Javascript: besides "use strict", what other "used" directives exist?

Question

Besides use strict , what other use directives exist?

Answer 1

A few more examples that might be in the "directory prolog" (a section potentially usable by JavaScript):

• 'use strict';
• 'use asm';
  Mozilla asm.js is a subset of the number-centric language.
• 'use stricter';
  Google SoundScript For quick OOP
  There are also several modes:
  • 'use stricter+types';
  • http://www.2ality.com/2015/02/soundscript.html
• 'use babel';
  Used by Atom.io. (Formerly: 'use 6to5'; )

Twitter Jeremy Ashkenas suggests some use of the + and - prefixes on the same line of 'use ...'; ( 'use stricter +types -tco +jsx +asm +es2019-pre'; ), but this, apparently, contradicts spec (specified in kangax answer and Bergi ).

Answer 2

If he is still interested in someone, I just came across the "use asm" directive, for use with asm.js.

So it seems that over time, additional "use" options may be added.

Answer 3

Although it is not declarative, like "use strict"; , the next version of ECMAScript (codenamed Harmony) is likely to have an option that you can use in the <script> .

From this article :

• select the MIME type in the script tag: <script type="application/javascript;version=next"> (where "next" is a placeholder for something that remains to be determined)

The article provides an overview of this presentation by David Herman , which is very worth a look.