1.2 Versions of JavaScript

JavaScript has evolved over the years, and Netscape has released several versions of the language. Microsoft has released similar versions of the JavaScript language under the name "JScript." And ECMA (http://www.ecma.ch) has published three versions of the ECMA-262 standard that standardize the JavaScript language under the awkward name "ECMAScript."

Table 1-1 lists these various versions and explains their key features and how they are related to one another. In this book, I often use the name "JavaScript" to refer to any implementation of the language, including Microsoft's JScript. When I'm specifically referring to ECMAScript, I often use the terms "ECMA-262" or "ECMA."

Table 1-1. Versions of JavaScript

Version	Description
JavaScript 1.0	The original version of the language. It was buggy and is now essentially obsolete. Implemented by Netscape 2.
JavaScript 1.1	Introduced a true Array object; most serious bugs resolved. Implemented by Netscape 3.
JavaScript 1.2	Introduced the switch statement, regular expressions, and a number of other features. Almost compliant with ECMA v1, but has some incompatibilities. Implemented by Netscape 4.
JavaScript 1.3	Fixed incompatibilities of JavaScript 1.2. Compliant with ECMA v1. Implemented by Netscape 4.5.
JavaScript 1.4	Implemented only in Netscape server products.
JavaScript 1.5	Introduced exception handling. Compliant with ECMA v3. Implemented by Mozilla and Netscape 6.
JScript 1.0	Roughly equivalent to JavaScript 1.0. Implemented by early releases of IE 3.
JScript 2.0	Roughly equivalent to JavaScript 1.1. Implemented by later releases of IE 3.
JScript 3.0	Roughly equivalent to JavaScript 1.3. Compliant with ECMA v1. Implemented by IE 4.
JScript 4.0	Not implemented by any web browser.
JScript 5.0	Supported exception handling. Partially compliant with ECMA v3. Implemented by IE 5.
JScript 5.5	Roughly equivalent to JavaScript 1.5. Fully compliant with ECMA v3. Implemented by IE 5.5 and IE 6. (IE 6 actually implements JScript 5.6, but 5.6 is not different from 5.5 in any way that is relevant to client-side JavaScript programmers.)
ECMA v1	The first standard version of the language. Standardized the basic features of JavaScript 1.1 and added a few new features. Did not standardize the switch statement or regular expression support. Conformant implementations are JavaScript 1.3 and JScript 3.0.
ECMA v2	A maintenance release of the standard that included clarifications but defined no new features.
ECMA v3	Standardized the switch statement, regular expressions, and exception handling. Conformant implementations are JavaScript 1.5 and JScript 5.5.