Util | Node.js v6.11.4 Documentation

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# Util#

Stability: 2 - Stable

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.debuglog(section)#

Added in: v0.11.3

- `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)#

Added in: v0.8.0

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])#

Added in: v0.5.3

- `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 (both integer and float).
- `%j` - JSON. Replaced with the string `'[Circular]'` if the argument contains circular references.
- `%%` - 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.inherits(constructor, superConstructor)#

Added in: v0.3.0

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 {
      constructor() {
        super();
      }
      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])#

Added in: v0.3.0

- `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. Defaults to `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. Defaults to `false`. Colors are customizable, see Customizing `util.inspect` colors.
  - `customInspect` <boolean> If `false`, then custom `inspect(depth, opts)` functions exported on the `object` being inspected will not be called. Defaults to `true`.
  - `showProxy` <boolean> If `true`, then objects and functions that are `Proxy` objects will be introspected to show their `target` and `handler` objects. Defaults to `false`.
  - `maxArrayLength` <number> Specifies the maximum number of array and `TypedArray` elements to include when formatting. Defaults to `100`. Set to `null` to show all array elements. Set to `0` or negative to show no array elements.
  - `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. Defaults to 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, equivalently `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;
      }

      inspect(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] = function(depth) {
      return { bar: 'baz' };
    };

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

A custom inspection method can alternatively be provided by exposing an `inspect(depth, opts)` method on the object:

    const util = require('util');

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

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

### util.inspect.defaultOptions#

Added in: v6.4.0

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);

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

### util.inspect.custom#

Added in: v6.6.0

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

## 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.debug(string)#

Added in: v0.3.0 Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use `console.error()` instead.

- `string` <string> The message to print to `stderr`

Deprecated predecessor of `console.error`.

### util.error([...strings])#

Added in: v0.3.0 Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use `console.error()` instead.

- `...strings` <string> The message to print to `stderr`

Deprecated predecessor of `console.error`.

### util.isArray(object)#

Added in: v0.6.0 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use `Buffer.isBuffer()` instead.

- `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)#

Added in: v0.6.0 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.6.0 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.6.0 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.11.5 Deprecated since: v4.0.0
Stability: 0 - Deprecated

- `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)#

Added in: v0.3.0 Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use a third party module instead.

- `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])#

Added in: v0.3.0 Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use `console.log()` instead.

Deprecated predecessor of `console.log`.

### util.puts([...strings])#

Added in: v0.3.0 Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use `console.log()` instead.

Deprecated predecessor of `console.log`.

### util._extend(target, source)#

Added in: v0.7.5 Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use `Object.assign()` instead.

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()`.