gistfile1.js

// data

var nums = [1, 2, 3, 4, 5, 6, 7, 8];

// What you would do if you did not have a functional mindset

var evens = [];
for (var i = 0; i < nums.length; ++i) {
  if (nums[i] % 2 === 0) {
    evens.push(nums[i]);
  }
}

console.log(evens);
  
// What you would do if you had a functional mindset

var odd = function(x) { return x % 2 === 0 ? true : false; };

var filter = function(coll, idd) {
  var res = [];
  for (var i = 0; i < coll.length; ++i) {
    if (idd(coll[i])) res.push(coll[i]);
  }
  return res;
};

var evens = filter(nums, odd);

console.log(evens);