Web ECMAScript

This page is for documenting the differences between the ES5 draft and the requirements for ECMAScript implementations in web browsers.

Identifiers

(this is very rough)

Identifiers containing escape sequences are not equivalent to fully unescaped identifiers in the case that, after fully unescaping identifier, it is a ReservedWord. In particular it is possible to create Identifiers that unescape to a reserved word so long as at least one character is fully escaped. Subsequent use of such identifiers must also have at least one character escaped (otherwise the reserved word will be used instead) but it need not be the same character(s) as that originally used to create the identifier.

15.5.4 Properties of the String Prototype Object

Several extra methods are found on String.prototype for wrapping text in HTML elements (these are all generic; the this value need not be a String object):

Algorithm ToHTMLTag(tag_name, content, attribute_name, attribute_value):

1. if attribute_name is undefined return "<" + tag_name + ">" + content + "</" + tag_name + ">"
2. otherwise return "<" + tag_name + " " + attribute_name + "=\"" + attribute_value + "\">" + content + "</" + tag_name + ">"

String.prototype.anchor(name)

1. Let content be ToString(this)
2. Let attribute_value be ToString(name)
3. Return ToHTMLTag("a", content, "name", attribute_value)

String.prototype.big()

1. Let content be ToString(this)
2. Return ToHTMLTag("big", content)

String.prototype.blink()

1. Let content be ToString(this)
2. Return ToHTMLTag("blink", content)

String.prototype.bold()

1. Let content be ToString(this)
2. Return ToHTMLTag("b", content)

String.prototype.fixed()

1. Let content be ToString(this)
2. Return ToHTMLTag("tt", content)

String.prototype.fontcolor(color)

1. Let content be ToString(this)
2. Let attribute_value be ToString(color)
3. Return ToHTMLTag("font", content, "color", attribute_value)

String.prototype.fontsize(size)

1. Let content be ToString(this)
2. Let attribute_value be ToString(size)
3. Return ToHTMLTag("font", content, "size", attribute_value)

String.prototype.italics()

1. Let content be ToString(this)
2. Return ToHTMLTag("i", content)

String.prototype.link(href)

1. Let content be ToString(this)
2. Let attribute_value be ToString(href)
3. Return ToHTMLTag("a", content, "href", attribute_value)

String.prototype.small()

1. Let content be ToString(this)
2. Return ToHTMLTag("small", content)

String.prototype.strike()

1. Let content be ToString(this)
2. Return ToHTMLTag("strike", content)

String.prototype.sub()

1. Let content be ToString(this)
2. Return ToHTMLTag("sub", content)

String.prototype.sup()

1. Let content be ToString(this)
2. Return ToHTMLTag("sup", content)

RegExp

After a regexp is executed the RegExp constructor object has properties $1...$9 which are assigned the values of the first 9 match groups from the previous regexp. (more detail here)

DecimalEscapes in CharacterRanges all behave like \0 rather than throwing syntax errors i.e. /[\1-Z]/ will match any character with a codepoint between 0 and 90.

RegExp.prototype.compile changes the regexp in place. In Carakan/Nitro/V8 the method returns undefined; in SpiderMonkey it returns the regexp object.

Juriy Zaytsev’s non-standard ES5 compatibility table lists some more proprietary extensions.

Date

15.9.4.3 Date.UTC

When called with fewer than 2 arguments Date.UTC must return NaN.

toString

TODO

Global scope

ES5 claims the global scope "this" is the same as the global object, which is not always true in HTML5.

var statements

The erratum in https://bugs.ecmascript.org/show_bug.cgi?id=78#c0 needs to be followed so that var statements at the top level of scripts can shadow any properties from the global object's prototype chain.

Eval

Use of eval not called "eval". Should work but implementations differ on the scope (Spidermonkey first tries global and then local if the object as not found globally, JScript uses local scope, others use global scope), may not have strong compat requirements

eval.apply(this, code) should work but scope again varies when "this" is not the global object

eval.apply({}, code) should throw EvalError

Eval and Global Scopes

See: http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2011-March/030798.html

Date Parsing

TODO:This is a mess

HTML comments

<!-- is a line comment (same as //)

In JScript, the following are ignored if they are at the end of the script (i.e. just followed by whitespace lines): either (1) a line that starts with just whitespace and comments and consists of "-->" followed by anything except new lines and finally followed by another "-->", or (2) one or two lines that start with just whitespace and comments and consist of just "-->". --> at the end of the last line of the script causes the line to be ignored unless the "-->" occurs within a comment. The last non-whitespace line of the script is ignored if it ends with "-->" and doesn't contain "//" or "<!--" (but not in eval).

In SpiderMonkey, "-->" on any line that starts with just whitespace and comments is treated as a line comment (same as //). (Also, ;version=1.6, 1.7 or 1.8 or ;e4x=1 enables E4X <!-- --> comments.)

In Carakan, Nitro and V8, "-->" on any line that starts with just whitespace (but not comments) is treated as a line comment (same as //).

TODO: test Chakra.

Property Enumeration

Enumeration of objects is in insertion order (but host property order compared to user defined property order does not seem to be significant). Property order seems to survive the use of "delete" (i.e. removing then readding a property doesn't change its position), at least in Chrome and Firefox.

Object Properties

All objects have a mutable __proto__ property that is a reference to the prototype of the object. Note that ES5 defines that objects with extensible:false must not have their prototype mutated. In the case that setting an objects __proto__ would cause a prototype chain to become cyclic, the setter must fail and throw Error().

Getters and Setters

* __lookupGetter__
* __lookupSetter__
* __defineGetter__
* __defineSetter__

foo.arguments

TODO http://www.w3.org/mid/[email protected]

Also see

• http://kangax.github.com/es5-compat-table/non-standard/
• https://bugs.ecmascript.org/buglist.cgi?product=ECMA-262&component=technical%20content&resolution=--- – open bugs on the ES spec