What happens if we set the value of undefined?

Question

What does this line below do?

undefined = \'A value\';

If it does not change the value of undefined then what happens behind t

Answer 1

Unlike things like true, 123 or null, undefined is not a literal. That means using the undefined identifier is not a foolproof way to obtain the undefined value. Instead, can use the void operator, e.g. void 0.

By default, undefined defined a property of the global object, that is, global variable. Before ECMAScript 5, that property was writable, so

undefined = "A value";

replaced the value of window.undefined, assuming it was not shadowed by a local variable. Then if you used "A value" === undefined, you would get true. And void 0 === undefined would produce false.

ECMAScript 5 changed this behavior, and now the property is not writable nor configurable. Therefore, assignments to undefined will be ignored in non-strict mode, and will throw an exception is strict mode. Under the hood,

1. undefined = "A value"; is a Simple Assignment
2. That uses PutValue to put the value "A value" in a reference with base the global object, referenced name "undefined", and strict flag if the assignment is made in strict mode.
3. It calls the [[Put]] internal method of the global object, passing "undefined" as the property name, "A value" as the value, and the strict flag as the throw flag.
4. It calls the [[DefineOwnProperty]] internal method of the global object, passing "undefined", the property descriptor {[[Value]]: "A value"}, and the throw flag as arguments.
5. It rejects, that is, throws a TypeError exception if the throw flag is true, otherwise returns false.

However, you are still able to declare local undefined variables:

(function() {
  var undefined = "A value";
  alert(undefined); // "A value";
})();