You Don't Know JS: Types & Grammar

Appendix A. Mixed Environment JavaScript

Beyond the core language mechanics we’ve fully explored in this book, there are several ways that your JS code can behave differently when it runs in the real world. If JS was executing purely inside an engine, it’d be entirely predictable based on nothing but the black-and-white of the spec. But JS pretty much always runs in the context of a hosting environment, which exposes your code to some degree of unpredictability.

For example, when your code runs alongside code from other sources, or when your code runs in different types of JS engines (not just browsers), there are some things that may behave differently.

We’ll briefly explore some of these concerns.

Annex B (ECMAScript)

It’s a little known fact that the official name of the language is ECMAScript (referring to the ECMA standards body that manages it). What then is “JavaScript”? JavaScript is the common tradename of the language, of course, but more appropriately, JavaScript is basically the browser implementation of the spec.

The official ECMAScript specification includes “Annex B,” which discusses specific deviations from the official spec for the purposes of JS compatibility in browsers.

The proper way to consider these deviations is that they are only reliably present/valid if your code is running in a browser. If your code always runs in browsers, you won’t see any observable difference. If not (like if it can run in node.js, Rhino, etc.), or you’re not sure, tread carefully. ...