// This FUNCTION DECLERATION...
function double(num) {
  return num * 2
}

// ...can be re-written as an ARROW FUNCTION....
const double = (num) => {
  return num * 2
}
// ...and shortened by removing the parenthases,
// since there's only one parameter, and by removing
// the curly brackets and "return" since it implicitly
// returns if there's only one line of code.
const double = num => num * 2