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

You can use delete only for deleting objects, object properties or array element.

delete expression

delete will be not working if expression can't be represented as property. So delete can remove global variable, but not variables inited by var.

So, let me explain:

var x = 5;

You create variable in global scope by var, not property of window object. This var is just linked to window.x. And then you compare window.x === x it will return true. But:

delete x; // deleting variable, not property, return false
delete window.x; // resolve link to x and also deleting variable, not property, return false

BUT

window.x = 5;//add property
delete window.x; // can delete property, return true

AND

window.x = 5;//add property
delete x; //resolve x. it's a propery of window, return true

and older

In ECMAScript 262/3 as @Peter explain is available DontDelete flag. But in ECMAScript 262/5.1 in strict mode deleting is regulated by Configurable flag:

When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.