Given a non-negative integer, find the indexes of all the bits which are set to 1

Question

Here is what I'm doing:

function getIndexes(n) {
    return n
        .toString(2)
        .split("")
        .map((c, i) => [c, i])
        .filter(([c, _]) => c == "1")
        .map(([_, i]) => i)
    ;
}

console.log(getIndexes(6));
console.log(getIndexes(7));
console.log(getIndexes(42));

Explanation:

1. Convert the input number into a binary string
2. Convert the binary string into an array of characters
3. Map each character to a [character, index] tuple
4. Filter (keep only) the tuples whose character is "1"
5. Map each tuple to the index of that tuple

It feels a bit of an overkill.

Is there a more standard way to achieve that?

I care less about performance and more about using something cleaner and/or custom.

Thanks!

Answer 1

If I well understood what you want to achieve, here is an alternative solution:

function getIndexes(n) {

  const result = [];

  const long_n = BigInt(n);

  for(let i = 0n; i < 64n; ++i) {

    if((long_n >> i) & 1n)  result.push(Number(i));

  }
  
  return result;
}