Assigning value to undefined in JavaScript

From MDN:

undefined is a property of the global object; i.e., it is a variable in global scope. The initial value of undefined is the primitive value undefined.

Hence, you can assign the value to undefined unlike true and null which are reserved keywords. Note that this is the same case with NaN as well which is again not a reserved keyword and hence, you can assign any value to it.

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Just to add more to this, it doesn't matter even if you are assigning a value to undefined, it will not write to it as it is a readonly property.

Quoting from MDN again.

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. Even when this is not the case, avoid overriding it.

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Prefer using strict-mode in your JavaScript by declaring "use strict" at the very top of the file or inside a function to avoid such things. Using something like

"use strict";
undefined = 'test'; //will raise an error, refer to [1]

^[1] VM1082:2 Uncaught TypeError: Cannot assign to read only property 'undefined' of object '#'

This is because undefined is not a reserved word in JavaScript, even though it has a special meaning. So it can be assigned a value and the whole statement is valid. Whereas true and null are reserved words and can't be assigned values.

For reference: JavaScript Reserved Words