September 2017 Update

Task/Fibonacci-sequence/JavaScript/fibonacci-sequence-9.js

@@ -0,0 +1,35 @@
+(() => {
+    'use strict';
+
+    // Nth member of fibonacci series
+
+    // fib :: Int -> Int
+    function fib(n) {
+        return mapAccumL(([a, b]) => [
+            [b, a + b], b
+        ], [0, 1], range(1, n))[0][0];
+    };
+
+    // GENERIC FUNCTIONS
+
+    // mapAccumL :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
+    let mapAccumL = (f, acc, xs) => {
+        return xs.reduce((a, x) => {
+            let pair = f(a[0], x);
+
+            return [pair[0], a[1].concat(pair[1])];
+        }, [acc, []]);
+    }
+
+    // range :: Int -> Int -> Maybe Int -> [Int]
+    let range = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+
+    // TEST
+    return fib(32);
+
+    // --> 2178309
+})();