Javascript bitstring to BigInt

bitstringToBigInt.js

const bitstringToBigInt = bstr => {
  let counter = 0n;
  let byteindex = 0n;
  
  for (let i = bstr.length - 1; i >= 0; i--) {
    if (bstr[i] === '1') {
      counter += 2n**byteindex;
    }

    byteindex++;
  }
  
  return counter;
}

const bitstring16 = '10101000000000001';
const expected16 = 86017n;

const bitstring64 = '10000000000000000000000000000000000000000000001000000000010000001';
const expected64 = 18446744073709813889n;

const bitstring500 = '100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000001';
const expected500 = 3273390607896141870013189696827599152216642046043064789483291368096133796404674554883271737830461478356928839096742628359722239281751026650784062242817n;

console.log(bitstringToBigInt(bitstring16), bitstringToBigInt(bitstring16) === expected16);
console.log(bitstringToBigInt(bitstring64), bitstringToBigInt(bitstring64) === expected64);
console.log(bitstringToBigInt(bitstring500), bitstringToBigInt(bitstring500) === expected500);