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doc: general improvements to util.md 

PR-URL: https://github.com/nodejs/node/pull/6897
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
v7.x
James M Snell 9 years ago
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      doc/api/util.md

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doc/api/util.md
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@ -2,40 +2,32 @@
Stability: 2 - Stable
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.js' own
internal APIs. However, many of the utilities are useful for application and
module developers as well. It can be accessed using:
The `util` module is primarily designed to support the needs of Node.js'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.js's internal functionality.
## util.debug(string)
Stability: 0 - Deprecated: Use [`console.error()`][] instead.
Deprecated predecessor of `console.error`.
```js
const util = require('util');
```
## util.debuglog(section)
* `section` {String} The section of the program to be debugged
* `section` {String} A string identifying the portion of the application for
which the `debuglog` function is being created.
* 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.
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 operate similar to
`console.error()`. If not, then the returned function is a no-op.
For example:
```js
var debuglog = util.debuglog('foo');
const debuglog = util.debuglog('foo');
var bar = 123;
debuglog('hello from foo [%d]', bar);
debuglog('hello from foo [%d]', 123);
```
If this program is run with `NODE_DEBUG=foo` in the environment, then
@ -48,12 +40,13 @@ 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`.
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)
Marks that a method should not be used any more.
The `util.deprecate()` method wraps the given `function` in such a way that
marks it as being deprecated.
```js
const util = require('util');
@ -65,35 +58,39 @@ exports.puts = util.deprecate(() => {
}, 'util.puts: Use console.log instead');
```
It returns a modified function which warns once by default.
When called, `util.deprecated()` 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 that it is called. After the warning is emitted, the wrapped `function`
is called.
This function does nothing if either the `--no-deprecation` command
line flag is used, or the `process.noDeprecation` property is set to
`true` *prior* to the first deprecation warning.
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 `--trace-deprecation` is set, a warning and a stack trace are logged
to the console the first time the deprecated API is used. Configurable
at run-time through the `process.traceDeprecation` boolean.
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 `--throw-deprecation` is set then the application throws an exception
when the deprecated API is used. Configurable at run-time through the
`process.throwDeprecation` boolean.
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.
`process.throwDeprecation` takes precedence over `process.traceDeprecation`.
## util.error([...])
Stability: 0 - Deprecated: Use [`console.error()`][] instead.
Deprecated predecessor of `console.error`.
The `--throw-deprecation` command line flag and `process.throwDeprecation`
property take precedence over `--trace-deprecation` and
`process.traceDeprecation`.
## util.format(format[, ...])
Returns a formatted string using the first argument as a `printf`-like format.
* `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 that contains zero or more *placeholders*.
Each placeholder is replaced with the converted value from its corresponding
argument. Supported placeholders are:
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).
@ -105,20 +102,22 @@ If the placeholder does not have a corresponding argument, the placeholder is
not replaced.
```js
util.format('%s:%s', 'foo'); // 'foo:%s'
util.format('%s:%s', 'foo');
// Returns 'foo:%s'
```
If there are more arguments than placeholders, the extra arguments are
coerced to strings (for objects and symbols, `util.inspect()` is used)
and then concatenated, delimited by a space.
If there are more arguments passed to the `util.format()` method than there are
placeholders, the extra arguments are coerced into strings (for objects and
symbols, `util.inspect()` is used) then concatenated to the returned string,
delimited by a space.
```js
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()`.
a string that is the concatenation of all arguments separated by spaces.
Each argument is converted to a string using `util.inspect()`.
```js
util.format(1, 2, 3); // '1 2 3'
@ -130,6 +129,9 @@ _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`.
@ -164,34 +166,33 @@ stream.write('It works!'); // Received data: "It works!"
## 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 and symbol
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 [Customizing
`util.inspect` colors][].
- `customInspect` - if `false`, then custom `inspect(depth, opts)` functions
defined on the objects being inspected won't be called. Defaults to `true`.
- `showProxy` - if `true`, then objects and functions that are Proxy objects
will be introspected to show their `target` and `hander` objects. Defaults to
`false`.
- `maxArrayLength` - 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.
Example of inspecting all properties of the `util` object:
* `object` {any} Any JavaScript primitive or Object.
* `options` {Object}
* `showHidden` {boolean} If `true`, the `object`'s non-enumerable and
symbol properties will be included in the formatted result. Defaults to
`false`.
* `depth` (number) Specifies 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` {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 `hander`
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.
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:
```js
const util = require('util');
@ -200,7 +201,7 @@ 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
called these receive the current `depth` in the recursive inspection, as well as
the options object passed to `util.inspect()`.
### Customizing `util.inspect` colors
@ -208,31 +209,36 @@ the options object passed to `util.inspect()`.
<!-- 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.
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
<!-- 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:
Objects may also define their own `inspect(depth, opts)` function that
`util.inspect()` will invoke and use the result of when inspecting the object:
