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

why does const and let not allow duplicate declaration?

There's a big difference between how c# or java (for example) handle duplicate variable names, where name collision returns a compilation error, and how it works in an interpreted language like js. Please, check the snippet below: The value of i isn't duplicated? Not really, still, in the function and block context the same variable name is referred as two different variables, depending on where those are declared.

function checkLetDuplication() {
  let i = 'function scope';
  for ( let i = 0 ; i < 3 ; i++ )
  {
    console.log('(for statement scope): inside the for loop i, equals: ', i);
  }
  console.log('("checkLetDuplication" function scope): outside the for loop i , equals: ', i);
}
checkLetDuplication();