Why does const work in some for-loops in JavaScript?

Question

I do know why const doesn't work in for-loops. We need to create a new scope and copy over a value into that. So this won't fly.

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

Whereas this will.

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

However, I noticed that both of them work when looping though the properties of an object like this.

for(let property in thingy) console.log(property);
for(const property in thingy) console.log(property);

I'm not sure why.

Answer 1

for (const property in object) works because with each iteration you get a new variable, which is scoped only to that iteration. You can easily check that by using a closure inside a loop:

 for (const property in {a: 1, b: 2}) { setTimeout(() => { console.log(property); }, 100); }

This logs a and b , but if you change const to var , it logs b twice.

Answer 2

In your first example, i is modified via the i++ . A const can't be modified, so you get an error.

In the second example, property is re-defined for each iteration (each instance falls out of scope and a new one created rather than just re-assigning to the same variable) of the for loop. Since you're actually re-defining rather than modifying the value, const works just fine.