Binary sum of leetcode questions

Binary sum
Here are two binary strings , Returns the sum of them （ In binary ）.

Input is Non empty String and contains only numbers 1 and 0.

Example 1:

Input : a = “11”, b = “1”
Output : “100”
Example 2:

Input : a = “1010”, b = “1011”
Output : “10101”

Tips ：

Each string consists only of characters ‘0’ or ‘1’ form .
1 <= a.length, b.length <= 10^4
If the string is not “0” , No leading zeros .

var addBinary = function(a, b) {
    
  let a_length = a.length
  let b_length = b.length
  let res = []
  if (a_length > b_length) {
    
    b = b.padStart(a_length, '0')
    b_length = a_length
  } else if (a_length < b_length) {
    
    a = a.padStart(b_length, '0')
    a_length = b_length
  }
  let temp = 0
  for (let k = a_length - 1; k >= 0; k--) {
    
    let temp_sum = temp + parseInt(a[k]) + parseInt(b[k])
    let end_sum = 0
    if (temp_sum === 0) {
    
      end_sum = 0
      temp = 0
    } else if (temp_sum === 1) {
    
      end_sum = 1
      temp = 0
    } else if (temp_sum === 2) {
    
      end_sum = 0
      temp = 1
    } else if (temp_sum === 3) {
    
      end_sum = 1
      temp = 1
    }
    res.unshift(end_sum)
  }
  if (temp === 1) {
    
    res.unshift(1)
  }

  return res.join('')
}

let a = "1", b = "111"

res = addBinary(a, b)
console.log(res);