undefined = true; then back to undefined?

Question

Some javascript tomfoolery:

If this works

undefined = true;

then how can you revert back to set undefined back to representing undefined?

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Sure, the easy way is to store undefined in another variable before setting undefined to true. What other way can you restore undefined?

First thought to set it back was delete undefined;, didn't work.

Answer 1

Alex's answer is the safe and practical way to ensure undefined is really undefined.

But JS is super flexible, so for fun, if you wish to revert the global undefined, then you can reassign window.undefined from the safety of a new function scope:

(function () {            // Create a new scope
   var a;                 // "a" is undefined in this scope
   window.undefined = a;  // Set the global "undefined" to "a"
})()                      // Execute immediately

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Taking this idea futher, you can reconfigure the above to become:

undefined = (function () {
   var a;                 // "a" is undefined
   return a;              // return "a"
})();

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Or really, just this:

undefined = (function () {
    return;
})();

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But in fact, you don't actually need to return anything:

undefined = function(){}();

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Of course, then there's also the void operator ;-) doh!

undefined = void 0;