Just different names for what is really ECMAScript. John Resig has a good explanation.
Here's the full version breakdown:
- IE 6-7 support JScript 5 (which is equivalent to ECMAScript 3, JavaScript 1.5)
- IE 8 supports JScript 6 (which is equivalent to ECMAScript 3, JavaScript 1.5 - more bug fixes over JScript 5)
- Firefox 1.0 supports JavaScript 1.5 (ECMAScript 3 equivalent)
- Firefox 1.5 supports JavaScript 1.6 (1.5 + Array Extras + E4X + misc.)
- Firefox 2.0 supports JavaScript 1.7 (1.6 + Generator + Iterators + let + misc.)
- Firefox 3.0 supports JavaScript 1.8 (1.7 + Generator Expressions + Expression Closures + misc.)
- The next version of Firefox will support JavaScript 1.9 (1.8 + To be determined)
- Opera supports a language that is equivalent to ECMAScript 3 + Getters and Setters + misc.
- Safari supports a language that is equivalent to ECMAScript 3 + Getters and Setters + misc.
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