Block-level variable redeclaration with var vs. let/const

Question

Part 1

Given this example:

var number = 10
{
   var number = 42
}
console.log(number) // 42

Why does line 4 not throw an Uncaught SyntaxError: Identifier 'number' has already been declared? It works with let / const because of block scoping (although the output is, of course, 10 not 42), but how come it works with var?

Part 2

Compare this to the following, which works with var:

var number = 10
var number = 42
console.log(number) // 42

but doesn't with let:

let number = 10
let number = 42 // SyntaxError
console.log(number)

Why is var given a "free pass"? Does it have to do with the number property being reassigned to the window object when var is used?

Answer 1

You are allowed to redeclare var variables in JavaScript, even in strict mode.

The scope of a variable declared with var is its current execution context, which is either the enclosing function or, for variables declared outside any function, global. If you re-declare a JavaScript variable, it will not lose its value.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var#var_hoisting

'use strict'
var someVar = 'Hello';
var someVar = 2 + 2;
console.log(someVar);