js function to add two binary number strings

addTwoBinaries.js

function addTwoBinaries(a,b) {
  ///we need to track the carry value
  //we also need to track the sum which will the be result
  let carry = 0;
  let result = "";
  //we will need to do the add and carry operation at least as many times as the
  //longest number, so let's keep track of two lengths for iteration
  let i = 0;
  let j = 0;
  //while we have digits left on each, lets do the sum and carry
  while(i<=a.length - 1 || j <=b.length-1) {
    //magic
    let num1 = i < 0 ? 0 : a[i] | 0;
    let num2 = j < 0 ? 0 : b[j] | 0;
    carry += num1 + num2; // sum of the two single digits
    result = carry % 2 + result; //concat string in proper order
    carry = carry / 2 | 0; // remove fractionals
    i++;
    j++;
  }
  //what if afterwards there is still carry leftover?
  //if we still have a carry??
  if(carry) {
    result = carry + result;
  }
  return result;
}

console.log(addTwoBinaries("1101", "1001"));

wrong with addTwoBinaries("100011", "1")

yes its coming wrong as well for addTwoBinaries("11011","1000"), but ya well try.