new Demo();
 

Enables experimental support for decorators, which is in stage 2 of the TC39 standardization process.

Decorators are a language feature which hasn’t yet been fully ratified into the JavaScript specification. This means that the implementation version in TypeScript may differ from the implementation in JavaScript when it it decided by TC39.

You can find out more about decorator support in TypeScript in the handbook.

Controls how JSX constructs are emitted in JavaScript files. This only affects output of JS files that started in .tsx files.

• react: Emit .js files with JSX changed to the equivalent React.createElement calls
• react-jsx: Emit .js files with the JSX changed to _jsx calls
• react-jsxdev: Emit .js files with the JSX to _jsx calls
• preserve: Emit .jsx files with the JSX unchanged
• react-native: Emit .js files with the JSX unchanged

For example

This sample code:

export const helloWorld = () => <h1>Hello world</h1>;

Default: "react"

import React from 'react';
export const helloWorld = () => React.createElement("h1", null, "Hello world");
 

Preserve: "preserve"

import React from 'react';
export const helloWorld = () => <h1>Hello world</h1>;
 

React Native: "react-native"

import React from 'react';
export const helloWorld = () => <h1>Hello world</h1>;
 

React 17 transform: "react-jsx"^[1]

import { jsx as _jsx } from "react/jsx-runtime";
import React from 'react';
export const helloWorld = () => _jsx("h1", { children: "Hello world" }, void 0);
 

React 17 dev transform: "react-jsxdev"^[1]

import { jsxDEV as _jsxDEV } from "react/jsx-dev-runtime";
const _jsxFileName = "/home/runner/work/TypeScript-Website/TypeScript-Website/index.tsx";
import React from 'react';
export const helloWorld = () => _jsxDEV("h1", { children: "Hello world" }, void 0, false, { fileName: _jsxFileName, lineNumber: 9, columnNumber: 32 }, this);
 

Changes the function called in .js files when compiling JSX Elements using the classic JSX runtime. The most common change is to use "h" or "preact.h" instead of the default "React.createElement" if using preact.

For example, this TSX file:

import { h } from "preact";

const HelloWorld = () => <div>Hello</div>;

With jsxFactory: "h" looks like:

const preact_1 = require("preact");
const HelloWorld = () => (0, preact_1.h)("div", null, "Hello");
 

This option can be used on a per-file basis too similar to Babel’s /** @jsx h */ directive.

/** @jsx h */
import { h } from "preact";
 
const HelloWorld = () => <div>Hello</div>;

The factory chosen will also affect where the JSX namespace is looked up (for type checking information) before falling back to the global one.

If the factory is defined as React.createElement (the default), the compiler will check for React.JSX before checking for a global JSX. If the factory is defined as h, it will check for h.JSX before a global JSX.

Specify the JSX fragment factory function to use when targeting react JSX emit with jsxFactory compiler option is specified, e.g. Fragment.

For example with this TSConfig:

{
  "compilerOptions": {
    "target": "esnext",
    "module": "commonjs",
    "jsx": "react",
    "jsxFactory": "h",
    "jsxFragmentFactory": "Fragment"
  }
}

This TSX file:

import { h, Fragment } from "preact";

const HelloWorld = () => (
  <>
    <div>Hello</div>
  </>
);

Would look like:

const preact_1 = require("preact");
const HelloWorld = () => ((0, preact_1.h)(preact_1.Fragment, null,
    (0, preact_1.h)("div", null, "Hello")));
 

This option can be used on a per-file basis too similar to Babel’s /* @jsxFrag h */ directive.

For example:

/** @jsx h */
/** @jsxFrag Fragment */
 
import { h, Fragment } from "preact";
 
const HelloWorld = () => (
  <>
    <div>Hello</div>
  </>
);

Declares the module specifier to be used for importing the jsx and jsxs factory functions when using jsx as "react-jsx" or "react-jsxdev" which were introduced in TypeScript 4.1.

With React 17 the library supports a new form of JSX transformation via a separate import.

For example with this code:

import React from "react";

function App() {
  return <h1>Hello World</h1>;
}

Using this TSConfig:

{
  "compilerOptions": {
    "target": "esnext",
    "module": "commonjs",
    "jsx": "react-jsx"
  }
}

The emitted JavaScript from TypeScript is:

"use strict";
var __importDefault = (this && this.__importDefault) || function (mod) {
    return (mod && mod.__esModule) ? mod : { "default": mod };
};
Object.defineProperty(exports, "__esModule", { value: true });
const jsx_runtime_1 = require("react/jsx-runtime");
const react_1 = __importDefault(require("react"));
function App() {
    return (0, jsx_runtime_1.jsx)("h1", { children: "Hello World" }, void 0);
}
 

For example if you wanted to use "jsxImportSource": "preact", you need a tsconfig like:

{
  "compilerOptions": {
    "target": "esnext",
    "module": "commonjs",
    "jsx": "react-jsx",
    "jsxImportSource": "preact",
    "types": ["preact"]
  }
}

Which generates code like:

function App() {
    return (0, jsx_runtime_1.jsx)("h1", { children: "Hello World" }, void 0);
}
exports.App = App;
 

Alternatively, you can use a per-file pragma to set this option, for example:

/** @jsxImportSource preact */

export function App() {
  return <h1>Hello World</h1>;
}

Would add preact/jsx-runtime as an import for the _jsx factory.

