February 1999 Draft JavaScript 2.0 Compatibility

Thursday, February 18, 1999

JavaScript 1.x

Existing JavaScript 1.x scripts will continue to run in a browser environment because browsers will be able to distinguish JavaScript 2.0 or later scripts from JavaScript 1.x scripts and treat each script according to the language in which it was written. JavaScript 2.0 scripts can be distinguished as follows:

• JavaScript 2.0 scripts will have a language attribute of JavaScript2.0.
• JavaScript 2.0 scripts will contain an import statement.

Migration

Some developers will want to incrementally modify their JavaScript 1.x scripts to take advantage of JavaScript 2.0 features. Many JavaScript 1.x scripts are compatible and will run unchanged in either JavaScript 1.x or JavaScript 2.0 (as long as one changes the language attribute or adds an import statement). The vast majority of other JavaScript 1.x scripts can be made compatible via the following process, which can be performed automatically:

• Perform semicolon insertion on the script.
• Move local and global var and function declarations in a function into a scope that encloses all of their references.
• Insert traditional and a newline before function declarations that are used as constructors or methods, or are called with fewer or more arguments than designated (JavaScript 1.x will treat traditional as an expression and inser a phantom semicolon after it).
• Declare global variables before their first reference.
• Replace void expr by void(expr).
• Replace expr[expr, expr] by expr[(expr, expr)] because commas are now significant inside brackets.
• Place regular expression literals in parentheses if they are found at the beginning of a statement.
• Rename uses of field or eval for identifiers (manual intervention may be needed if these are used to name fields).

We intend to provide a tool that will perform such transformations.

The main situations where manual intervention may be required are:

• Code that dynamically generates code for use in eval.
• Code that modifies the standard JavaScript 1.x objects such as Object and String.

Waldemar Horwat Last modified Thursday, February 18, 1999