Javascript: besides “use strict”, which other “use” directives are there?

Question

Besides use strict, which other use directives are there?

Answer 1

Some more examples that can be in the “directive prologue” (a section potentially usable by JavaScript engines):

• 'use strict';
• 'use asm';
  Mozilla's asm.js is a subset of the language, geared towards crunching numbers.
• 'use stricter';
  Google's SoundScript. For fast OOP
  Has also some modes like:
  • 'use stricter+types';
  • http://www.2ality.com/2015/02/soundscript.html
• 'use babel';
  Used in Atom.io. (Was previously: 'use 6to5';)

A tweet by Jeremy Ashkenas suggests some further use of + and - prefixes in the same 'use ...'; string ('use stricter +types -tco +jsx +asm +es2019-pre';), but that seems to contradict the spec (further clarified in kangax's answer and Bergi's answer).

Answer 2

In case it's still of interest to anyone, I just ran across a "use asm" directive, for use with asm.js.

So it seems like there may be additional "use" options added over time.

Answer 3

Although it isn't a declarative like "use strict";, the next version of ECMAScript (codenamed Harmony) will apparently have an opt-in capability you can use in the <script> tag.

From this article:

• opt-in via MIME type in script tag: <script type="application/javascript;version=next"> (where “next” is a placeholder for something that has still to be determined)

The article is an overview of this presentation by David Herman, which is very much worth watching.