unable to understand usage of const in javascript

Question

I am confused about how const is used in javascript.

for (let stat of allStats){
        const index = allStats.indexOf(stat);

        console.log(filenames[index], stat.isFile());
    }

For example in the above code, "const index" within the for loop is being assigned different values in each iteration. How is that possible. I mean once a value is assigned to a const variable, can it be changed?

What have I misunderstood here..?

This works because with each iteration you get a new variable, which is scoped only to that iteration. — Bishan, Sep 02 '21 at 03:06
Does this answer your question? [Why does const work in some for-loops in JavaScript?](https://stackoverflow.com/questions/41067790/why-does-const-work-in-some-for-loops-in-javascript) — Bishan, Sep 02 '21 at 03:07
@Bishan this is a not a duplicate of that question. That question is referring to the use of `const` in the header of a for loop (traditional for loop, for-of loops, and for-in loops), while this question pertains to the body of the for loop, regardless of what type it is — Samathingamajig, Sep 02 '21 at 03:20

score 5 · Accepted Answer · answered Sep 02 '21 at 02:55

const only means that the identifier declared with const in the scope in which it's visible can't be reassigned.

Here, each iteration of the loop has a different binding for index - you have a separate index for every iteration, so no variable is being reassigned.

Another way of looking at it:

function parseStat(stat) {
    const index = allStats.indexOf(stat);
    console.log(filenames[index], stat.isFile());
}
for (let stat of allStats){
    parseStat(stat);
}

would be just fine too - the index identifier is local to the parseStat function, just like in your original code, the index identifier is local to each iteration of the loop.

I think I understand it now but the concept that each iteration of the loop has a different "scope" is a different idea than what I am used to. — theju112t, Sep 02 '21 at 04:09

score 1 · Answer 2 · answered Sep 02 '21 at 03:04

1) let and const are true block scope. they are only visible only within the block they are declared in.

const arr = [1, 2, 3, 4];
for (let val of arr) {
  const double = val * 2;
  console.log(double);
}

console.log(double); // ReferenceError: double is not defined

2) Each loop iteration gets its own block variables. It is similar like you get its own local variables each time you invoke a function.

const arr = [1, 2, 3, 4];
for (let val of arr) {
  const double = val * 2;
  console.log(double);
}

unable to understand usage of const in javascript

2 Answers2