Deprecated and obsolete features

This page lists features of JavaScript that are deprecated (that is, still available but planned for removal) and obsolete (that is, no longer usable).

Deprecated features

These deprecated features can still be used, but should be used with caution because they are expected to be removed entirely sometime in the future. You should work to remove their use from your code.

RegExp properties

The following properties are deprecated. This does not affect their use in replacement strings:

Property	Description
`$1-$9`	Parenthesized substring matches, if any. Warning: Using these properties can result in problems, since browser extensions can modify them. Avoid them!
`$_`	See `input`.
`$*`	See `multiline`.
`$&`	See `lastMatch`.
`$+`	See `lastParen`.
$`	See `leftContext`.
`$'`	See `rightContext`.
`input`	The string against which a regular expression is matched.
`lastMatch`	The last matched characters.
`lastParen`	The last parenthesized substring match, if any.
`leftContext`	The substring preceding the most recent match.
`rightContext`	The substring following the most recent match.

The following are now properties of RegExp instances, no longer of the RegExp object:

Property	Description
`global`	Whether or not to test the regular expression against all possible matches in a string, or only against the first.
`ignoreCase`	Whether or not to ignore case while attempting a match in a string.
`lastIndex`	The index at which to start the next match.
`multiline`	Whether or not to search in strings across multiple lines.
`source`	The text of the pattern.

RegExp methods

The compile() method is deprecated.
The valueOf method is no longer specialized for RegExp. Use Object.valueOf().

Function properties

The caller and arguments properties are deprecated, because they leak the function caller. Instead of the arguments property, you should use the arguments object inside function closures.

Legacy generator

Legacy generator function statement and Legacy generator function expression are deprecated. Use function* statement and function* expression instead.
JS1.7/JS1.8 Array comprehension and JS1.7/JS1.8 Generator comprehension are deprecated.

Iterator

StopIteration is deprecated.
Iterator is deprecated.

Object methods

watch and unwatch are deprecated. Use Proxy instead.
__iterator__ is deprecated.
__noSuchMethod__ is deprecated. Use Proxy instead.

Date methods

getYear and setYear are affected by the Year-2000-Problem and have been subsumed by getFullYear and setFullYear.
You should use toISOString instead of the deprecated toGMTString method in new code.
toLocaleFormat is deprecated.

Functions

Expression closures are deprecated. Use regular functions or arrow functions instead.

Proxy

Proxy.create and Proxy.createFunction are deprecated. Use Proxy instead.
The following traps are obsolete:
- hasOwn (bug 980565, Firefox 33).
- getEnumerablePropertyKeys (bug 783829, Firefox 37)
- getOwnPropertyNames (bug 1007334, Firefox 33)
- keys (bug 1007334, Firefox 33)

Escape sequences

Octal escape sequences (\ followed by one, two, or three octal digits) are deprecated in string and regular expression literals.
The escape and unescape functions are deprecated. Use encodeURI, encodeURIComponent, decodeURI or decodeURIComponent to encode and decode escape sequences for special characters.

String methods

HTML wrapper methods like String.prototype.fontsize and String.prototype.big.
String.prototype.quote is removed from Firefox 37.
non standard flags parameter in String.prototype.search, String.prototype.match, and String.prototype.replace are deprecated.

Obsolete features

These obsolete features have been entirely removed from JavaScript and can no longer be used as of the indicated version of JavaScript.

Object

Property	Description
`__count__`	Returns the number of enumerable properties directly on a user-defined object.
`__parent__`	Points to an object's context.
`Object.prototype.eval()`	Evaluates a string of JavaScript code in the context of the specified object.
`Object.observe()`	Asynchronously observing the changes to an object.
`Object.unobserve()`	Remove observers.
`Object.getNotifier()`	Creates an object that allows to synthetically trigger a change.

Function

Property	Description
`arity`	Number of formal arguments.

Array

Property	Description
`Array.observe()`	Asynchronously observing changes to Arrays.
`Array.unobserve()`	Remove observers.

Number

Number.toInteger()

ParallelArray

ParallelArray

Statements

for each...in is deprecated. Use for...of instead.
Destructuring for...in is deprecated. Use for...of instead.
let blocks and let expressions are obsolete.

E4X

See E4X for more information.

Sharp variables

See Sharp variables in JavaScript for more information.

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Contributors to this page: fscholz, cdr, arai, ethertank, Sheppy, ziyunfei, CacycleWP, evilpie, Nanto vi, Mgjbot, Ptak82, Dria

Last updated by: fscholz, Oct 6, 2016, 3:04:16 AM