How dangerous is it in JavaScript, really, to assume undefined is not overwritten?

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No, I never have. This is mostly because I develop on modern browsers, which are mostly ECMAScript 5 compliant. The ES5 standard dictates that undefined is now readonly. If you use strict mode (you should), an error will be thrown if you accidentally try to modify it.

undefined = 5;
alert(undefined); // still undefined

'use strict';
undefined = 5; // throws TypeError

What you should not do is create your own scoped, mutable undefined:

(function (undefined) {
    // don't do this, because now `undefined` can be changed
    undefined = 5;
})();

Constant is fine. Still unnecessary, but fine.

(function () {
    const undefined = void 0;
})();

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