const keyword scope in Javascript

Question

1. >>> const a = 2
2. >>> var a = 3
3. >>> a = 4
4. >>> a // print 2

Why the operation line 3 is allowed? const

Answer 1

const scope is defined as 'block scoped' (the scope of which, is restricted to the block in which it is declared).

MDN documentation:

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

Regarding your specific issue: First as comments said const is relevant in ES6. I don't know about you but i get (typing your line 2: var a = 3;): SyntaxError: Identifier 'a' has already been declared so your example is not quite possible.