What is the purpose of the delete operator in Javascript?

Question

The behaviour of the delete operator seems very complicated and there are many misunderstandings about what it actually does. To me, it seems that reassigning something to <

Answer 1

The other answers are explaining the motivation behind the delete keyword. I would like to add that as of 2017, browser do deallocate memory both when deleting a property and when setting the property to undefined.

Consider this example (source of roughSizeOfObject()):

> var obj = {a:42,b:"b"}; roughSizeOfObject(obj)
26
> obj.a = undefined; roughSizeOfObject(obj)
18
> delete obj.a; roughSizeOfObject(obj)
10
> obj.b = undefined; roughSizeOfObject(obj)
8
> delete obj.b; roughSizeOfObject(obj)
0

The example comes from Chrome 61 (64-bit) console (note that all characters in String are internally encoded as 16-bit unsigned integer).