Node.js v6.17.1 Documentation

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)#

Added in: v0.11.3
  • section <string> A string identifying the portion of the application for which the debuglog 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)#

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 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])#

Added in: v0.5.3
  • format <string> 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 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)#

Added in: v0.3.0

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.

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])#

Added in: v0.3.0
  • object <any> Any JavaScript primitive or Object.
  • options <Object>
    • showHidden <boolean> If true, the object's non-enumerable symbols and properties will be included in the formatted result. Defaults to false.
    • depth <number> 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 <boolean> If true, the output will be styled with ANSI color codes. Defaults to false. Colors are customizable, see Customizing util.inspect colors.
    • customInspect <boolean> If false, then custom inspect(depth, opts) functions exported on the object being inspected will not be called. Defaults to true.
    • showProxy <boolean> If true, then objects and functions that are Proxy objects will be introspected to show their target and handler objects. Defaults to false.
    • maxArrayLength <number> Specifies the maximum number of array and TypedArray elements to include when formatting. Defaults to 100. Set to null to show all array elements. Set to 0 or negative to show no array elements.
    • breakLength <number> 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 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