calculator.js

class Calculator {
  evaluate (s) {
    s = s.replace(/\s/g, '');
    const l = s.length;
    const stack = [];
    let i = 0;
    let o = '+';
    while (i < l) {
      const x = s[i];
      if (/\d/.test(x)) {
        const start = i;
        while (i < l && /\d|\./.test(s[i])) i++;
        i--;
        const temp = +s.slice(start, i + 1);
        if (o === '+') stack.push(temp);
        if (o === '-') stack.push(-temp);
        if (o === '*') stack.push(stack.pop() * temp);
        if (o === '/') stack.push(stack.pop() / temp);
      } else o = x;
      i++;
    }
    return stack.reduce((r, e) => r + e, 0);
  }
};