filter-with-reduce.js

// using native JS
function filter(list, predicate) {
  list.reduce(function(prev, curr, i, arr) {
    if (!predicate(curr)) arr.splice(i, 1);
  }, []);

  return list;
}

// using Underscore
function filter(list, predicate) {
  _.reduce(list, function(prev, curr, i, arr) {
    if (!predicate(curr)) arr.splice(i, 1);
  }, []);

  return list;
}

filter([1,2,3,4,5,6], function(sub) { return sub % 2 === 0 }); // [2,4,6]

Also not a fan of the approach because I'm mutating the original collection. This is not FP (functional programming), but yeah.

In FP, we always create a brand new structure.