JS : delete

When you declare a global JavaScript variable, what you are actually doing is defining a property of the global object (§3.5).

If you use var to declare the variable, the property that is created is nonconfigurable (see §6.7), which means that it cannot be deleted with the delete
operator. We’ve already noted that if you’re not using strict mode and you assign a value to an undeclared variable, JavaScript automatically creates a global variable for you. Variables
created in this way are regular, configurable properties of the global object and they can be deleted:

var truevar = 1; // A properly declared global variable, nondeletable.
fakevar = 2; // Creates a deletable property of the global object.
this.fakevar2 = 3; // This does the same thing.
delete truevar // => false: variable not deleted
delete fakevar // => true: variable deleted
delete this.fakevar2 // => true: variable deleted