Why does var allow duplicate declaration but why does const and let not allow duplicate declaration?

Question

Why does var allow duplicate declaration but why does const and let not allow duplicate declaration?

var is allow duplicate declaration

Answer 1

Assuming you want to know whether this behavior is as per spec, check this 13.3.2

Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collective define only one variable.

let and const are the recent editions, while var is probably as old as Javascript itself.

In old days Javascript code-base didn't used to be too big to bother about programming mistakes and most probably focus was to ensure that instead of reporting the error of re-declaration of variable JS engine should handle it.