ECMAScript 4 Netscape Proposal Compatibility

Friday, March 14, 2003

ECMAScript 4 is intended to be upwards compatible with almost all ECMAScript 3 and earlier scripts. The following are the current compatibility issues:

• Commas are now significant inside brackets, so expr[expr, expr] should be replaced by expr[(expr, expr)].
• Initializers are not allowed on a for-in variable; the =b in the statement for (var a=b in c) ... is not allowed.
• Uses of the identifiers as, is, namespace, and use need to be renamed or escaped with \_ because these are now reserved words.
• ECMAScript 4 drops the wrapper classes Boolean, Number, and String. Boolean, Number, and String now refer to classes that have primitive booleans, numbers, and strings as instances. The methods of these new classes correspond to the methods of ECMAScript 3’s wrapper classes. The results of calling new on Boolean, Number, or String are now implementation-defined, so an implementation may choose to retain the wrappers for compatibility with ECMAScript 3, but it is not required to do so.
• ECMAScript 4 permits but does not require implementations to allow array indices greater than 4294967295. Code that relies on indices greater than 4294967295 not being recognized as valid array indices may not work.
• Code that modifies the standard ECMAScript 3 objects such as Object and String may not work.
• ECMAScript 4 defines additional global constants, which may clash with top-level identifiers in programs.
• Invalid ECMAScript 3 code may parse as valid ECMAScript 4 code. ECMAScript 3 programs which relied on getting an exception for such formerly invalid code may not work.

For applications such as browsers where 100% compatibility is needed, scripts will be assumed to be written ECMAScript 3 unless explicitly marked as being ECMAScript 4 through either an HTML or XML attribute or by including a use ecmascript(4) statement.

Waldemar Horwat Last modified Friday, March 14, 2003