Simple calculator

Calculator.js

const ops = "+-*/";
const nums = "0123456789";

function split(expr) {
  let rtn = [];
  let stack = '';
  for (let i = 0; i < expr.length; ++i) {
    if (ops.includes(expr[i])) {
      rtn.push(stack);
      rtn.push(expr[i]);
      stack = '';
    } else if (nums.includes(expr[i])) {
      stack = stack + expr[i];
    } else {
      return undefined;
    }
  }
  if (stack.length != 0)
    rtn.push(stack);
  return rtn;
}

function calc(expr) {
  if (!expr)
    return undefined;

  if (expr.length % 2 == 0)
    return undefined;

  let sum = 0;
  let target = Number(expr[0]);
  let scale = 1;

  for (let i = 1; i < expr.length; i += 2) {
    if (expr[i] == '*') {
      target *= Number(expr[i + 1]);
    } else if (expr[i] == '/') {
      target /= Number(expr[i + 1]);
    } else if (expr[i] == '+') {
      sum += target * scale;
      target = expr[i + 1];
      scale = 1;
    } else {
      sum += target * scale;
      target = expr[i + 1];
      scale = -1;
    }
  }
  sum += target * scale;;

  return sum;
}

function run() {
  const input = prompt('Put an expression: ');
  const result = calc(split(input));

  if (result != undefined)
    alert(input + '=' + result);
  else
    alert('Invalid input!');
}