Reassign javascript with let

Question

Are there any differences between the two ways described below? Which one should I use any why?

while (true) {
    let test = getValue();
    ....
}

And

let test;
while (true) {
    test = getValue();
    ....
}

Answer 1

let is block-scoped that means it will only exist within a {...} block.

You should use this form (note that here let is not necessary if you don't modify the value within the same block) if you only use test within that while block (and not outside).

while (true) {
    const test = getValue();
    ....
}

You should use this form if you need to access test from outside the while loop.

let test;
while (true) {
    test = getValue();
    ....
}

Answer 2

If you use test variable outside your while loop, go with the second:

let test;
while (true) {
    test = getValue();
    ....
}
if (test) { ... }   // <--- Use test outside the while loop

Otherwise, the first one is better