Util

The util module is primarily designed to support the needs of Node.js' own internal APIs. However, many of the utilities are useful for application and module developers as well. It can be accessed using:

const util = require('util');

util.callbackify(original)

• original <Function> An async function
• Returns: <Function> a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking a (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

For example:

const util = require('util');

async function fn() {
  return 'hello world';
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});

Will print:

hello world

Note:

• The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

• Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

  function fn() {
    return Promise.reject(null);
  }
  const callbackFunction = util.callbackify(fn);
  
  callbackFunction((err, ret) => {
    // When the Promise was rejected with `null` it is wrapped with an Error and
    // the original value is stored in `reason`.
    err && err.hasOwnProperty('reason') && err.reason === null;  // true
  });
  

util.debuglog(section)

• section <string> A string identifying the portion of the application for which the debuglog function is being created.
• Returns: <Function> The logging function

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

For example:

const util = require('util');
const debuglog = util.debuglog('foo');

debuglog('hello from foo [%d]', 123);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable. For example: NODE_DEBUG=fs,net,tls.

util.deprecate(function, string)

The util.deprecate() method wraps the given function or class in such a way that it is marked as deprecated.

const util = require('util');

exports.puts = util.deprecate(function() {
  for (let i = 0, len = arguments.length; i < len; ++i) {
    process.stdout.write(arguments[i] + '\n');
  }
}, 'util.puts: Use console.log instead');

When called, util.deprecate() will return a function that will emit a DeprecationWarning using the process.on('warning') event. By default, this warning will be emitted and printed to stderr exactly once, the first time it is called. After the warning is emitted, the wrapped function is called.

If either the --no-deprecation or --no-warnings command line flags are used, or if the process.noDeprecation property is set to true prior to the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to stderr the first time the deprecated function is called.

If the --throw-deprecation command line flag is set, or the process.throwDeprecation property is set to true, then an exception will be thrown when the deprecated function is called.

The --throw-deprecation command line flag and process.throwDeprecation property take precedence over --trace-deprecation and process.traceDeprecation.

util.format(format[, ...args])

• format <string> A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format.

The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:

• %s - String.
• %d - Number (integer or floating point value).
• %i - Integer.
• %f - Floating point value.
• %j - JSON. Replaced with the string '[Circular]' if the argument contains circular references.
• %o - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() with options { showHidden: true, depth: 4, showProxy: true }. This will show the full object including non-enumerable symbols and properties.
• %O - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() without options. This will show the full object not including non-enumerable symbols and properties.
• %% - single percent sign ('%'). This does not consume an argument.

If the placeholder does not have a corresponding argument, the placeholder is not replaced.

util.format('%s:%s', 'foo');
// Returns: 'foo:%s'

If there are more arguments passed to the util.format() method than the number of placeholders, the extra arguments are coerced into strings then concatenated to the returned string, each delimited by a space. Excessive arguments whose typeof is 'object' or 'symbol' (except null) will be transformed by util.inspect().

util.format('%s:%s', 'foo', 'bar', 'baz'); // 'foo:bar baz'

If the first argument is not a string then util.format() returns a string that is the concatenation of all arguments separated by spaces. Each argument is converted to a string using util.inspect().

util.format(1, 2, 3); // '1 2 3'

If only one argument is passed to util.format(), it is returned as it is without any formatting.

util.format('%% %s'); // '%% %s'

util.getSystemErrorName(err)

• err <number>
• Returns: <string>

Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

fs.access('file/that/does/not/exist', (err) => {
  const name = util.getSystemErrorName(err.errno);
  console.error(name);  // ENOENT
});

util.inherits(constructor, superConstructor)

Note: Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

• constructor <Function>
• superConstructor <Function>

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

const util = require('util');
const EventEmitter = require('events');

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"

ES6 example using class and extends

const EventEmitter = require('events');

class MyStream extends EventEmitter {
  write(data) {
    this.emit('data', data);
  }
}

const stream = new MyStream();

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('With ES6');

util.inspect(object[, options])

• object <any> Any JavaScript primitive or Object.
• options <Object>
  • showHidden <boolean> If true, the object's non-enumerable symbols and properties will be included in the formatted result. Default: false.
  • depth <number> Specifies the number of times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2. To make it recurse indefinitely pass null.
  • colors <boolean> If true, the output will be styled with ANSI color codes. Colors are customizable, see Customizing util.inspect colors. Default: false.
  • customInspect <boolean> If false, then custom inspect(depth, opts) functions exported on the object being inspected will not be called. Default: true.
  • showProxy <boolean> If true, then objects and functions that are Proxy objects will be introspected to show their target and handler objects. Default: false.
  • maxArrayLength <number> Specifies the maximum number of array and TypedArray elements to include when formatting. Set to null to show all array elements. Set to 0 or negative to show no array elements. Default: 100.
  • breakLength <number> The length at which an object's keys are split across multiple lines. Set to Infinity to format an object as a single line. Default: 60 for legacy compatibility.

