Can't understand the behavior of deleting vars in JavaScript

Question

Here is the issue:

var x = 5;
window.x === x // true. x, as it seems, is a property of window
delete x; // false
delete window.x; // false;

Answer 1

This is how I understand that:

var x = 5; declared in the global scope creates the new window property x.

window.x = 5; declared (whereever) creates the new window property x as well. That's why window.x === x gives you true.

The difference is that javascript by default sets different descriptors for x property according to the way (one of two above) it is declared.

var x = 5 is equal to:

Object.defineProperty(window,'x',{
  value: 5,
  writable: true,
  enumerable: true,
  configurable: false
});

while window.x = 5 is equal to:

Object.defineProperty(window,'x',{
  value: 5,
  writable: true,
  enumerable: true,
  configurable: true
});

The configurable descriptor, if false, forbides to delete the property. We can assume, that javascript use Object.defineProperty with different descriptor settings under the hood when we declare variables in a simple way with var keyword or without it (automatically assigned to window). You can simply check that:

var x = 5;
window.y = 5;

console.log(Object.getOwnPropertyDescriptor(window,'x')); //configurable:false
console.log(Object.getOwnPropertyDescriptor(window,'y')); //configurable:true