Util | Node.js v6.17.1 Documentation (original) (raw)

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.debuglog(section)#

Added in: v0.11.3

section A string identifying the portion of the application for which the debuglog function is being created.
Returns: 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_DEBUGenvironment variable. If the section name appears within the value of that environment variable, then the returned function operates similar toconsole.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_DEBUGenvironment 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 aDeprecationWarning 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 functionis 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 theprocess.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.throwDeprecationproperty take precedence over --trace-deprecation andprocess.traceDeprecation.

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

Added in: v0.5.3

format 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.
%% - 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 whosetypeof is 'object' or 'symbol' (except null) will be transformed byutil.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 andextends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

constructor
superConstructor

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

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 JavaScript primitive or Object.
options
- showHidden If true, the object's non-enumerable symbols and properties will be included in the formatted result. Defaults to false.
- depth 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 If true, the output will be styled with ANSI color codes. Defaults to false. Colors are customizable, seeCustomizing util.inspect colors.
- customInspect If false, then custom inspect(depth, opts)functions exported on the object being inspected will not be called. Defaults to true.
- showProxy If true, then objects and functions that areProxy objects will be introspected to show their target and handlerobjects. Defaults to false.
- maxArrayLength Specifies the maximum number of array andTypedArray elements to include when formatting. Defaults to 100. Set tonull to show all array elements. Set to 0 or negative to show no array elements.
- breakLength 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 fromutil.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 byutil.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 byutil.inspect. This is useful for functions like console.log orutil.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, seeCustom 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

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

...strings The message to print to stderr

Deprecated predecessor of console.error.

util.isArray(object)#

Added in: v0.6.0 Deprecated since: v4.0.0

object

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

object

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

object

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

object

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

object

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

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

object

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

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

object

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

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

object

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

object

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

object

Returns true if the given object is strictly an Object and not aFunction. 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

object

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

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

object

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

object

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

object

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

object

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

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

Deprecated predecessor of console.log.

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

Added in: v0.3.0 Deprecated since: v0.11.3

Deprecated predecessor of console.log.

util._extend(target, source)#

Added in: v0.7.5 Deprecated since: v6.0.0

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 | Node.js v6.17.1 Documentation (original) (raw)

Util#

util.debuglog(section)#

util.deprecate(function, string)#

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

util.inherits(constructor, superConstructor)#

util.inspect(object[, options])#

Customizing util.inspect colors#

Custom inspection functions on Objects#

util.inspect.defaultOptions#

util.inspect.custom#

Deprecated APIs#

util.debug(string)#

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

util.isArray(object)#

util.isBoolean(object)#

util.isBuffer(object)#

util.isDate(object)#

util.isError(object)#

util.isFunction(object)#

util.isNull(object)#

util.isNullOrUndefined(object)#

util.isNumber(object)#

util.isObject(object)#

util.isPrimitive(object)#

util.isRegExp(object)#

util.isString(object)#

util.isSymbol(object)#

util.isUndefined(object)#

util.log(string)#

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

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

util._extend(target, source)#

Customizing `util.inspect` colors#