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Stability: 4 - API Frozen

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'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's internal functionality.


  • 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.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 converted to strings with util.inspect() and these strings are 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'


Output with timestamp on stdout.

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

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 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:

var 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:

var util = require('util');

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

  // "{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' };

  // "{ bar: 'baz' }"


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

var util = require('util');

  // true
util.isArray(new Array)
  // true
  // false


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

var util = require('util');

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


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

var util = require('util');

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


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

var util = require('util');

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

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.

var util = require("util");
var events = require("events");

function MyStream() {;

util.inherits(MyStream, events.EventEmitter);

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

var stream = new MyStream();

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

stream.on("data", function(data) {
    console.log('Received data: "' + data + '"');
stream.write("It works!"); // Received data: "It works!"


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

Deprecated predecessor of console.error.


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

Deprecated predecessor of console.error.


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

Deprecated predecessor of console.log.


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().