Quick refactor of random code on http://brooks.io/syntax-highlight/v2/

random-refactor.js

// Orignal ------------------------------------------------------------
var isFancy = true;

var fancy = document.querySelector("#commentsbtn");
fancy.addEventListener("click", function(e){
  if (isFancy) {
    document.body.classList.remove("fancy-comments");
    isFancy = false;
  }
  else {
    document.body.classList.add("fancy-comments");
    isFancy = true;
  }
}, false);

var isFancy = true;

// Step 1 -------------------------------------------------------------
// What actually change? and turn it into an expression (pure data in this case)
var
  isFancy = true,
  fancy = document.querySelector("#commentsbtn");

fancy.addEventListener("click", function(e){
  const
    [method, b] =
        isFancy? ['remove', false]
               : ['add', true] 
  
  document.body.classList[method]("fancy-comments")
  isFancy = b
}, false);


// Step 2 -------------------------------------------------------------
//-- Util -------------------------------------------------------------
// Would be libraries just have a self-contained example
const
  elseIf =
    (f, t) => pred =>
      pred? t : f,
  compose =
    (f,g,h) => x => f(g(h(x))),

//-- Main -------------------------------------------------------------
// Pull out the data, make simpler functions and compose them.
// If it was more complex I would change the Array to
// Object { Method::String, isShown::Bool }
// Disclaimer
// Not tested, did a small babel repl test so added `getModel` and reversed
// the arguments to `when` and renamed it to `elseIf` to see that it do update.
let
  // Our simple model, using `let` we know it can be mutated
  isFancy = true;

const
  SELECTOR = 
    "#commentsbtn",
  getModel =
    _ => isFancy,
  getArgs =
    elseIf(['add', true], ['remove', false]),
  applyArgs =
    ([method, b]) => {
      document.body.classList[method]("fancy-comments");
      isFancy = b;
    },
  clickHandler =
    compose(applyArgs, getArgs, getModel);

document
  .querySelector(SELECTOR)
  .addEventListener("click", clickHandler, false);
  
// Step 2.1 - Added pseudo types --------------------------------------
//-- Util -------------------------------------------------------------
const
  elseIf = // :: (a,b) -> Bool -> a `or` b
    (f, t) => pred =>
      pred? t : f,
  compose = // :: ((a -> b), (b -> c), (c -> d)) -> (a -> d) -> d
    (f,g,h) => x => f(g(h(x)));

//-- Main -------------------------------------------------------------
let
  // Our simple model, using `let` we know it can be mutated
  isFancy = true; // :: Bool

const
  SELECTOR = // :: String
    "#commentsbtn",
  getModel = // :: () -> Bool
    _ => isFancy,
  getArgs = // :: Bool -> (String, Bool)
    elseIf(['add', true], ['remove', false]),
  applyArgs = // ::  (String, Bool) -> IO ()
    ([method, b]) => {
      document.body.classList[method]("fancy-comments");
      isFancy = b;
    },
  clickHandler = // :: () -> IO ()
    compose(applyArgs, getArgs, getModel);

document
  .querySelector(SELECTOR)
  .addEventListener("click", clickHandler, false);