If I declare 'i' above for loop I am getting the output as 3 3 3 and if I declare 'i' inside for loop I am getting output as 1 2 3

Question

Declaring i before the for loop gives output as 3 3 3.

let i;
for (i = 0; i < 3; i++) {
  const log = () => {
    console.log(i);
  }
  setTimeout(log, 100);
}

//Output 3 3 3

Declaring i in the for loop gives output as 1 2 3:

for (let i = 0; i < 3; i++) {
  const log = () => {
    console.log(i);
  }
  setTimeout(log, 100);
}

//Output 1 2 3

When declared in the head of a `for` loop you get a new `i` variable for every round of the loop. — Andreas, Jan 13 '21 at 07:16
Duplicate of [JavaScript closure inside loops – simple practical example](https://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) — FZs, Jan 13 '21 at 07:22

score 1 · Accepted Answer · answered Jan 13 '21 at 07:16

When you use let i = 0;, the variable i has its scope restricted to the block scope of the for loop, e.g. between the two curly braces.

When you have let i; outside of the for loop, the variable i is restricted to the block scope enclosing the for loop, not the scope of the for loop itself, and you see this behavior occur.

If I declare 'i' above for loop I am getting the output as 3 3 3 and if I declare 'i' inside for loop I am getting output as 1 2 3

1 Answers1