This book has mostly talked about ECMAScript 3 because it’s the most widely distributed version of the language. However, the latest standard is ECMAScript 5.1 (version 4 was a dead-end branch that was eventually scrapped). Most modern browsers support ES5. You should know about what’s new in ES5 even if you currently have to support legacy browsers.
The three main areas of updates are:
Strict mode is not backward compatible, and is therefore an opt-in feature. Once per function or once per program you can opt into strict mode by using:
Examples of features that trigger an error in strict mode are:
window.Infinity = 0;)