Fibonacci function

fibonacci-function.js

/** ES 5 */
function f (n) { return !!~[1,2].indexOf(n) ? 1 : f(n-1) + f(n-2) } 

/** ES 6 */
const f = n => !!~[1,2].indexOf(n) ? 1 : f(n-1) + f(n-2)

/** more crazy   */
const f = n => !!({1:1, 2:1})[n] ? 1 : f(n-1) + f(n-2)

/**
 * Optimized function
 */
function fib2(n) {
    var arr = Array.apply(null, {length: n + 1})
        .map(function(val, i) { return i });
    
    for (var i = 2; i <= n; i++) {
        arr[i] = arr[i - 1] + arr[i - 2];
    }
    
    return arr[n];
}