https://twitter.com/peterspeller/status/630813259558486016

pete.js

function mystery (input) {              // input is passed in as 3
  var secret = 4                        // secret doesn't change in all of this. It stays as 4 and is used in the mystery2 function
  input += 2                            // 3 += 2 [input = input + 2] which is 5. 
  function mystery2 (multiplier) {
    multiplier *= input                 /* multiplier = 6 * 5 : multiplier gets set when this returned function is
                                           finally called as param(6) [after getting temporarily set as the variable `hidden`] */
    return secret * multiplier          // secret is still just 4 and multiplier is now 30 [6 * 5] from the line above.
  }
  return mystery2                       // becomes variable `hidden` and then actually called as param(6)
}

function mystery3 (param) {             // hidden is passed in as the param
  function mystery4 (bonus) {           // bonus gets set as `2` from when it's finally called as jumble(2)
    return param(6) + bonus             // param is our returned `mystery2` function
                                        // bonus 
  }
  return mystery4                       // becomes jumble()
}

var hidden = mystery(3)
var jumble = mystery3(hidden)
var result = jumble(2)

console.log(result)