Note: In order for this to work like you would expect, your tsx file must include an export or import so that it is considered a module.

TypeScript includes a default set of type definitions for built-in JS APIs (like Math), as well as type definitions for things found in browser environments (like document). TypeScript also includes APIs for newer JS features matching the target you specify; for example the definition for Map is available if target is ES6 or newer.

You may want to change these for a few reasons:

• Your program doesn’t run in a browser, so you don’t want the "dom" type definitions
• Your runtime platform provides certain JavaScript API objects (maybe through polyfills), but doesn’t yet support the full syntax of a given ECMAScript version
• You have polyfills or native implementations for some, but not all, of a higher level ECMAScript version

In TypeScript 4.5, lib files can be overriden by npm modules, find out more in the blog.

High Level libraries

Individual library components

This list may be out of date, you can see the full list in the TypeScript source code.

Disables the automatic inclusion of any library files. If this option is set, lib is ignored.

TypeScript cannot compile anything without a set of interfaces for key primitives like: Array, Boolean, Function, IArguments, Number, Object, RegExp, and String. It is expected that if you use noLib you will be including your own type definitions for these.

Use jsxFactory instead. Specify the object invoked for createElement when targeting react for TSX files.

Modern browsers support all ES6 features, so ES6 is a good choice. You might choose to set a lower target if your code is deployed to older environments, or a higher target if your code is guaranteed to run in newer environments.

The target setting changes which JS features are downleveled and which are left intact. For example, an arrow function () => this will be turned into an equivalent function expression if target is ES5 or lower.

Changing target also changes the default value of lib. You may “mix and match” target and lib settings as desired, but you could just set target for convenience.

For developer platforms like Node there are baselines for the target, depending on the type of platform and its version. You can find a set of community organized TSConfigs at tsconfig/bases, which has configurations for common platforms and their versions.

The special ESNext value refers to the highest version your version of TypeScript supports. This setting should be used with caution, since it doesn’t mean the same thing between different TypeScript versions and can make upgrades less predictable.

This flag is used as part of migrating to the upcoming standard version of class fields. TypeScript introduced class fields many years before it was ratified in TC39. The latest version of the upcoming specification has a different runtime behavior to TypeScript’s implementation but the same syntax.

This flag switches to the upcoming ECMA runtime behavior.

You can read more about the transition in the 3.7 release notes.

Used to output diagnostic information for debugging. This command is a subset of extendedDiagnostics which are more user-facing results, and easier to interpret.

If you have been asked by a TypeScript compiler engineer to give the results using this flag in a compile, in which there is no harm in using extendedDiagnostics instead.

Print names of files which TypeScript sees as a part of your project and the reason they are part of the compilation.

For example, with this project of just a single index.ts file

example
├── index.ts
├── package.json
└── tsconfig.json

Using a tsconfig.json which has explainFiles set to true:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "explainFiles": true
  }
}

Running TypeScript against this folder would have output like this:

❯ tsc
node_modules/typescript/lib/lib.d.ts
  Default library for target 'es5'
node_modules/typescript/lib/lib.es5.d.ts
  Library referenced via 'es5' from file 'node_modules/typescript/lib/lib.d.ts'
node_modules/typescript/lib/lib.dom.d.ts
  Library referenced via 'dom' from file 'node_modules/typescript/lib/lib.d.ts'
node_modules/typescript/lib/lib.webworker.importscripts.d.ts
  Library referenced via 'webworker.importscripts' from file 'node_modules/typescript/lib/lib.d.ts'
node_modules/typescript/lib/lib.scripthost.d.ts
  Library referenced via 'scripthost' from file 'node_modules/typescript/lib/lib.d.ts'
index.ts
  Matched by include pattern '**/*' in 'tsconfig.json'

The output above show:

• The initial lib.d.ts lookup based on target, and the chain of .d.ts files which are referenced
• The index.ts file located via the default pattern of include

This option is intended for debugging how a file has become a part of your compile.

You can use this flag to discover where TypeScript is spending its time when compiling. This is a tool used for understanding the performance characteristics of your codebase overall.

