Angular Metadata and AOT

The Angular AOT compiler turns your TypeScript source code into runnable JavaScript. As part of that process, the compiler extracts and interprets metadata about the parts of the application that Angular is supposed to manage.

You write metadata in a subset of TypeScript. This guide explains why a subset is necessary, describes the subset constraints, and what happens when you step outside of those constraints.

Angular metadata

Angular metadata tells Angular how to construct instances of your application classes and interact with them at runtime.

You specify the metadata with decorators such as @Component() and @Input(). You also specify metadata implicitly in the constructor declarations of these decorated classes.

In the following example, the @Component() metadata object and the class constructor tell Angular how to create and display an instance of TypicalComponent.

@Component({
  selector: 'app-typical',
  template: 'div>A typical component for {{data.name}}</div>
)}
export class TypicalComponent {
  @Input() data: TypicalData;
  constructor(private someService: SomeService) { ... }
}

The Angular compiler extracts the metadata once and generates a factory for TypicalComponent. When it needs to create a TypicalComponent instance, Angular calls the factory, which produces a new visual element, bound to a new instance of the component class with its injected dependency.

Compile ahead-of-time (AOT)

You should use AOT to compile an application that must launch quickly. With AOT, there is no runtime compile step. The client doesn't need the compiler library at all and excluding it significantly reduces the total payload. The browser downloads a smaller set of safely-compiled, application module(s) and libraries that it can parse quickly and run almost immediately.

The AOT compiler produces a number of files, including the application JavaScript that ultimately runs in the browser. It then statically analyzes your source code and interprets the Angular metadata without actually running the application.

To compile the app, run the ngc stand-alone tool as part of your build process. When using the CLI, run the ng build command.

For more information on AOT, see Ahead-of-Time Compilation.

Metadata restrictions

Angular metadata expressions must conform to the following general constraints:

1. Limit expression syntax to the supported subset of JavaScript.
2. Only reference exported symbols after code folding.
3. Only call functions supported by the compiler.
4. Decorated and data-bound class members must be public.

The next sections elaborate on these points.

How AOT works

It helps to think of the AOT compiler as having two phases: a code analysis phase in which it simply records a representation of the source; and a code generation phase in which the compiler's StaticReflector handles the interpretation as well as places restrictions on what it interprets.

Phase 1: analysis

The TypeScript compiler does some of the analytic work of the first phase. It emits the .d.ts type definition files with type information that the AOT compiler needs to generate application code.

At the same time, the AOT collector analyzes the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.

You can think of .metadata.json as a diagram of the overall structure of a decorator's metadata, represented as an abstract syntax tree (AST).

Angular's schema.ts describes the JSON format as a collection of TypeScript interfaces.

Expression syntax

The collector only understands a subset of JavaScript. Define metadata objects with the following limited syntax:

Syntax	Example
Literal object	{cherry: true, apple: true, mincemeat: false}
Literal array	['cherries', 'flour', 'sugar']
Spread in literal array	['apples', 'flour', ...the_rest]
Calls	bake(ingredients)
New	new Oven()
Property access	pie.slice
Array index	ingredients[0]
Identifier reference	Component
A template string	` pie is ${multiplier} times better than cake `
Literal string	'pi'
Literal number	3.14153265
Literal boolean	true
Literal null	null
Supported prefix operator	!cake
Supported Binary operator	a + b
Conditional operator	a ? b : c
Parentheses	(a + b)

If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. The compiler later reports the error if it needs that piece of metadata to generate the application code.

If you want ngc to report syntax errors immediately rather than produce a .metadata.json file with errors, set the strictMetadataEmit option in tsconfig.

"angularCompilerOptions": {
   ...
   "strictMetadataEmit" : true
 }

Angular libraries have this option to ensure that all Angular .metadata.json files are clean and it is a best practice to do the same when building your own libraries.

No arrow functions

The AOT compiler does not support function expressions and arrow functions, also called lambda functions.

Consider the following component decorator:

@Component({
  ...
  providers: [{provide: server, useFactory: () => new Server()}]
})

The AOT collector does not support the arrow function, () => new Server(), in a metadata expression. It generates an error node in place of the function.

When the compiler later interprets this node, it reports an error that invites you to turn the arrow function into an exported function.

You can fix the error by converting to this:

export function serverFactory() {
  return new Server();
}

@Component({
  ...
  providers: [{provide: server, useFactory: serverFactory}]
})

Limited function calls

The collector can represent a function call or object creation with new as long as the syntax is valid. The collector only cares about proper syntax.

But beware. The compiler may later refuse to generate a call to a particular function or creation of a particular object. The compiler only supports calls to a small set of functions and will use new for only a few designated classes. These functions and classes are in a table of below.

