<p>we could be doing space complexity O(1) if in Js was possible to replace char in 
the string by index without creating a new string..</p>
<pre><code class="language-javascript">
/**
 * @param {string} num1
 * @param {string} num2
 * @return {string}
 */
function add(num1, num2) {
    let x = num1.length - 1
    let y = num2.length - 1
    let remainder = 0
    let result = ''

    while (x &gt;= 0 || y &gt;= 0 || remainder) {
      let sum = Number(num1[x] || 0) + Number(num2[y] || 0) + remainder
      remainder = 0
      if (sum &gt; 9) {
        result = (sum - 10) + result
        remainder = 1
      } else {
        result = sum + result
      }
      x--
      y--
    }
    return result
  }
</code></pre>


This is my accepted solution(time&space complexity in worse case both max(num1, num2))

Luckily we have BigInt in JavaScript so handle big numbers. What if we need to do it by ourselves for older browsers? You are asked to implement a str…

63. create `_.cloneDeep()`

63. `_.cloneDeep()`を実装する

63. 手写`_.cloneDeep()`

61. create your own `Function.prototype.call`

61. `Function.prototype.call`を実装する

61. 实现`Function.prototype.call`

62. implement BigInt addition

62. BigIntの足し算を実装する

62.  实现大整数加法(BigInt)