Example of poormans pattern matching in Javascript. It useful in some cases. Beware of recursive definitions like this. It will blow the stack.

fib.js

// Classic Javascript
function fib(n) {
  if(n === 0) { return 0 }
  if(n === 1) { return 1 }
  return fib(n-1) + fib(n-2)}

// Non-auto-curry
const mkCases = // :: Object Functions -> a -> b
  cases => key =>
    (cases[key]? cases[key] : cases["_"])(key)

const fib = // :: Number -> Number
  mkCases({
    0: _ => 0,
    1: _ => 1,
    _: n => fib(n-1) + fib(n-2)
  })

/*
-- Haskell
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

fib n =
  case n of
    0 -> 0
    1 -> 1
    _ -> fib (n-1) + fib (n-2)
*/