undefined is not a keyword, it is just an identifier.

In the global scope, undefined is read only.

In the scope of the IIFE you have, you can create a new undefined variable that masks the global one.

Don't do that. It confuses people trying to maintain the code.

(function() {
  function undefined() {
    alert('d')
  };
  undefined()

  alert(undefined)
})()

alert(undefined)

This works because you are naming the function undefined inside a function. This means, inside the function's local scope, undefined = the function. As you can see if you run the code snippet, outside the function's scope, undefined = undefined once again.