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)
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(() => { 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([...])
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
- iftrue
then the object's non-enumerable and symbol properties will be shown too. Defaults tofalse
. -
depth
- tellsinspect
how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to2
. To make it recurse indefinitely passnull
. -
colors
- iftrue
, then the output will be styled with ANSI color codes. Defaults tofalse
. Colors are customizable, see Customizingutil.inspect
colors. -
customInspect
- iffalse
, then custominspect(depth, opts)
functions defined on the objects being inspected won't be called. Defaults totrue
.
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)
Internal alias for Array.isArray
.
Returns true
if the given "object" is an Array
. Otherwise, returns false
.
const util = require('util'); util.isArray([]) // true util.isArray(new Array) // true util.isArray({}) // false
util.isBoolean(object)
Returns true
if the given "object" is a Boolean
. Otherwise, returns false
.
const util = require('util'); util.isBoolean(1) // false util.isBoolean(0) // false util.isBoolean(false) // true
util.isBuffer(object)
Buffer.isBuffer()
instead.Returns true
if the given "object" is a Buffer
. Otherwise, returns false
.
const util = require('util'); util.isBuffer({ length: 0 }) // false util.isBuffer([]) // false util.isBuffer(new Buffer('hello world')) // true
util.isDate(object)
Returns true
if the given "object" is a Date
. Otherwise, returns false
.
const util = require('util'); util.isDate(new Date()) // true util.isDate(Date()) // false (without 'new' returns a String) util.isDate({}) // false
util.isError(object)
Returns true
if the given "object" is an Error
. Otherwise, returns false
.
const util = require('util'); util.isError(new Error()) // true util.isError(new TypeError()) // true util.isError({ name: 'Error', message: 'an error occurred' }) // 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
.
// This example requires the `--harmony-tostring` flag const util = require('util'); const obj = { name: 'Error', message: 'an error occurred' }; util.isError(obj); // false obj[Symbol.toStringTag] = 'Error'; util.isError(obj); // true
util.isFunction(object)
Returns true
if the given "object" is a Function
. Otherwise, returns false
.
const util = require('util'); function Foo() {} var Bar = function() {}; util.isFunction({}) // false util.isFunction(Foo) // true util.isFunction(Bar) // true
util.isNull(object)
Returns true
if the given "object" is strictly null
. Otherwise, returns false
.
const util = require('util'); util.isNull(0) // false util.isNull(undefined) // false util.isNull(null) // true
util.isNullOrUndefined(object)
Returns true
if the given "object" is null
or undefined
. Otherwise, returns false
.
const util = require('util'); util.isNullOrUndefined(0) // false util.isNullOrUndefined(undefined) // true util.isNullOrUndefined(null) // true
util.isNumber(object)
Returns true
if the given "object" is a Number
. Otherwise, returns false
.
const util = require('util'); util.isNumber(false) // false util.isNumber(Infinity) // true util.isNumber(0) // true util.isNumber(NaN) // true
util.isObject(object)
Returns true
if the given "object" is strictly an Object
and not a Function
. Otherwise, returns false
.
const util = require('util'); util.isObject(5) // false util.isObject(null) // false util.isObject({}) // true util.isObject(function(){}) // false
util.isPrimitive(object)
Returns true
if the given "object" is a primitive type. Otherwise, returns false
.
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)
Returns true
if the given "object" is a RegExp
. Otherwise, returns false
.
const util = require('util'); util.isRegExp(/some regexp/) // true util.isRegExp(new RegExp('another regexp')) // true util.isRegExp({}) // false
util.isString(object)
Returns true
if the given "object" is a String
. Otherwise, returns false
.
const util = require('util'); util.isString('') // true util.isString('foo') // true util.isString(String('foo')) // true util.isString(5) // false
util.isSymbol(object)
Returns true
if the given "object" is a Symbol
. Otherwise, returns false
.
const util = require('util'); util.isSymbol(5) // false util.isSymbol('foo') // false util.isSymbol(Symbol('foo')) // true
util.isUndefined(object)
Returns true
if the given "object" is undefined
. Otherwise, returns false
.
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([...])
console.log()
instead.Deprecated predecessor of console.log
.
util.pump(readableStream, writableStream[, callback])
Deprecated predecessor of stream.pipe()
.
util.puts([...])
console.log()
instead.Deprecated predecessor of console.log
.