util

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'

util.log(string)

Output with timestamp on stdout.

util.inspect(object[, options])

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().

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 .
Highlighted styles and their default values are:

• 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 '{' + 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)

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

var util = require('util');
util.isArray([])
  // true
util.isArray(new Array)
  // true
util.isArray({})
  // false

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

var util = require('util');
util.isRegExp(/some regexp/)
util.isRegExp(new RegExp('another regexp'))
  // true
util.isRegExp({})

util.isDate(object)

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

var 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. false otherwise.

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() {
    events.EventEmitter.call(this);
}
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!"

util.deprecate(function, string)

Marks that a method should not be used any more.

exports.puts = exports.deprecate(function() {
  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. If
--throw-deprecation is set then the application will throw an exception
if the deprecated API is used.

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.puts([…])

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

Deprecated predecessor of console.log.

util.print([…])

Stability: 0 - Deprecated: Use `console.log` instead.

Deprecated predecessor of console.log.

util.pump(readableStream, writableStream[, callback])