Undefined = true; then go back to undefined?

Question

Some javascript tomfoolery:

If it works

undefined = true;

then how can you go back to set undefinedback to the view undefined?

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Of course, an easy way is to save undefinedin another variable before setting undefinedto true. What other way can you recover undefined?

At first I thought that it was delete undefined;not possible to return it .

Answer 1

Alex's answer is a safe and practical way to guarantee undefinedtruly undefined.

JS , , undefined, window.undefined :

(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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, , :

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

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, :

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

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:

undefined = function(){}();

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, void operator;-) doh!

undefined = void 0;

Answer 2

(function(undefined) {

   // undefined is undefined again!

})();

jQuery .

delete . , , Array undefined (, in). splice() Array.