Javascript map, reduce and filter

map-reduce-filter.js

function map(f, a) {
  return a.length ? [].concat(f(a[0]), map(f, a.slice(1))) : []
}

function filter(f, a) {
  return map(function (x) { return f(x) ? x : [] }, a)
}

var even = function (x) { return x % 2 == 0 };

console.log(filter(even, [1, 2, 3, 4]))

Re-implementing map and filter on arrays, just for fun.