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Node.js v6.11.3-rc.1 Documentation
Table of Contents
- Util
- util.debuglog(section)
- util.deprecate(function, string)
- util.format(format[, ...args])
- util.inherits(constructor, superConstructor)
- util.inspect(object[, options])
- 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)
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)#
section
<string> A string identifying the portion of the application for which thedebuglog
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)#
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])#
format
<string> Aprintf
-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)#
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])#
object
<any> Any JavaScript primitive or Object.options
<Object>showHidden
<boolean> Iftrue
, theobject
's non-enumerable symbols and properties will be included in the formatted result. Defaults tofalse
.depth
<number> Specifies the number of times to recurse while formatting theobject
. This is useful for inspecting large complicated objects. Defaults to2
. To make it recurse indefinitely passnull
.colors
<boolean> Iftrue
, the output will be styled with ANSI color codes. Defaults tofalse
. Colors are customizable, see Customizingutil.inspect
colors.customInspect
<boolean> Iffalse
, then custominspect(depth, opts)
functions exported on theobject
being inspected will not be called. Defaults totrue
.showProxy
<boolean> Iftrue
, then objects and functions that areProxy
objects will be introspected to show theirtarget
andhandler
objects. Defaults tofalse
.maxArrayLength
<number> Specifies the maximum number of array andTypedArray
elements to include when formatting. Defaults to100
. Set tonull
to show all array elements. Set to0
or negative to show no array elements.breakLength
<number> The length at which an object's keys are split across multiple lines. Set toInfinity
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#
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#
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)#
Stability: 0 - Deprecated: Use console.error()
instead.
string
<string> The message to print tostderr
Deprecated predecessor of console.error
.
util.error([...strings])#
Stability: 0 - Deprecated: Use console.error()
instead.
...strings
<string> The message to print tostderr
Deprecated predecessor of console.error
.
util.isArray(object)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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)#
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])#
Stability: 0 - Deprecated: Use console.log()
instead.
Deprecated predecessor of console.log
.
util.puts([...strings])#
Stability: 0 - Deprecated: Use console.log()
instead.
Deprecated predecessor of console.log
.
util._extend(target, source)#
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()
.