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

I've made a little POC with and without strict mode.

The effect is that, if you're not using strict mode everything goes fine. If you're using strict mode you'll have a nice:

TypeError: Cannot assign to read only property 'undefined'

Now let's get to the POC:

"use strict"
var c;

if (c === undefined) {
  console.log("nothing happened")
}

undefined = "goofy"

c = "goofy"

if (c === undefined) {
  console.log("c is 'goofy' and it's equal to undefined.. gosh.. we broke js")
}

Now, as I said, with strict mode you obtain a TypeError while removing the "use strict" the script goes fine and the output is simply nothing happened.

I've found this Q/A that could be useful if you want to know more

NOTE: I've tested this code using Node.js.