Functional JS

fun.js

"use strict";

function toArray(thing) {
	if (Array.isArray(thing)) return thing;
	return Array.prototype.slice.call(thing);
}

function head(arr) {
	return arr[0];
}

function tail(arr) {
	return toArray(arr).slice(1);
}

function take(arr, num) {
	return toArray(arr).slice(0, num);
}

function concat(head, tail) {
	return [head].concat(tail);
}

function curry(fn) {
	var args = tail(arguments);
	return function() {
		return fn.apply(this, args.concat(toArray(arguments)));
	};
}

function foldr(fn, init, xs) {
	if (xs.length === 0) return init;
	return fn(head(xs), foldr(fn, init, tail(xs)));
}

function foldl(fn, init, xs) {
	if (xs.length === 0) return init;
	return fn(foldr(fn, init, tail(xs)), head(xs));
}

function map(fn, xs) {
	var m = function(head, tail) {
		return concat(fn(head), tail);
	};
	return foldr(m, [], xs);
}

function filter(fn, xs) {
	var f = function(head, tail) {
		if (fn(head)) return concat(head, tail);
		return tail;
	};
	return foldr(f, [], xs);
}

function append(xs, ys) {
	return foldr(concat, ys, xs);
}

function zipWith(fn, xs, ys) {
	var step = function(xs, ys, init) {
		if (xs.length === 0 || ys.length === 0) return init;
		return concat(fn(head(xs), head(ys)), step(tail(xs), tail(ys), init));
	};
	return step(xs, ys, []);
}

function pair(x, y) {
	return [x, y];
}

var zip = curry(zipWith, pair);

function lte(a, b) {
	return b <= a;
}

function gt(a, b) {
	return b > a;
}

function quicksort(arr) {
	if (arr.length === 0) return [];
	var x = head(arr), xs = tail(arr);
	if (xs.length === 0) return [x];
	var smallerSorted = quicksort(filter(curry(lte, x), xs));
	var biggerSorted  = quicksort(filter(curry(gt, x), xs));
	return smallerSorted.concat(x, biggerSorted);
}

function add(a, b) {
	return a + b;
}

var sum = curry(foldr, add, 0);
var add2 = curry(map, curry(add, 2));
var max = curry(foldr, Math.max, -Infinity);
var min = curry(foldr, Math.min, Infinity);
var biggerThan2 = curry(gt, 2);

console.log(sum([1, 2, 3, 4, 5, 6]));
console.log(add2([1, 2, 3, 4, 5, 6]));
console.log(filter(biggerThan2,[1, 2, 3, 4, 5, 6]));
console.log(append([1,2,3,4], [4,5,6,7]));
console.log(zip([1,2,3,4], [4,5,6,7]));
console.log(zipWith(add, [1,2,3,4], [4,5,6,7]));
console.log('max', max([1,2,3,4,5, 3]));
console.log('min', min([3]));

console.log(zipWith(Math.max, [1,2,3,4], [4,3,2,1]));

console.log(filter(curry(gt, 5), [1,2,3]));
console.log(quicksort([4,2,5,7,2,7,9,1,2,3]));