ECMAScript 5

Revision 3 of ECMAScript is the one you can take for granted to be implemented in all browsers and environments. Revision 4 was skipped and revision 5 (let's call it ES5 for short) was officially accepted in December 2009.

ES5 introduces some new objects and properties and also the so-called "strict mode". Strict mode is a subset of the language that excludes deprecated features. The strict mode is opt-in and not required, meaning that if you want your code to run in the strict mode, you declare your intention using (once per function, or once for the whole program) the following string:

"use strict";

This is just a JavaScript string, and it's OK to have strings floating around unassigned to any variable. As a result, older browsers that don't "speak" ES5 will simply ignore it, so this strict mode is backwards compatible and won't break older browsers.

In future versions, strict mode is likely to become the default or the only mode. For the time being, it's optional.

For backwards compatibility, all the examples in this book work in ES3, but at the same time, all the code in the book is written so that it will run without warnings in ES5's strict mode. Additionally, any ES5-specific parts will be clearly marked. Appendix C, Built-in Objects, lists the new additions to ES5 in detail.