Add Strings LeetCode solution in JavaScript

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var addStrings = function (num1, num2) {
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  let i = num1.length - 1;
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  let j = num2.length - 1;
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  let carry = 0;
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  let sum = "";
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  for (; i >= 0 || j >= 0 || carry > 0; i--, j--) {
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    const digit1 = i < 0 ? 0 : num1.charAt(i) - "0";
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    const digit2 = j < 0 ? 0 : num2.charAt(j) - "0";
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    const digitsSum = digit1 + digit2 + carry;
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    sum = `${digitsSum % 10}${sum}`;
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    carry = Math.floor(digitsSum / 10);
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  }
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  return sum;
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};