javascript - var let is ok but let let isn't?

Question

//Version A:
var let = true;
console.log(let);//true

Answer 1

Because the specification says so.

From https://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations-static-semantics-early-errors:

13.3.1 Let and Const Declarations

13.3.1.1 Static Semantics: Early Errors

LexicalDeclaration : LetOrConst BindingList ;

• It is a Syntax Error if the BoundNames of BindingList contains "let".

The variable statement specification has no such limitation, probably because let was not in use at the time var was defined, and changing the specification for var would be a breaking change.