The util.inspect() method returns a string representation of object that is primarily useful for debugging. Additional options may be passed that alter certain aspects of the formatted string.

The following example inspects all properties of the util object:

const util = require('util');

console.log(util.inspect(util, { showHidden: true, depth: null }));

Values may supply their own custom inspect(depth, opts) functions, when called these receive the current depth in the recursive inspection, as well as the options object passed to util.inspect().

Customizing util.inspect colors

Color output (if enabled) of util.inspect is customizable globally via the util.inspect.styles and util.inspect.colors properties.

util.inspect.styles is a map associating a style name to a color from util.inspect.colors.

The default styles and associated colors are:

• number - yellow
• boolean - yellow
• string - green
• date - magenta
• regexp - red
• null - bold
• undefined - grey
• special - cyan (only applied to functions at this time)
• name - (no styling)

The predefined color codes are: white, grey, black, blue, cyan, green, magenta, red and yellow. There are also bold, italic, underline and inverse codes.

Color styling uses ANSI control codes that may not be supported on all terminals.

Custom inspection functions on Objects

Objects may also define their own [util.inspect.custom](depth, opts) (or the equivalent but deprecated inspect(depth, opts)) function that util.inspect() will invoke and use the result of when inspecting the object:

const util = require('util');

class Box {
  constructor(value) {
    this.value = value;
  }

  [util.inspect.custom](depth, options) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = util.inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

util.inspect(box);
// Returns: "Box< true >"

Custom [util.inspect.custom](depth, opts) functions typically return a string but may return a value of any type that will be formatted accordingly by util.inspect().

const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = (depth) => {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"

util.inspect.custom

A Symbol that can be used to declare custom inspect functions, see Custom inspection functions on Objects.

util.inspect.defaultOptions

The defaultOptions value allows customization of the default options used by util.inspect. This is useful for functions like console.log or util.format which implicitly call into util.inspect. It shall be set to an object containing one or more valid util.inspect() options. Setting option properties directly is also supported.

const util = require('util');
const arr = Array(101).fill(0);

console.log(arr); // logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array

util.promisify(original)

• original <Function>
• Returns: <Function>

Takes a function following the common error-first callback style, i.e. taking a (err, value) => ... callback as the last argument, and returns a version that returns promises.

For example:

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Handle the error.
});

Or, equivalently using async functions:

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);

async function callStat() {
  const stats = await stat('.');
  console.log(`This directory is owned by ${stats.uid}`);
}

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Custom promisified functions

Using the util.promisify.custom symbol one can override the return value of util.promisify():

const util = require('util');

function doSomething(foo, callback) {
  // ...
}

doSomething[util.promisify.custom] = (foo) => {
  return getPromiseSomehow();
};

const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'

This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.

For example, with a function that takes in (foo, onSuccessCallback, onErrorCallback):

doSomething[util.promisify.custom] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};

If promisify.custom is defined but is not a function, promisify() will throw an error.

util.promisify.custom

• <symbol>

A Symbol that can be used to declare custom promisified variants of functions, see Custom promisified functions.

Class: util.TextDecoder

An implementation of the WHATWG Encoding Standard TextDecoder API.

const decoder = new TextDecoder('shift_jis');
let string = '';
let buffer;
while (buffer = getNextChunkSomehow()) {
  string += decoder.decode(buffer, { stream: true });
}
string += decoder.decode(); // end-of-stream

WHATWG Supported Encodings

Per the WHATWG Encoding Standard, the encodings supported by the TextDecoder API are outlined in the tables below. For each encoding, one or more aliases may be used.

Different Node.js build configurations support different sets of encodings. While a very basic set of encodings is supported even on Node.js builds without ICU enabled, support for some encodings is provided only when Node.js is built with ICU and using the full ICU data (see Internationalization).

