util Node.js v4.2.6 Manual & Documentation

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# util#

    Stability: 2 - Stable

These functions are in the module `'util'`. Use `require('util')` to access them.

The `util` module is primarily designed to support the needs of Node.js's internal APIs. Many of these utilities are useful for your own programs. If you find that these functions are lacking for your purposes, however, you are encouraged to write your own utilities. We are not interested in any future additions to the `util` module that are unnecessary for Node.js's internal functionality.

## util.debug(string)#

    Stability: 0 - Deprecated: use console.error() instead.

Deprecated predecessor of `console.error`.

## util.debuglog(section)#

- `section` String The section of the program to be debugged
- Returns: Function The logging function

This is used to create a function which conditionally writes to stderr based on the existence of a `NODE_DEBUG` environment variable. If the `section` name appears in that environment variable, then the returned function will be similar to `console.error()`. If not, then the returned function is a no-op.

For example:

    var debuglog = util.debuglog('foo');

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

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.

You may separate multiple `NODE_DEBUG` environment variables with a comma. For example, `NODE_DEBUG=fs,net,tls`.

## util.deprecate(function, string)#

Marks that a method should not be used any more.

    const util = require('util');

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

It returns a modified function which warns once by default.

If `--no-deprecation` is set then this function is a NO-OP. Configurable at run-time through the `process.noDeprecation` boolean (only effective when set before a module is loaded.)

If `--trace-deprecation` is set, a warning and a stack trace are logged to the console the first time the deprecated API is used. Configurable at run-time through the `process.traceDeprecation` boolean.

If `--throw-deprecation` is set then the application throws an exception when the deprecated API is used. Configurable at run-time through the `process.throwDeprecation` boolean.

`process.throwDeprecation` takes precedence over `process.traceDeprecation`.

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

    Stability: 0 - Deprecated: Use console.error() instead.

Deprecated predecessor of `console.error`.

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

Returns a formatted string using the first argument as a `printf`-like format.

The first argument is a string that contains zero or more placeholders. Each placeholder is replaced with the converted value from its 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'); // 'foo:%s'

If there are more arguments than placeholders, the extra arguments are coerced to strings (for objects and symbols, `util.inspect()` is used) and then concatenated, delimited by a space.

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

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

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

## util.inherits(constructor, superConstructor)#

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);
    }

    var 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!"

## util.inspect(object[, options])#

Return a string representation of `object`, which is useful for debugging.

An optional options object may be passed that alters certain aspects of the formatted string:

- `showHidden` - if `true` then the object's non-enumerable and symbol properties will be shown too. Defaults to `false`.

- `depth` - tells `inspect` how many 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` - if `true`, then the output will be styled with ANSI color codes. Defaults to `false`. Colors are customizable, see below.

- `customInspect` - if `false`, then custom `inspect(depth, opts)` functions defined on the objects being inspected won't be called. Defaults to `true`.

Example of inspecting 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 they 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 `util.inspect.styles` and `util.inspect.colors` objects.

`util.inspect.styles` is a map assigning each style a color from `util.inspect.colors`. Highlighted styles and their default values are: `number` (yellow) `boolean` (yellow) `string` (green) `date` (magenta) `regexp` (red) `null` (bold) `undefined` (grey) `special` - only function at this time (cyan) * `name` (intentionally no styling)

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

### Custom `inspect()` function on Objects#

Objects also may define their own `inspect(depth)` function which `util.inspect()` will invoke and use the result of when inspecting the object:

    const util = require('util');

    var obj = { name: 'nate' };
    obj.inspect = function(depth) {
      return `{${this.name}}`;
    };

    util.inspect(obj);
      // "{nate}"

You may also return another Object entirely, and the returned String will be formatted according to the returned Object. This is similar to how `JSON.stringify()` works:

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

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

## util.isArray(object)#

    Stability: 0 - Deprecated

Internal alias for `Array.isArray`.

Returns `true` if the given "object" is an `Array`. `false` otherwise.

    const util = require('util');

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

## util.isBoolean(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `Boolean`. `false` otherwise.

    const util = require('util');

    util.isBoolean(1)
      // false
    util.isBoolean(0)
      // false
    util.isBoolean(false)
      // true

## util.isBuffer(object)#

    Stability: 0 - Deprecated

Use `Buffer.isBuffer()` instead.

Returns `true` if the given "object" is a `Buffer`. `false` otherwise.

    const util = require('util');

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

## util.isDate(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `Date`. `false` otherwise.

    const util = require('util');

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

## util.isError(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is an `Error`. `false` otherwise.

    const util = require('util');

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

## util.isFunction(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `Function`. `false` otherwise.

    const util = require('util');

    function Foo() {}
    var Bar = function() {};

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

## util.isNull(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is strictly `null`. `false` otherwise.

    const util = require('util');

    util.isNull(0)
      // false
    util.isNull(undefined)
      // false
    util.isNull(null)
      // true

## util.isNullOrUndefined(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is `null` or `undefined`. `false` otherwise.

    const util = require('util');

    util.isNullOrUndefined(0)
      // false
    util.isNullOrUndefined(undefined)
      // true
    util.isNullOrUndefined(null)
      // true

## util.isNumber(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `Number`. `false` otherwise.

    const util = require('util');

    util.isNumber(false)
      // false
    util.isNumber(Infinity)
      // true
    util.isNumber(0)
      // true
    util.isNumber(NaN)
      // true

## util.isObject(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is strictly an `Object` and not a `Function`. `false` otherwise.

    const util = require('util');

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

## util.isPrimitive(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a primitive type. `false` otherwise.

    const util = require('util');

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

## util.isRegExp(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `RegExp`. `false` otherwise.

    const util = require('util');

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

## util.isString(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `String`. `false` otherwise.

    const util = require('util');

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

## util.isSymbol(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is a `Symbol`. `false` otherwise.

    const util = require('util');

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

## util.isUndefined(object)#

    Stability: 0 - Deprecated

Returns `true` if the given "object" is `undefined`. `false` otherwise.

    const util = require('util');

    var foo;
    util.isUndefined(5)
      // false
    util.isUndefined(foo)
      // true
    util.isUndefined(null)
      // false

## util.log(string)#

Output with timestamp on `stdout`.

    require('util').log('Timestamped message.');

## util.print([...])#

    Stability: 0 - Deprecated: Use console.log instead.

Deprecated predecessor of `console.log`.

## util.pump(readableStream, writableStream[, callback])#

    Stability: 0 - Deprecated: Use readableStream.pipe(writableStream)

Deprecated predecessor of `stream.pipe()`.

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

    Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of `console.log`.