Is there any way to check if strict mode is enforced?

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The fact that this inside a function called in the global context will not point to the global object can be used to detect strict mode:

var isStrict = (function() { return !this; })();

Demo:

> echo '"use strict"; var isStrict = (function() { return !this; })(); console.log(isStrict);' | node
true
> echo 'var isStrict = (function() { return !this; })(); console.log(isStrict);' | node
false

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