Folding

The compiler can only resolve references to exported symbols. Fortunately, the collector enables limited use of non-exported symbols through folding.

The collector may be able to evaluate an expression during collection and record the result in the .metadata.json instead of the original expression.

For example, the collector can evaluate the expression 1 + 2 + 3 + 4 and replace it with the result, 10.

This process is called folding. An expression that can be reduced in this manner is foldable.

The collector can evaluate references to module-local const declarations and initialized var and let declarations, effectively removing them from the .metadata.json file.

Consider the following component definition:

const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template
})
class HeroComponent {
  @Input() hero: Hero;
}

The compiler could not refer to the template constant because it isn't exported.

But the collector can fold the template constant into the metadata definition by inlining its contents. The effect is the same as if you had written:

@Component({
  selector: 'app-hero',
  template: '<div>{{hero.name}}</div>'
})
class HeroComponent {
  @Input() hero: Hero;
}

There is no longer a reference to template and, therefore, nothing to trouble the compiler when it later interprets the collector's output in .metadata.json.

You can take this example a step further by including the template constant in another expression:

const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template + '<div>{{hero.title}}</div>'
})
class HeroComponent {
  @Input() hero: Hero;
}

The collector reduces this expression to its equivalent folded string:

'<div>{{hero.name}}</div><div>{{hero.title}}</div>'.

Foldable syntax

The following table describes which expressions the collector can and cannot fold:

Syntax	Foldable
Literal object	yes
Literal array	yes
Spread in literal array	no
Calls	no
New	no
Property access	yes, if target is foldable
Array index	yes, if target and index are foldable
Identifier reference	yes, if it is a reference to a local
A template with no substitutions	yes
A template with substitutions	yes, if the substitutions are foldable
Literal string	yes
Literal number	yes
Literal boolean	yes
Literal null	yes
Supported prefix operator	yes, if operand is foldable
Supported binary operator	yes, if both left and right are foldable
Conditional operator	yes, if condition is foldable
Parentheses	yes, if the expression is foldable

If an expression is not foldable, the collector writes it to .metadata.json as an AST for the compiler to resolve.

Phase 2: code generation

The collector makes no attempt to understand the metadata that it collects and outputs to .metadata.json. It represents the metadata as best it can and records errors when it detects a metadata syntax violation.

It's the compiler's job to interpret the .metadata.json in the code generation phase.

The compiler understands all syntax forms that the collector supports, but it may reject syntactically correct metadata if the semantics violate compiler rules.

The compiler can only reference exported symbols.

Decorated component class members must be public. You cannot make an @Input() property private or internal.

Data bound properties must also be public.

// BAD CODE - title is private
@Component({
  selector: 'app-root',
  template: '<h1>{{title}}</h1>'
})
class AppComponent {
  private title = 'My App'; // Bad
}

Most importantly, the compiler only generates code to create instances of certain classes, support certain decorators, and call certain functions from the following lists.

New instances

The compiler only allows metadata that create instances of these Angular classes.

Class	Module
OpaqueToken	@angular/core
InjectionToken	@angular/core

Annotations/Decorators

The compiler only supports metadata for these Angular decorators.

Decorator	Module
Attribute	@angular/core
Component	@angular/core
ContentChild	@angular/core
ContentChildren	@angular/core
Directive	@angular/core
Host	@angular/core
HostBinding	@angular/core
HostListener	@angular/core
Inject	@angular/core
Injectable	@angular/core
Input	@angular/core
NgModule	@angular/core
Optional	@angular/core
Output	@angular/core
Pipe	@angular/core
Self	@angular/core
SkipSelf	@angular/core
ViewChild	@angular/core

Macro-functions and macro-static methods

The compiler also supports macros in the form of functions or static methods that return an expression.

For example, consider the following function:

export function wrapInArray<T>(value: T): T[] {
  return [value];
}

You can call the wrapInArray in a metadata definition because it returns the value of an expression that conforms to the compiler's restrictive JavaScript subset.

You might use wrapInArray() like this:

@NgModule({
  declarations: wrapInArray(TypicalComponent)
})
class TypicalModule {}

The compiler treats this usage as if you had written:

@NgModule({
  declarations: [TypicalComponent]
})
class TypicalModule {}

The collector is simplistic in its determination of what qualifies as a macro function; it can only contain a single return statement.

The Angular RouterModule exports two macro static methods, forRoot and forChild, to help declare root and child routes. Review the source code for these methods to see how macros can simplify configuration of complex Angular modules.

Metadata Errors

The following are metadata errors you may encounter, with explanations and suggested corrections.

