AJAX - The Complete Reference

552 Part IV: Appendixes

• Parameters are passed to functions by value except in the case of composite types
  like arrays or objects, which are passed by reference.


• JavaScript is a prototype-based object-oriented language (not class-based in its
  current incarnation).


• There are four kinds of available objects: built-in objects, host (browser) objects,
  document objects, and user-defined objects.


• JavaScript supports functional style and higher order programming styles.


• Features such as browser interaction, document manipulation, and even basic I/O
  are provided by related objects and are not directly built-in to the language.


• Standard security policy defines scripts to be able to interact only with server-side
  scripts from the same origin (serving domain) and restricts access to local resources
  without explicit grant by the user. However, we note that scripts can be combined
  from different resources and will share the same name space which leads to the
  security concerns presented in Chapter 7.

Language Versions and Conformance

A common criticism voiced about JavaScript is the wide range of versions. This is a

legitimate concern for JavaScript developers looking for the widest range of compatibility.

Table A-1 presents the current versions of ECMAScript that JavaScript implementations

may conform to, as well as notes on the emerging Edition 4.

Each browser vendor has evolved its implementation of JavaScript a bit differently.

Netscape/Mozilla/Firefox often serves as the reference implementation, at least for the core

language. The evolution of JavaScript in this family of browsers is detailed in Table A-2.

TABLE A-1 Standard Versions of JavaScript

Standard Version Description

ECMAScript Edition 1 First standardized version of JavaScript, based loosely on

JavaScript 1.0 and JScript 1.0.

ECMAScript Edition 2 Standard version correcting errors within Edition 1 (and some

very minor improvements).

ECMAScript Edition 3 More advanced language standard based on ECMAScript Edition 2.

Includes regular expressions and exception handling. In

widespread use.

ECMAScript for XML (E4X) ECMA-357 (www.ecma-international.org/publications/standards/

Ecma-357.htm) is a modification of ECMAScript that adds native

XML support to the language. It is supported in Firefox 1.5+ and

later versions of ActionScript. Given the focus on ECMAScript

Version 4 and JavaScript 2.0, this is likely to be

a little discussed version of the language.

ECMAScript Edition 4 New standard, still unfinished at the time of this writing. Likely

to include features such as optional static typing, packages and

namespaces, generators and iterators, JSON, and class-based OOP.