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Editorial Team
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Asked: 2026-05-23T13:55:30+00:00

Do we need to put use strict in external js files if our html

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Do we need to put “use strict” in external js files if our html file (which imports the external js files) already has “use strict” ?

And if our external js files do not have “use strict”, are they still “strict” within a HTML file that has “use strict” ?

Example:

<!DOCTYPE html>
<html>
<head>
    <title></title>
    <script>
        "use strict";
        function f() {
            // calling File1 functions (File1 does not have "use strict"; at the top)
            // are the File1 functions "strict"?
        }
    </script>
    <script src="File1.js"></script>
    <script>
        //by the way.. is it strict here ?
    </script>
</head>
<body>
</body>
</html>
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1 Answer

  1. Editorial Team

    You must put "use strict"; (or 'use strict';) at the top of each script (or function) to make them strict. In your example, the functions in File1.js will not be strict, nor will the second block. See https://developer.mozilla.org/en/JavaScript/Strict_mode#Invoking_strict_mode for details.

    If this wasn’t the case, using strict mode could invalidate third-party scripts that you import, so it makes sense that strictness only applies to the scripts and individual functions that you explicitly specify.

    For example:

    external.js:

    console.log("C: This script is non-strict.");

var g = function (x) {
    console.log("g is non-strict regardless of caller.");
    return 2 * x;
};

    test.html:

    <!DOCTYPE html>
<html>
<head>
    <title></title>
    <script>
        "use strict";
        console.log("A: This script element is strict.");
        function f() {
            console.log("The strictness of a script does not affect" +
                    " the strictness of external scripts, so g is" +
                    " still non-strict when called from f.");
            return g(3);
        }
    </script>
    <script src="external.js"></script>
    <script>
        f();
        console.log("B: This script element is non-strict.")
    </script>
</head>
<body>
</body>
</html>
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