```js
const util = require('util');
@ -246,9 +252,9 @@ 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:
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()`.
```js
var obj = { foo: 'this will not show up in the inspect() output' };
@ -260,13 +266,36 @@ util.inspect(obj);
// "{ bar: 'baz' }"
```
## util.isArray(object)
## 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 to `stderr`
Deprecated predecessor of `console.error`.
### util.error([...])
Stability: 0 - Deprecated: Use [`console.error()`][] instead.
* `string` {string} The message to print to `stderr`
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`.
Returns `true` if the given `object` is an `Array`. Otherwise, returns `false`.
```js
const util = require('util');
@ -279,11 +308,13 @@ util.isArray({})
// false
```
## util.isBoolean(object)
### util.isBoolean(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `Boolean`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `Boolean`. Otherwise, returns `false`.
```js
const util = require('util');
@ -296,11 +327,13 @@ util.isBoolean(false)
// true
```
## util.isBuffer(object)
### util.isBuffer(object)
Stability: 0 - Deprecated: Use [`Buffer.isBuffer()`][] instead.
Returns `true` if the given "object" is a `Buffer`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `Buffer`. Otherwise, returns `false`.
```js
const util = require('util');
@ -313,11 +346,13 @@ util.isBuffer(Buffer.from('hello world'))
// true
```
## util.isDate(object)
### util.isDate(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `Date`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `Date`. Otherwise, returns `false`.
```js
const util = require('util');
@ -330,11 +365,13 @@ util.isDate({})
// false
```
## util.isError(object)
### util.isError(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is an [`Error`][]. Otherwise, returns
* `object` {any}
Returns `true` if the given `object` is an [`Error`][]. Otherwise, returns
`false`.
```js
@ -350,7 +387,7 @@ util.isError({ name: 'Error', message: 'an error occurred' })
Note that this method relies on `Object.prototype.toString()` behavior. It is
possible to obtain an incorrect result when the `object` argument manipulates
`@@toStringTag`.
the `@@toStringTag` or overrides the `toString()` method.
```js
const util = require('util');
@ -363,11 +400,13 @@ util.isError(obj);
// true
```
## util.isFunction(object)
### util.isFunction(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `Function`. Otherwise, returns
* `object` {any}
Returns `true` if the given `object` is a `Function`. Otherwise, returns
`false`.
```js
@ -384,11 +423,13 @@ util.isFunction(Bar)
// true
```
## util.isNull(object)
### util.isNull(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is strictly `null`. Otherwise, returns
* `object` {any}
Returns `true` if the given `object` is strictly `null`. Otherwise, returns
`false`.
```js
@ -402,11 +443,13 @@ util.isNull(null)
// true
```
## util.isNullOrUndefined(object)
### util.isNullOrUndefined(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is `null` or `undefined`. Otherwise,
* `object` {any}
Returns `true` if the given `object` is `null` or `undefined`. Otherwise,
returns `false`.
```js
@ -420,11 +463,13 @@ util.isNullOrUndefined(null)
// true
```
## util.isNumber(object)
### util.isNumber(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `Number`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `Number`. Otherwise, returns `false`.
```js
const util = require('util');
@ -439,11 +484,13 @@ util.isNumber(NaN)
// true
```
## util.isObject(object)
### util.isObject(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is strictly an `Object` __and__ not a
* `object` {any}
Returns `true` if the given `object` is strictly an `Object` __and__ not a
`Function`. Otherwise, returns `false`.
```js
@ -459,11 +506,13 @@ util.isObject(function(){})
// false
```
## util.isPrimitive(object)
### util.isPrimitive(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a primitive type. Otherwise, returns
* `object` {any}
Returns `true` if the given `object` is a primitive type. Otherwise, returns
`false`.
```js
@ -489,11 +538,13 @@ util.isPrimitive(new Date())
// false
```
## util.isRegExp(object)
### util.isRegExp(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `RegExp`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `RegExp`. Otherwise, returns `false`.
```js
const util = require('util');
@ -506,11 +557,13 @@ util.isRegExp({})
// false
```
## util.isString(object)
### util.isString(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `String`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `string`. Otherwise, returns `false`.
```js
const util = require('util');
@ -529,7 +582,9 @@ util.isString(5)
Stability: 0 - Deprecated
Returns `true` if the given "object" is a `Symbol`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is a `Symbol`. Otherwise, returns `false`.
```js
const util = require('util');
@ -542,11 +597,13 @@ util.isSymbol(Symbol('foo'))
// true
```
## util.isUndefined(object)
### util.isUndefined(object)
Stability: 0 - Deprecated
Returns `true` if the given "object" is `undefined`. Otherwise, returns `false`.
* `object` {any}
Returns `true` if the given `object` is `undefined`. Otherwise, returns `false`.
```js
const util = require('util');
@ -560,35 +617,42 @@ util.isUndefined(null)
// false
```
## util.log(string)
### util.log(string)
Stability: 0 - Deprecated: Use a third party module instead.
Output with timestamp on `stdout`.
* `string` {string}
The `util.log()` method prints the given `string` to `stdout` with an included
timestamp.
require('util').log('Timestamped message.');
```js
const util = require('util')
util.log('Timestamped message.');
```
## util.print([...])
### util.print([...])
Stability: 0 - Deprecated: Use [`console.log()`][] instead.
Deprecated predecessor of `console.log`.
## util.puts([...])
### util.puts([...])
Stability: 0 - Deprecated: Use [`console.log()`][] instead.
Deprecated predecessor of `console.log`.
## util._extend(obj)
### util._extend(obj)
Stability: 0 - Deprecated: Use Object.assign() instead.
`_extend` was never intended to be used outside of internal NodeJS modules. The
community found and used it anyway.
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`.
similar built-in functionality through `Object.assign()`.
[`Array.isArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
[constructor]: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/constructor

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