Basic Calculator II LeetCode solution in JavaScript

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/**
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 * @param {string} s
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 * @return {number}
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 */
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var calculate = function (s) {
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  const n = s.length;
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  let currNum = 0,
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    lastNum = 0,
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    res = 0;
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  let op = "+";
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  for (let i = 0; i < n; i++) {
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    let currChar = s[i];
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    if (currChar !== " " && !isNaN(Number(currChar))) {
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      currNum = currNum * 10 + Number(currChar);
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    }
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    if ((isNaN(Number(currChar)) && currChar !== " ") || i === n - 1) {
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      if (op === "+" || op === "-") {
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        res += lastNum;
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        lastNum = op === "+" ? currNum : -currNum;
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      } else if (op === "*") {
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        lastNum *= currNum;
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      } else if (op === "/") {
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        lastNum =
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          Math.floor(Math.abs(lastNum) / currNum) * (lastNum < 0 ? -1 : 1);
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      }
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      op = currChar;
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      currNum = 0;
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    }
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  }
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  res += lastNum;
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  return res;
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};