You can use const anywhere you can use let to declare a variable. You use it for variables whose value will not change.

Many programmers consider it a best practice to use it for any variable that that does not specifically need to be mutable to avoid unintended side effects from accidentally reassigning a value.

const s = "constant";
console.log(s, typeof s);

// error: can't assign value to a constant
// s = "nope";

const n = 500000000;
console.log(n, typeof n);

const d = 3e20 / n;
console.log(d, typeof d);

constant string
500000000 number
600000000000 number