text: // ES5
  const add = function(n1, n2) {
    return n1 + n2;
  };
  console.log(add(4, 8));
  
  // ES6 - fat arrow
  const add2 = (n1, n2) => {
    return n1 + n2;
  };
  console.log(add2(4, 8));
  
  // ES6 - single line in arrow function; auto return.
  const add3 = (n1, n2) => n1 + n2;
  console.log(add3(4, 8));
  
  // ES6 - single argument, no need ()
  const double = number => number * 2;
  console.log(double(5));
  
  // ES6 - zero argument, use ()
  const today = () => new Date();
  console.log(today());
  
  // ES6 - advanced
  const numbers = [1, 2, 3];
  const doubleNumbers1 = numbers.map(function(number) {
    return number * 2;
  });
  console.log(doubleNumbers1);
  
  //
  const doubleNumbers2 = numbers.map(number => number * 2);
  console.log(doubleNumbers2);
  