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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features below:

Deprecated and obsolete features - JavaScript

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

These deprecated features can still be used, but should be used with caution because they are not required to be implemented by every JavaScript engine. You should work to remove their use from your code.

Some of these deprecated features are listed in the Annex B section of the ECMAScript specification. This section is described as normative optional — that is, web browser hosts must implement these features, while non-web hosts may not. These features are likely stable because removing them will cause backward compatibility issues and break legacy websites. (JavaScript has the design goal of "don't break the web".) Still, they are not cross-platform portable and may not be supported by all analysis tools, so you are advised to not use them, as the introduction of Annex B states:

… All of the language features and behaviors specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. …

… Programmers should not use or assume the existence of these features and behaviors when writing new ECMAScript code. …

Some others, albeit in the main spec body, are also marked as normative optional and should not be depended on.

JavaScript source, if parsed as scripts, allows HTML-like comments, as if the script is part of a <script> tag.

The following is valid JavaScript when running in a web browser (or Node.js, which uses the V8 engine powering Chrome):

<!-- comment
console.log("a"); <!-- another comment
console.log("b");
--> More comment
// Logs "a" and "b"

<!-- and --> both act like //, i.e., starting line comments. --> is only valid at the start of a line (to avoid ambiguity with a postfix decrement followed by a greater than operator), while <!-- can occur anywhere in the line.

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

$1–$9

Parenthesized substring matches, if any.

input, $_

The string against which a regular expression is matched.

lastMatch, $&

The last matched substring.

lastParen, $+

The last parenthesized substring match, if any.

leftContext, $`

The substring preceding the most recent match.

rightContext, $'

The substring following the most recent match.

Warning: Avoid using these static properties, as they can cause issues when interacting with external code!

The compile() method is deprecated. Construct a new RegExp instance instead.

The following regex syntaxes are deprecated and only available in Unicode-unaware mode. In Unicode-aware mode, they are all syntax errors:

• Lookahead assertions can have quantifiers.
• Backreferences that do not refer to an existing capturing group become legacy octal escapes.
• In character classes, character ranges where one boundary is a character class makes the - become a literal character.
• An escape sequence that's not recognized becomes an "identity escape".
• Escape sequences within character classes of the form \cX where X is a number or _ are decoded in the same way as those with ASCII letters: \c0 is the same as \cP when taken modulo 32. In addition, if the form \cX is encountered anywhere where X is not one of the recognized characters, then the backslash is treated as a literal character.
• The sequence \k within a regex that doesn't have any named capturing groups is treated as an identity escape.
• The syntax characters ], {, and } may appear literally without escaping if they cannot be interpreted as the end of a character class or quantifier delimiters.

• The caller property of functions and the arguments.callee property are deprecated and unavailable in strict mode.
• Instead of accessing arguments as a property of a function, you should use the arguments object inside function closures.

• HTML wrapper methods like String.prototype.fontsize and String.prototype.big.
• String.prototype.substr probably won't be removed anytime soon, but it's defined in Annex B and hence normative optional.
• String.prototype.trimLeft and String.prototype.trimRight should be replaced with String.prototype.trimStart and String.prototype.trimEnd.

• The getYear() and setYear() methods are affected by the Year-2000-Problem and have been subsumed by getFullYear and setFullYear.
• The toGMTString() method is deprecated. Use toUTCString() instead.

• 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.

The with statement is deprecated and unavailable in strict mode.

Initializers in var declarations of for...in loops headers are deprecated and produce syntax errors in strict mode. The initializer expression is evaluated and assigned to the variable, but the value would be immediately reassigned on the first iteration of the loop.

Normally, the catch block of a try...catch statement cannot contain any variable declaration with the same name as the variables bound in the catch(). An extension grammar allows the catch block to contain a var declared variable with the same name as the catch-bound identifier, but only if the catch binding is a simple identifier, not a destructuring pattern. However, this variable's initialization and assignment would only act on the catch-bound identifier, instead of the upper scope variable, and the behavior could be confusing.

var a = 2;
try {
  throw new Error();
} catch (a) {
  var a = 1; // This 1 is assigned to the caught `a`, not the outer `a`.
}
console.log(a); // 2

try {
  throw new Error();
  // Note: identifier changed to `err` to avoid conflict with
  // the inner declaration of `a`.
} catch (err) {
  var a = 1; // This 1 is assigned to the upper-scope `a`.
}
console.log(a); // 1

This is listed in Annex B of the spec and hence may not be implemented everywhere. Avoid any name conflicts between the catch-bound identifier and variables declared in the catch block.

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

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

The valueOf() method is no longer specialized for RegExp. It uses Object.prototype.valueOf(), which returns itself.

• Functions' arity property is obsolete. Use length instead.

• Non-standard String generic methods like String.slice(myStr, 0, 12), String.replace(myStr, /\./g, "!"), etc. have been introduced in Firefox 1.5 (JavaScript 1.6), deprecated in Firefox 53, and removed in Firefox 68. You can use methods on String.prototype together with Function.call instead.
• String.prototype.quote is removed from Firefox 37.
• Non-standard flags parameter in String.prototype.search, String.prototype.match, and String.prototype.replace are obsolete.

• WeakMap.prototype.clear() was added in Firefox 20 and removed in Firefox 46. It is not possible to traverse all keys in a WeakMap.

• Date.prototype.toLocaleFormat(), which used a format string in the same format expected by the strftime() function in C, is obsolete. Use toLocaleString() or Intl.DateTimeFormat instead.

• Non-standard Array generic methods like Array.slice(myArr, 0, 12), Array.forEach(myArr, myFn), etc. have been introduced in Firefox 1.5 (JavaScript 1.6), deprecated in Firefox 68, and removed in Firefox 71. You can use methods on Array.prototype together with Function.call instead.

• Number.toInteger() is obsolete. Use Math.floor, Math.round, or other methods instead.

• Proxy.create and Proxy.createFunction are obsolete. Use the Proxy() constructor 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)

• ParallelArray is obsolete.

• for each...in is obsolete. Use for...of instead.
• let blocks and let expressions are obsolete.
• Expression closures (function () 1 as a shorthand of function () { return 1; }) are obsolete. Use regular functions or arrow functions instead.

The toSource() methods of arrays, numbers, strings, etc. and the uneval() global function are obsolete. Use toString(), or write your own serialization method instead.

Legacy generator function statements and legacy generator function expressions are removed. The legacy generator function syntax reuses the function keyword, which automatically becomes a generator function when there are one or more yield expressions in the body — this is now a syntax error. Use function* statements and function* expressions instead.

Array comprehensions and generator comprehensions are removed.

// Legacy array comprehensions
[for (x of iterable) x]
[for (x of iterable) if (condition) x]
[for (x of iterable) for (y of iterable) x + y]

// Legacy generator comprehensions
(for (x of iterable) x)
(for (x of iterable) if (condition) x)
(for (x of iterable) for (y of iterable) x + y)

Firefox, prior to version 26, implemented another iterator protocol that is similar to the standard Iterator protocol. An object is an legacy iterator when it implements a next() method, which produces a value on each call and throws a StopIteration object at the end of iteration. This legacy iterator protocol differs from the standard iterator protocol:

• The value was returned directly as the return value of calls to next(), instead of the value property of the IteratorResult object.
• Iteration termination was expressed by throwing a StopIteration object, instead of through the done property of the IteratorResult object.

This feature, along with the StopIteration global constructor, was removed in Firefox 58+. For future-facing usages, consider using for...of loops and the iterator protocol.

Sharp variables are obsolete. To create circular structures, use temporary variables instead.

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