useful.js

// sum an array, optionally providing a function to call on each element of the
// array to retrieve the value to sum
Array.prototype.sum = function(fn) {
  return this.reduce(function(accum, elem) {
    return accum + (fn ? fn(elem) : elem);
  }, 0);
};

// flatten an array
// [1,2,[3,4]] -> [1,2,3,4]
Array.prototype.flatten = function() {
  return this.reduce(function(accum, elem) { return accum.concat(elem); }, []);
};