You can learn more about how to measure and understand the output in the performance section of the wiki.

This option gives you the chance to have TypeScript emit a v8 CPU profile during the compiler run. The CPU profile can provide insight into why your builds may be slow.

This option can only be used from the CLI via: --generateCpuProfile tsc-output.cpuprofile.

npm run tsc --generateCpuProfile tsc-output.cpuprofile

This file can be opened in a chromium based browser like Chrome or Edge Developer in the CPU profiler section. You can learn more about understanding the compilers performance in the TypeScript wiki section on performance.

Print names of generated files part of the compilation to the terminal.

This flag is useful in two cases:

• You want to transpile TypeScript as a part of a build chain in the terminal where the filenames are processed in the next command.
• You are not sure that TypeScript has included a file you expected, as a part of debugging the file inclusion settings.

For example:

example
├── index.ts
├── package.json
└── tsconfig.json

With:

{
  "compilerOptions": {
    "declaration": true,
    "listFiles": true
  }
}

Would echo paths like:

$ npm run tsc

path/to/example/index.js
path/to/example/index.d.ts

Normally, TypeScript would return silently on success.

Print names of files part of the compilation. This is useful when you are not sure that TypeScript has included a file you expected.

For example:

example
├── index.ts
├── package.json
└── tsconfig.json

With:

{
  "compilerOptions": {
    "listFiles": true
  }
}

Would echo paths like:

$ npm run tsc
path/to/example/node_modules/typescript/lib/lib.d.ts
path/to/example/node_modules/typescript/lib/lib.es5.d.ts
path/to/example/node_modules/typescript/lib/lib.dom.d.ts
path/to/example/node_modules/typescript/lib/lib.webworker.importscripts.d.ts
path/to/example/node_modules/typescript/lib/lib.scripthost.d.ts
path/to/example/index.ts

Note if using TypeScript 4.2, prefer explainFiles which offers an explanation of why a file was added too.

When you are trying to debug why a module isn’t being included. You can set traceResolutions to true to have TypeScript print information about its resolution process for each processed file.

You can read more about this in the handbook.

The composite option enforces certain constraints which make it possible for build tools (including TypeScript itself, under --build mode) to quickly determine if a project has been built yet.

When this setting is on:

• The rootDir setting, if not explicitly set, defaults to the directory containing the tsconfig.json file.

• All implementation files must be matched by an include pattern or listed in the files array. If this constraint is violated, tsc will inform you which files weren’t specified.

• declaration defaults to true

You can find documentation on TypeScript projects in the handbook.

In multi-project TypeScript programs, TypeScript will load all of the available projects into memory in order to provide accurate results for editor responses which require a full knowledge graph like ‘Find All References’.

If your project is large, you can use the flag disableReferencedProjectLoad to disable the automatic loading of all projects. Instead, projects are loaded dynamically as you open files through your editor.

When working with composite TypeScript projects, this option provides a way to declare that you do not want a project to be included when using features like find all references or jump to definition in an editor.

This flag is something you can use to increase responsiveness in large composite projects.

When working with composite TypeScript projects, this option provides a way to go back to the pre-3.7 behavior where d.ts files were used to as the boundaries between modules. In 3.7 the source of truth is now your TypeScript files.

Tells TypeScript to save information about the project graph from the last compilation to files stored on disk. This creates a series of .tsbuildinfo files in the same folder as your compilation output. They are not used by your JavaScript at runtime and can be safely deleted. You can read more about the flag in the 3.4 release notes.

To control which folders you want to the files to be built to, use the config option tsBuildInfoFile.

This setting lets you specify a file for storing incremental compilation information as a part of composite projects which enables faster building of larger TypeScript codebases. You can read more about composite projects in the handbook.

This option offers a way to configure the place where TypeScript keeps track of the files it stores on the disk to indicate a project’s build state — by default, they are in the same folder as your emitted JavaScript.

Do not truncate error messages.

With false, the default.

var x: {
  propertyWithAnExceedinglyLongName1: string;
  propertyWithAnExceedinglyLongName2: string;
  propertyWithAnExceedinglyLongName3: string;
  propertyWithAnExceedinglyLongName4: string;
  propertyWithAnExceedinglyLongName5: string;
  propertyWithAnExceedinglyLongName6: string;
  propertyWithAnExceedinglyLongName7: string;
  propertyWithAnExceedinglyLongName8: string;
};
 
// String representation of type of 'x' should be truncated in error message
var s: string = x;

With true

var x: {
  propertyWithAnExceedinglyLongName1: string;
  propertyWithAnExceedinglyLongName2: string;
  propertyWithAnExceedinglyLongName3: string;
  propertyWithAnExceedinglyLongName4: string;
  propertyWithAnExceedinglyLongName5: string;
  propertyWithAnExceedinglyLongName6: string;
  propertyWithAnExceedinglyLongName7: string;
  propertyWithAnExceedinglyLongName8: string;
};
 
// String representation of type of 'x' should be truncated in error message
var s: string = x;

Whether to keep outdated console output in watch mode instead of clearing the screen every time a change happened.