Expression form not supported Reference to a local (non-exported) symbol Only initialized variables and constants Reference to a non-exported class Reference to a non-exported function Function calls are not supported Destructured variable or constant not supported Could not resolve type Name expected Unsupported enum member name Tagged template expressions are not supported Symbol reference expected

Expression form not supported

The compiler encountered an expression it didn't understand while evalutating Angular metadata.

Language features outside of the compiler's restricted expression syntax can produce this error, as seen in the following example:

// ERROR
export class Fooish { ... }
...
const prop = typeof Fooish; // typeof is not valid in metadata
  ...
  // bracket notation is not valid in metadata
  { provide: 'token', useValue: { [prop]: 'value' } }; 
  ...

You can use typeof and bracket notation in normal application code. You just can't use those features within expressions that define Angular metadata.

Avoid this error by sticking to the compiler's restricted expression syntax when writing Angular metadata and be wary of new or unusual TypeScript features.

Reference to a local (non-exported) symbol

Reference to a local (non-exported) symbol 'symbol name'. Consider exporting the symbol.

The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized.

Here's a provider example of the problem.

// ERROR
let foo: number; // neither exported nor initialized

@Component({
  selector: 'my-component',
  template: ... ,
  providers: [
    { provide: Foo, useValue: foo }
  ]
})
export class MyComponent {}

The compiler generates the component factory, which includes the useValue provider code, in a separate module. That factory module can't reach back to this source module to access the local (non-exported) foo variable.

You could fix the problem by initializing foo.

let foo = 42; // initialized

The compiler will fold the expression into the provider as if you had written this.

providers: [
    { provide: Foo, useValue: 42 }
  ]

Alternatively, you can fix it by exporting foo with the expectation that foo will be assigned at runtime when you actually know its value.

// CORRECTED
export let foo: number; // exported

@Component({
  selector: 'my-component',
  template: ... ,
  providers: [
    { provide: Foo, useValue: foo }
  ]
})
export class MyComponent {}

Adding export often works for variables referenced in metadata such as providers and animations because the compiler can generate references to the exported variables in these expressions. It doesn't need the values of those variables.

Adding export doesn't work when the compiler needs the actual value in order to generate code. For example, it doesn't work for the template property.

// ERROR
export let someTemplate: string; // exported but not initialized

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

The compiler needs the value of the template property right now to generate the component factory. The variable reference alone is insufficient. Prefixing the declaration with export merely produces a new error, "Only initialized variables and constants can be referenced".

Only initialized variables and constants

Only initialized variables and constants can be referenced because the value of this variable is needed by the template compiler.

The compiler found a reference to an exported variable or static field that wasn't initialized. It needs the value of that variable to generate code.

The following example tries to set the component's template property to the value of the exported someTemplate variable which is declared but unassigned.

// ERROR
export let someTemplate: string;

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

You'd also get this error if you imported someTemplate from some other module and neglected to initialize it there.

// ERROR - not initialized there either
import { someTemplate } from './config';

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

The compiler cannot wait until runtime to get the template information. It must statically derive the value of the someTemplate variable from the source code so that it can generate the component factory, which includes instructions for building the element based on the template.

To correct this error, provide the initial value of the variable in an initializer clause on the same line.

// CORRECTED
export let someTemplate = '<h1>Greetings from Angular</h1>';

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

Reference to a non-exported class

Reference to a non-exported class . Consider exporting the class.

Metadata referenced a class that wasn't exported.

For example, you may have defined a class and used it as an injection token in a providers array but neglected to export that class.

// ERROR
abstract class MyStrategy { }

  ...
  providers: [
    { provide: MyStrategy, useValue: ... }
  ]
  ...

Angular generates a class factory in a separate module and that factory can only access exported classes. To correct this error, export the referenced class.

// CORRECTED
export abstract class MyStrategy { }

  ...
  providers: [
    { provide: MyStrategy, useValue: ... }
  ]
  ...

Reference to a non-exported function

Metadata referenced a function that wasn't exported.

For example, you may have set a providers useFactory property to a locally defined function that you neglected to export.

// ERROR
function myStrategy() { ... }

  ...
  providers: [
    { provide: MyStrategy, useFactory: myStrategy }
  ]
  ...

Angular generates a class factory in a separate module and that factory can only access exported functions. To correct this error, export the function.

// CORRECTED
export function myStrategy() { ... }

  ...
  providers: [
    { provide: MyStrategy, useFactory: myStrategy }
  ]
  ...

Function calls are not supported

Function calls are not supported. Consider replacing the function or lambda with a reference to an exported function.

The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider's useFactory to an anonymous function or arrow function like this.

// ERROR
  ...
  providers: [
    { provide: MyStrategy, useFactory: function() { ... } },
    { provide: OtherStrategy, useFactory: () => { ... } }
  ]
  ...

You also get this error if you call a function or method in a provider's useValue.

