maxBy_minBy.js

// Thank you http://stackoverflow.com/a/32234086/813665

var extremumBy = function(pluck, extremum, arr) {
  return arr.reduce(function(best, next) {
    var pair = [pluck(next), next];
    if (best === null) {
       return pair;
    }
    if (extremum(best[0], pair[0]) === best[0]) {
       return best;
    }
    return pair;
  }, null)[1];
};

minBy = function(fn, arr) {
  return extremumBy(fn, Math.min, arr);
};

maxBy = function(fn, arr) {
  return extremumBy(fn, Math.max, arr);
};