Web Developer's Reference Guide

JavaScript subset and extensions

Subsets are mostly defined for security purposes; scripts written using secure language subsets can be executed safely even if its source is untrusted, for instance, an ad server. Some of these subsets will be described later.

As JavaScript continued to evolve and allowed explicit extensions, newer versions were released. Many of the features were standardized. These extensions are compatible with modern browsers such as Firefox and Chrome. However, the implementation of non-standard extensions may require an external compiler because these features are being updated in major JavaScript engines now.

JavaScript subsets

As stated earlier, for execution security of untrusted code, we use subsets in JavaScript. For example, ...