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The ECMAScript specification defines the Use Strict Directive, but permits implementations to define their own directives. From here:

Implementations may define implementation specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue.

What are the known implementation-specific directives? A quick Google search yields nothing other than use strict.

I can think of use asm and use strong. Are there any others in use?

Jackson
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    Related: [Javascript: besides “use strict”, which other “use” directives are there?](http://stackoverflow.com/q/6999768/1529630) – Oriol Mar 28 '16 at 17:49
  • I think I saw a library or framework which used its own directive, but I can't remember now. – Oriol Mar 28 '16 at 17:51
  • I have seen "use babel" –  Mar 28 '16 at 18:12
  • Possible duplicate of [Javascript: besides "use strict", which other "use" directives are there?](http://stackoverflow.com/questions/6999768/javascript-besides-use-strict-which-other-use-directives-are-there) – Alpha Mar 26 '17 at 17:07

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For Spidermonkey I found only source:

macro(useAsm, useAsm, "use asm")
macro(useStrict, useStrict, "use strict")

V8 same source:

F(use_asm, "use asm")                            
F(use_strict, "use strict")
Stan Kondrat
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