// ERROR
import { calculateValue } from './utilities';

  ...
  providers: [
    { provide: SomeValue, useValue: calculateValue() }
  ]
  ...

To correct this error, export a function from the module and refer to the function in a useFactory provider instead.

// CORRECTED
import { calculateValue } from './utilities';

export function myStrategy() { ... }
export function otherStrategy() { ... }
export function someValueFactory() { 
  return calculateValue();
}
  ...
  providers: [
    { provide: MyStrategy, useFactory: myStrategy },
    { provide: OtherStrategy, useFactory: otherStrategy },
    { provide: SomeValue, useFactory: someValueFactory }
  ]
  ...

Destructured variable or constant not supported

Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring.

The compiler does not support references to variables assigned by destructuring.

For example, you cannot write something like this:

// ERROR
import { configuration } from './configuration';

// destructured assignment to foo and bar
const {foo, bar} = configuration;
  ...
  providers: [
    {provide: Foo, useValue: foo},
    {provide: Bar, useValue: bar},
  ]
  ...

To correct this error, refer to non-destructured values.

// CORRECTED
import { configuration } from './configuration';
  ...
  providers: [
    {provide: Foo, useValue: configuration.foo},
    {provide: Bar, useValue: configuration.bar},
  ]
  ...

Could not resolve type

The compiler encountered a type and can't determine which module exports that type.

This can happen if you refer to an ambient type. For example, the Window type is an ambiant type declared in the global .d.ts file.

You'll get an error if you reference it in the component constructor, which the compiler must statically analyze.

// ERROR
@Component({ })
export class MyComponent {
  constructor (private win: Window) { ... }
}

TypeScript understands ambiant types so you don't import them. The Angular compiler does not understand a type that you neglect to export or import.

In this case, the compiler doesn't understand how to inject something with the Window token.

Do not refer to ambient types in metadata expressions.

If you must inject an instance of an ambiant type, you can finesse the problem in four steps:

1. Create an injection token for an instance of the ambiant type.
2. Create a factory function that returns that instance.
3. Add a useFactory provider with that factory function.
4. Use @Inject to inject the instance.

Here's an illustrative example.

// CORRECTED
import { Inject } from '@angular/core';

export const WINDOW = new InjectionToken('Window');
export function _window() { return window; }

@Component({
  ...
  providers: [
    { provide: WINDOW, useFactory: _window }
  ]
})
export class MyComponent {
  constructor (@Inject(WINDOW) private win: Window) { ... }
}

The Window type in the constructor is no longer a problem for the compiler because it uses the @Inject(WINDOW) to generate the injection code.

Angular does something similar with the DOCUMENT token so you can inject the browser's document object (or an abstraction of it, depending upon the platform in which the application runs).

import { Inject }   from '@angular/core';
import { DOCUMENT } from '@angular/platform-browser';

@Component({ ... })
export class MyComponent {
  constructor (@Inject(DOCUMENT) private doc: Document) { ... }
}

Name expected

The compiler expected a name in an expression it was evaluating. This can happen if you use a number as a property name as in the following example.

// ERROR
provider: [{ provide: Foo, useValue: { 0: 'test' } }]

Change the name of the property to something non-numeric.

// CORRECTED
provider: [{ provide: Foo, useValue: { '0': 'test' } }]

Unsupported enum member name

Angular couldn't determine the value of the enum member that you referenced in metadata.

The compiler can understand simple enum values but not complex values such as those derived from computed properties.

// ERROR
enum Colors {
  Red = 1,
  White,
  Blue = "Blue".length // computed
}

  ...
  providers: [
    { provide: BaseColor,   useValue: Colors.White } // ok
    { provide: DangerColor, useValue: Colors.Red }   // ok
    { provide: StrongColor, useValue: Colors.Blue }  // bad
  ]
  ...

Avoid referring to enums with complicated initializers or computed properties.

Tagged template expressions are not supported

Tagged template expressions are not supported in metadata.

The compiler encountered a JavaScript ES2015 tagged template expression such as,

// ERROR
const expression = 'funky';
const raw = String.raw`A tagged template ${expression} string`;
 ...
 template: '<div>' + raw + '</div>'
 ...

String.raw() is a tag function native to JavaScript ES2015.

The AOT compiler does not support tagged template expressions; avoid them in metadata expressions.

Symbol reference expected

The compiler expected a reference to a symbol at the location specified in the error message.

This error can occur if you use an expression in the extends clause of a class.

Conclusion

This page covered:

• What the AOT compiler does.
• Why metadata must be written in a subset of JavaScript.
• What that subset is.
• Other restrictions on metadata definition.
• Macro-functions and macro-static methods.
• Compiler errors related to metadata.