Enabling strict mode for multiple JavaScript files

Question

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!

Answer 1

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>