Encodings Supported Without ICU

Encoding	Aliases
'utf-8'	'unicode-1-1-utf-8', 'utf8'
'utf-16le'	'utf-16'

Encodings Supported by Default (With ICU)

Encoding	Aliases
'utf-8'	'unicode-1-1-utf-8', 'utf8'
'utf-16le'	'utf-16'
'utf-16be'

Encodings Requiring Full ICU Data

Encoding	Aliases
'ibm866'	'866', 'cp866', 'csibm866'
'iso-8859-2'	'csisolatin2', 'iso-ir-101', 'iso8859-2', 'iso88592', 'iso_8859-2', 'iso_8859-2:1987', 'l2', 'latin2'
'iso-8859-3'	'csisolatin3', 'iso-ir-109', 'iso8859-3', 'iso88593', 'iso_8859-3', 'iso_8859-3:1988', 'l3', 'latin3'
'iso-8859-4'	'csisolatin4', 'iso-ir-110', 'iso8859-4', 'iso88594', 'iso_8859-4', 'iso_8859-4:1988', 'l4', 'latin4'
'iso-8859-5'	'csisolatincyrillic', 'cyrillic', 'iso-ir-144', 'iso8859-5', 'iso88595', 'iso_8859-5', 'iso_8859-5:1988'
'iso-8859-6'	'arabic', 'asmo-708', 'csiso88596e', 'csiso88596i', 'csisolatinarabic', 'ecma-114', 'iso-8859-6-e', 'iso-8859-6-i', 'iso-ir-127', 'iso8859-6', 'iso88596', 'iso_8859-6', 'iso_8859-6:1987'
'iso-8859-7'	'csisolatingreek', 'ecma-118', 'elot_928', 'greek', 'greek8', 'iso-ir-126', 'iso8859-7', 'iso88597', 'iso_8859-7', 'iso_8859-7:1987', 'sun_eu_greek'
'iso-8859-8'	'csiso88598e', 'csisolatinhebrew', 'hebrew', 'iso-8859-8-e', 'iso-ir-138', 'iso8859-8', 'iso88598', 'iso_8859-8', 'iso_8859-8:1988', 'visual'
'iso-8859-8-i'	'csiso88598i', 'logical'
'iso-8859-10'	'csisolatin6', 'iso-ir-157', 'iso8859-10', 'iso885910', 'l6', 'latin6'
'iso-8859-13'	'iso8859-13', 'iso885913'
'iso-8859-14'	'iso8859-14', 'iso885914'
'iso-8859-15'	'csisolatin9', 'iso8859-15', 'iso885915', 'iso_8859-15', 'l9'
'koi8-r'	'cskoi8r', 'koi', 'koi8', 'koi8_r'
'koi8-u'	'koi8-ru'
'macintosh'	'csmacintosh', 'mac', 'x-mac-roman'
'windows-874'	'dos-874', 'iso-8859-11', 'iso8859-11', 'iso885911', 'tis-620'
'windows-1250'	'cp1250', 'x-cp1250'
'windows-1251'	'cp1251', 'x-cp1251'
'windows-1252'	'ansi_x3.4-1968', 'ascii', 'cp1252', 'cp819', 'csisolatin1', 'ibm819', 'iso-8859-1', 'iso-ir-100', 'iso8859-1', 'iso88591', 'iso_8859-1', 'iso_8859-1:1987', 'l1', 'latin1', 'us-ascii', 'x-cp1252'
'windows-1253'	'cp1253', 'x-cp1253'
'windows-1254'	'cp1254', 'csisolatin5', 'iso-8859-9', 'iso-ir-148', 'iso8859-9', 'iso88599', 'iso_8859-9', 'iso_8859-9:1989', 'l5', 'latin5', 'x-cp1254'
'windows-1255'	'cp1255', 'x-cp1255'
'windows-1256'	'cp1256', 'x-cp1256'
'windows-1257'	'cp1257', 'x-cp1257'
'windows-1258'	'cp1258', 'x-cp1258'
'x-mac-cyrillic'	'x-mac-ukrainian'
'gbk'	'chinese', 'csgb2312', 'csiso58gb231280', 'gb2312', 'gb_2312', 'gb_2312-80', 'iso-ir-58', 'x-gbk'
'gb18030'
'big5'	'big5-hkscs', 'cn-big5', 'csbig5', 'x-x-big5'
'euc-jp'	'cseucpkdfmtjapanese', 'x-euc-jp'
'iso-2022-jp'	'csiso2022jp'
'shift_jis'	'csshiftjis', 'ms932', 'ms_kanji', 'shift-jis', 'sjis', 'windows-31j', 'x-sjis'
'euc-kr'	'cseuckr', 'csksc56011987', 'iso-ir-149', 'korean', 'ks_c_5601-1987', 'ks_c_5601-1989', 'ksc5601', 'ksc_5601', 'windows-949'

