If I use delete in Javascript, what is the difference in var x, and simple x declaration?

Question

x = 42;         // creates the property x on the global object
var y = 43;     // creates the property y on the global object, and marks it as non-configurable


// x is a property of the global object and can be deleted
delete x;       // returns true

// y is not configurable, so it cannot be deleted                
delete y;       // returns false 

I dont understand what's mean the non-configurable. Why I can't delete the y?

Answer 1

When you add a property to an object, you can make it configurable or non-configurable. The long hand version of your example:

x = 42;

Is

Object.defineProperty(window, 'x', {
  value: 42,
  writable: true,
  configurable: true,
  enumerable: true
});

Configurable properties can be deleted, which removes them from the object (this may then lead to the memory being recovered, but it is not direct).

You could also write:

window.x = 42;

Which makes it more obvious when we come to the next issue.

window.x = 42; // x is a property
var y = 43; // y is not a property

And this is the real reason you can't delete y. It isn't a property and it isn't attached to an object. The delete keyword is for deleting a property from an object.

In the case of y - it will naturally be available for garbage collection when it cannot be reached (or when the reference count is 0 in older browsers).

You can also prevent properties from being deleted:

Object.defineProperty(window, 'x', {
  value: 42,
  writable: true,
  configurable: false,
  enumerable: true
});

This will cause either the false return from the attempt to delete it, or an error if you are running in strict mode.