map-reduce-filter.es6.js

const map = (xs, fn) => {
	const _map = ([x, ...xs], fn, result) => (
		x === undefined 
		? result
		: _map(xs, fn, [...result, fn(x)]) 
	)
	return _map(xs, fn, []);
}

const filter = (xs, fn) => {
	const _filter = ([x, ...xs], fn, result) => (
		x === undefined 
		? result
		: _filter(xs, fn, fn(x) ? [...result, x] : result) 
	)
	return _filter(xs, fn, []);
}

const reduce = (xs, fn, initial) => {
	const _reduce = ([x, ...xs], fn, accumulator) => (
		x === undefined
		? accumulator
		: _reduce(xs, fn, fn(accumulator, x))
	)
	return _reduce(xs, fn, initial);
}

// Tests
console.log("map", map([1,2,3,4,5,6], n => n * n))
console.log("filter", filter([1,2,3,4,5,6], n => n % 2 == 0))
console.log("reduce", reduce([1,2,3,4,5,6], (sum, n) => sum + n, 0))