Table of Contents

• util
  • util.debuglog(section)
  • util.deprecate(function, string)
  • util.format(format[, ...])
  • util.inherits(constructor, superConstructor)
  • util.inspect(object[, options])
    • Customizing util.inspect colors
    • Custom inspect() function on Objects
  • Deprecated APIs
    • util.debug(string)
    • util.error([...])
    • 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([...])
    • util.puts([...])
    • util._extend(obj)

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)#

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 in such a way that it is marked as deprecated.

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');

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[, ...])#

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 (for objects and symbols, util.inspect() is used) then concatenated to the returned string, each 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 arguments separated by spaces. Each argument is converted to a string using util.inspect().

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

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!"

util.inspect(object[, options])#

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 inspect() function on Objects#

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

const util = require('util');

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

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

Custom inspect(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.inspect = function(depth) {
  return { bar: 'baz' };
};

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

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.

Deprecated predecessor of console.error.

util.error([...])#

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

Deprecated predecessor of console.error.

util.isArray(object)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated: Use 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(Buffer.from('hello world'));
  // true

util.isDate(object)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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.

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)#

Stability: 0 - Deprecated

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

const util = require('util');

function Foo() {}
const 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. Otherwise, returns false.

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. Otherwise, returns false.

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. 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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

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)#

Stability: 0 - Deprecated

Returns true if the given object is undefined. Otherwise, returns false.

const util = require('util');

const foo = undefined;
util.isUndefined(5);
  // false
util.isUndefined(foo);
  // true
util.isUndefined(null);
  // false

util.log(string)#

Stability: 0 - Deprecated: Use a third party module instead.

The util.log() method prints the given string to stdout with an included timestamp.

const util = require('util');

util.log('Timestamped message.');

util.print([...])#

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

Deprecated predecessor of console.log.

util.puts([...])#

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

Deprecated predecessor of console.log.

util._extend(obj)#

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().