Stylize errors and messages using color and context, this is on by default — offers you a chance to have less terse, single colored messages from the compiler.

Use skipLibCheck instead. Skip type checking of default library declaration files.

Skip type checking of declaration files.

This can save time during compilation at the expense of type-system accuracy. For example, two libraries could define two copies of the same type in an inconsistent way. Rather than doing a full check of all d.ts files, TypeScript will type check the code you specifically refer to in your app’s source code.

A common case where you might think to use skipLibCheck is when there are two copies of a library’s types in your node_modules. In these cases, you should consider using a feature like yarn’s resolutions to ensure there is only one copy of that dependency in your tree or investigate how to ensure there is only one copy by understanding the dependency resolution to fix the issue without additional tooling.

When this option is enabled, TypeScript will avoid rechecking/rebuilding all truly possibly-affected files, and only recheck/rebuild files that have changed as well as files that directly import them.

This can be considered a ‘fast & loose’ implementation of the watching algorithm, which can drastically reduce incremental rebuild times at the expense of having to run the full build occasionally to get all compiler error messages.

The strategy for how individual files are watched.

• fixedPollingInterval: Check every file for changes several times a second at a fixed interval.
• priorityPollingInterval: Check every file for changes several times a second, but use heuristics to check certain types of files less frequently than others.
• dynamicPriorityPolling: Use a dynamic queue where less-frequently modified files will be checked less often.
• useFsEvents (the default): Attempt to use the operating system/file system’s native events for file changes.
• useFsEventsOnParentDirectory: Attempt to use the operating system/file system’s native events to listen for changes on a file’s parent directory

The strategy for how entire directory trees are watched under systems that lack recursive file-watching functionality.

• fixedPollingInterval: Check every directory for changes several times a second at a fixed interval.
• dynamicPriorityPolling: Use a dynamic queue where less-frequently modified directories will be checked less often.
• useFsEvents (the default): Attempt to use the operating system/file system’s native events for directory changes.

When using file system events, this option specifies the polling strategy that gets used when the system runs out of native file watchers and/or doesn’t support native file watchers.

• fixedPollingInterval: Check every file for changes several times a second at a fixed interval.
• priorityPollingInterval: Check every file for changes several times a second, but use heuristics to check certain types of files less frequently than others.
• dynamicPriorityPolling: Use a dynamic queue where less-frequently modified files will be checked less often.
• synchronousWatchDirectory: Disable deferred watching on directories. Deferred watching is useful when lots of file changes might occur at once (e.g. a change in node_modules from running npm install), but you might want to disable it with this flag for some less-common setups.

Synchronously call callbacks and update the state of directory watchers on platforms that don`t support recursive watching natively. Instead of giving a small timeout to allow for potentially multiple edits to occur on a file.

{
  "watchOptions": {
    "synchronousWatchDirectory": true
  }
}

You can use excludeFiles to drastically reduce the number of files which are watched during --watch. This can be a useful way to reduce the number of open file which TypeScript tracks on Linux.

{
  "watchOptions": {
    "excludeDirectories": ["**/node_modules", "_build", "temp/*"]
  }
}

You can use excludeFiles to remove a set of specific files from the files which are watched.

{
  "watchOptions": {
    "excludeFiles": ["temp/file.ts"]
  }
}

Offers a config for disabling type-acquisition in JavaScript projects:

{
  "typeAcquisition": {
    "enable": false
  }
}

This could potentially remove all of the editor auto-completion for your project, if you want to get them back, you can use the Type Search to find @types packages or packages with types in them.

If you have a JavaScript project where TypeScript needs additional guidance to understand global dependencies, or have disabled the built-in inference via disableFilenameBasedTypeAcquisition.

You can use include to specify which types should be used from DefinitelyTyped:

{
  "typeAcquisition": {
    "include": ["jquery"]
  }
}

Offers a config for disabling the type-acquisition for a certain module in JavaScript projects. This can be useful for projects which include other libraries in testing infrastructure which aren’t needed in the main application.

{
  "typeAcquisition": {
    "exclude": ["jest", "mocha"]
  }
}

TypeScript’s type acquisition can infer what types should be added based on filenames in a project. This means that having a file like jquery.js in your project would automatically download the types for JQuery from DefinitelyTyped.

You can disable this via disableFilenameBasedTypeAcquisition.

{
  "typeAcquisition": {
    "disableFilenameBasedTypeAcquisition": true
  }
}