Discussion on: Daily Challenge #10 - Calculator

const calculator = operation => {

  // verify that the string with the operation has the right format:
  //   - a number
  //   - optionally followed by 0 or more:
  //       - a space
  //       - an operator (+-*/)
  //       - a space
  //       - another number
  if (!operation.match(/^\d+( [\+\-\/\*] \d+)*$/)) return null;

  // easy solution: now that we know the string has the format that we expect, 
  // return (eval(operation));
  // ...but someone did it already, so let's go the long way :P


  // break the string by spaces
  let ops = operation.split(" ");
  let opsSimple = [];

  // multiplication and division take priority
  // we create a new array that will only have numbers and + or -
  for (x = 0; x < ops.length; x++) {
    if (ops[x] === '*') {
      const val = opsSimple.pop();
      opsSimple.push(val * ops[x+1]);
      x++;
    } else if (ops[x] === '/') {
      // do not allow division by zero!
      if (ops[x+1] === "0") return "Error! Division by zero!";
      const val = opsSimple.pop();
      opsSimple.push(val / ops[x+1]);
      x++;
    } else {
      opsSimple.push(ops[x]);
    }
  }

  // calculate the addtiions and substractions sequentially
  let result = parseInt(opsSimple[0]);
  for (x = 1; x < opsSimple.length; x = x + 2) {
    if (opsSimple[x] === '+') {
      result += opsSimple[x+1];
    } else {
      result -= opsSimple[x+1];
    }
  }

  return result;
}