Basic Calculator LeetCode solution in JavaScript

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var calculate = function (s) {
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  const numStack = [[]];
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  const opStack = [];
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  const isNumber = (char) => !isNaN(char);
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  function calculate(a, b, op) {
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    if (op === "+") return a + b;
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    if (op === "-") return a - b;
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  }
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  let number = "";
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  for (let i = 0; i < s.length; i++) {
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    const char = s[i];
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    if (char === " ") continue;
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    if (isNumber(char)) {
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      number += char;
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      continue;
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    }
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    if (number) {
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      // i.e. char is not a number so the number has ended
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      if (numStack[numStack.length - 1].length === 0) {
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        numStack[numStack.length - 1].push(+number);
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      } else {
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        const a = numStack[numStack.length - 1].pop();
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        const op = opStack.pop();
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        numStack[numStack.length - 1].push(calculate(a, +number, op));
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      }
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      number = "";
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    }
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    if (char === "(") {
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      if (numStack[numStack.length - 1].length === 0) {
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        numStack[numStack.length - 1].push(0);
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        opStack.push("+");
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      }
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      numStack.push([]); // Start a new stack for this parenthesis
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      continue;
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    }
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    if (char === ")") {
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      const b = numStack.pop().pop();
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      const a = numStack[numStack.length - 1].pop();
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      const op = opStack.pop();
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      numStack[numStack.length - 1].push(calculate(a, b, op));
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      continue;
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    }
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    if (char === "+" || char === "-") {
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      if (numStack[numStack.length - 1].length === 0) {
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        numStack[numStack.length - 1].push(0);
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      }
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      opStack.push(char);
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      continue;
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    }
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    // We should never reach here
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    throw new Error("Unexpected input: " + char);
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  }
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  if (number) {
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    if (numStack[numStack.length - 1].length === 0) return +number;
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    const a = numStack[numStack.length - 1].pop();
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    const op = opStack.pop();
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    numStack[0].push(calculate(a, +number, op));
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  }
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  return numStack.pop().pop();
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};