JavaScript function pattern matching inspired by F#'s & renanpvaz gist

f.js

fun = (variations, matchIndex = 0) => (...appliedArgs) => (variations[appliedArgs[matchIndex]] || variations._)(...appliedArgs)


luck = fun({
  7: () => "LUCKY NUMBER SEVEN",
  _: () => "Sorry, you're out of luck, pal!",
})

luck(7)
// "LUCKY NUMBER SEVEN"
luck(1)
// "Sorry, you\'re out of luck, pal!"


// if the function is recursive:
fib = fun({
  0: () => 0,
  1: () => 1,
  _: n => fib(n - 1) + fib(n - 2),
})

fib(0)
// 0
fib(1)
// 1
fib(7)
// 13


fibonacciTail = n => fibonacciIter(n, 0, 1)
fibonacciIter = fun({
  0: (_n, prevFib) => prevFib,
  _: (n, prevFib, fib) => fibonacciIter(n - 1, fib, prevFib + fib),
})

fibonacciTail(0)
// 0
fibonacciTail(1)
// 1
fibonacciTail(78)
// 8944394323791464


factorial = fun({
  0: () => 1,
  _: n => n * factorial(n - 1),
})
 
factorial(6)
// 720