Note: The 'iso-8859-16' encoding listed in the WHATWG Encoding Standard is not supported.

new TextDecoder([encoding[, options]])

Creates an new TextDecoder instance. The encoding may specify one of the supported encodings or an alias.

textDecoder.decode([input[, options]])

Decodes the input and returns a string. If options.stream is true, any incomplete byte sequences occurring at the end of the input are buffered internally and emitted after the next call to textDecoder.decode().

If textDecoder.fatal is true, decoding errors that occur will result in a TypeError being thrown.

textDecoder.encoding

The encoding supported by the TextDecoder instance.

textDecoder.fatal

The value will be true if decoding errors result in a TypeError being thrown.

textDecoder.ignoreBOM

The value will be true if the decoding result will include the byte order mark.

Class: util.TextEncoder

An implementation of the WHATWG Encoding Standard TextEncoder API. All instances of TextEncoder only support UTF-8 encoding.

const encoder = new TextEncoder();
const uint8array = encoder.encode('this is some data');

textEncoder.encode([input])

UTF-8 encodes the input string and returns a Uint8Array containing the encoded bytes.

textEncoder.encoding

The encoding supported by the TextEncoder instance. Always set to 'utf-8'.

Deprecated APIs

The following APIs have been deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.

util._extend(target, source)

The util._extend() method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through Object.assign().

util.debug(string)

• string <string> The message to print to stderr

Deprecated predecessor of console.error.

util.error([...strings])

• ...strings <string> The message to print to stderr

Deprecated predecessor of console.error.

util.isArray(object)

• object <any>

Internal alias for Array.isArray.

Returns true if the given object is an Array. Otherwise, returns false.

const util = require('util');

util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false

util.isBoolean(object)

• object <any>

Returns true if the given object is a Boolean. Otherwise, returns false.

const util = require('util');

util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true

util.isBuffer(object)

• object <any>

Returns true if the given object is a Buffer. Otherwise, returns false.

const util = require('util');

util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true

util.isDate(object)

• object <any>

Returns true if the given object is a Date. Otherwise, returns false.

const util = require('util');

util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false

util.isError(object)

• object <any>

Returns true if the given object is an Error. Otherwise, returns false.

const util = require('util');

util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false

Note that this method relies on Object.prototype.toString() behavior. It is possible to obtain an incorrect result when the object argument manipulates @@toStringTag.

const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };

util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true

util.isFunction(object)

• object <any>

Returns true if the given object is a Function. Otherwise, returns false.

const util = require('util');

function Foo() {}
const Bar = () => {};

util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true

util.isNull(object)

• object <any>

Returns true if the given object is strictly null. Otherwise, returns false.

const util = require('util');

util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true

util.isNullOrUndefined(object)

• object <any>

Returns true if the given object is null or undefined. Otherwise, returns false.

const util = require('util');

util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true

util.isNumber(object)

• object <any>

Returns true if the given object is a Number. Otherwise, returns false.

const util = require('util');

util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true

util.isObject(object)

• object <any>

Returns true if the given object is strictly an Object and not a Function. Otherwise, returns false.

const util = require('util');

util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(function() {});
// Returns: false

util.isPrimitive(object)

• object <any>

Returns true if the given object is a primitive type. Otherwise, returns false.

const util = require('util');

util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(function() {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false

util.isRegExp(object)

• object <any>

Returns true if the given object is a RegExp. Otherwise, returns false.

const util = require('util');

util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false

util.isString(object)

• object <any>

Returns true if the given object is a string. Otherwise, returns false.

const util = require('util');

util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false

util.isSymbol(object)

• object <any>

Returns true if the given object is a Symbol. Otherwise, returns false.

const util = require('util');

util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true

util.isUndefined(object)

• object <any>

Returns true if the given object is undefined. Otherwise, returns false.

const util = require('util');

const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false

util.log(string)

• string <string>

The util.log() method prints the given string to stdout with an included timestamp.

const util = require('util');

util.log('Timestamped message.');

util.print([...strings])

Deprecated predecessor of console.log.

util.puts([...strings])

Deprecated predecessor of console.log.