Simple calculator

calculator.js

var operators = {
    '+': (a, b) => a + b,
    '-': (a, b) => a - b,
    '*': (a, b) => a * b,
    '/': (a, b) => a / b,
};

function calculator (str) {
  return str.split(/\s*(\+|-)\s*/g)
    .map(substr => {
      parts = substr.split(/\s*(\/|\*)\s*/g);
      
      return 1 === parts.length ?
        parts[0] :
        parts
        .map(cleanup)
        .reduce(compute);
    })
    .map(cleanup)
    .reduce(compute);
}

function cleanup (substr, i) {
  if(0 === i % 2)
    return parseInt(substr, 10);
  return substr;
}

function compute (result, part, i, parts) {
  if(0 === i)
    return part;
  if(i % 2)
    return operators[part](result, parts[i + 1]);
  return result;
}

calculator('1 + 1 - 4 * 6') // -22