Enabling strict mode for multiple JavaScript files - Stack Overflow

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To enable strict mode for all JavaScript, does the "use strict" setting need to be at the top of every imported JavaScript file, at the top of the first file, or the top of any file?

There doesn't seem to be ay documentation on this aspect.

Thanks!

To enable strict mode for all JavaScript, does the "use strict" setting need to be at the top of every imported JavaScript file, at the top of the first file, or the top of any file?

There doesn't seem to be ay documentation on this aspect.

Thanks!

• javascript

Share Improve this question Follow edited May 4, 2012 at 15:29 Donald T asked May 4, 2012 at 15:10 Donald TDonald T 10.7k1717 gold badges6565 silver badges9393 bronze badges

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2 Answers 2

Sorted by: Reset to default 11

It needs to be at the top of each script that you want strict applied to.

But, if the scripts were concatenated through minification, then "use strict" at the top of the first file would apply to all files (as they'd then be in the same file).

Because of this perceived danger (3rd party libraries?), it's advised not to do this, and instead apply it inside an IIFE for each script.

<script src="foo.js">
    (function () {
        "use strict";

        // Your code, don't forget you've now got to make things global via `window.blah = blah`
    }());
</script>

It goes at the top of each script, or if you only want it to apply to certain functions you can do something like:

// Non-strict code...

(function(){
  "use strict";

  // Your strict library goes here.
})();

// More non-strict code... 

Here's a good article about it: http://ejohn/blog/ecmascript-5-strict-mode-json-and-more/

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