index.ts

// HELP:

export const calculate2 = (s: string) => {
  s = s.replace(/\s/g, '')

  const stack = []
  let sign = '+'
  let res = 0
  let num = 0

  for (let i = 0; i < s.length; i++) {
    if (s[i] >= '0' && s[i] <= '9') {
      num = num * 10 + +s[i]
    }

    if (i == s.length - 1 || !(s[i] >= '0' && s[i] <= '9')) {
      if (sign == '+') {
        stack.push(num)
      } else if (sign == '-') {
        stack.push(-num)
      } else if (sign == '*') {
        stack.push(stack.pop() * num)
      } else if (sign == '/') {
        stack.push(~~(stack.pop() / num))
      }
      num = 0
      sign = s[i]
    }
  }

  for (var i = 0; i < stack.length; i++) {
    res += stack[i]
  }

  return res
}