Reverse Polish Notation Calculator

rpnCalc.js

rpnCalc = (val) => {
  const ops = {
    "+": (a, b) => a + b,
    "-": (a, b) => a - b,
    "*": (a, b) => a * b,
    "/": (a, b) => a / b,
  };
  return val.split(" ")
    .reduce((stack, val) => (
      parseFloat(val) ?
        [...stack, parseFloat(val)] :
        [...stack.slice(0, -2), ops[val].apply(null, stack.slice(-2))]
    ), [])[0];
}

test = () => {
  let res;
  
  res = rpnCalc("5 1 2 + 4 * + 3 -");
  if (res !== 14)
    console.error(res);
  else
    console.log(res);
  
  res = rpnCalc("1 2 + 4 * 5 + 3 -");
  if (res !== 14)
    console.error(res);
  else
    console.log(res);
}

test()