doc/api/util.markdown
# util
Stability: 4 - API Frozen
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'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.
## 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:
```javascript
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`.
require('util').log('Timestamped message.');
## 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` - 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()`.
### Customizing `util.inspect` colors
<!-- type=misc -->
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
<!-- type=misc -->
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)
Internal alias for Array.isArray.
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
## util.isRegExp(object)
Returns `true` if the given "object" is a `RegExp`. `false` otherwise.
var util = require('util');
util.isRegExp(/some regexp/)
// true
util.isRegExp(new RegExp('another regexp'))
// true
util.isRegExp({})
// false
## 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.
var util = require('util');
util.isError(new Error())
// true
util.isError(new TypeError())
// true
util.isError({ name: 'Error', message: 'an error occurred' })
// false
## util.inherits(constructor, superConstructor)
Inherit the prototype methods from one
[constructor](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/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 = util.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.
## 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.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])
Stability: 0 - Deprecated: Use readableStream.pipe(writableStream)
Deprecated predecessor of `stream.pipe()`.