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Editorial Team
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Asked: 2026-06-08T13:39:29+00:00

With this page: <!DOCTYPE html> <html> <head> <script> use strict; var foo = 2;

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With this page:

<!DOCTYPE html>
<html>
  <head>
    <script>
        "use strict";
        var foo = 2;
        delete foo;
    </script>
  </head>
  <body></body>
</html>

Firebug console gives:

applying the 'delete' operator to an unqualified name is deprecated
>>> foo
ReferenceError: foo is not defined
foo

But then this is successful:

>>> var bar = 2;
undefined
>>> delete bar;
true

Even if you comment out delete foo; so that the script does not break, deleting bar is still successful despite the fact it “is a property of a Global object as it is created via variable declaration and so has DontDelete attribute“:

>>> foo
2
>>> delete foo
false
>>> var bar = 2;
undefined
>>> delete bar
true

Is it possible to enable “strict mode” in FireBug and or Chrome’s console?

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1 Answer

  1. Editorial Team

    The firebug console works by wrapping all the code in an “eval” call so the first statement in your script is no longer “use strict” – hence it is disabled. You could try wrapping your code in a function to enforce “use strict” for that particular function but the best solution I know of is to skip the console and test straight in the page itself.

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