Why can a for..of / for..in loop use const while a normal for loop can only use let or var for its variable in JS?

Question

In JavaScript, you can do something like this:

const arr = [1, 2, 3, 4, 5];

for (const v of arr) console.log(v);

But in normal for loop, it gives a TypeError:

for (const i = 0; i < 5; i++) console.log(i);

// TypeError: Assignment to constant variable.

Shouldn't the for..of get an error as well?

Answer 1

No the for(...of...) loop won't get an error. Here is why:

In the second for loop, you are editing the constant variable which is NOT allowed. This throws a TypeError. In the first for loop, it doesn't matter what type of variable assignment you use. The variable (const,let, var, or no identifier) calls on the iterator which creates a sort of isolated temporary scope. This is why you can't access the variable outside of the for loop. For example:

const example = 1;
example = 2;
//TypeError

for (const someVar of Array(5)) someVar = 12;
//TypeError

{
    const num = 200;
}

const num = 150;
//This is fine because of scope