When and how I mostly use Array.prototype.reduce.

reduceUsecases.js

/**
 * Here are the kind of cases I like to use Array.prototype.reduce to deal
 * with:
 */

/**
 * We have an array of users sorted by name. The "reduce" method can be really
 * useful here:
 */
var users = [
  {
    id: 'wbruc',
    name: 'Bruce Wayne'
  },
  {
    id: 'djohn',
    name: 'John Doe'
  },
  {
    id: 'wjohn',
    name: 'John Wayne'
  }
];

/**
 * 1. Indexing the array by id:
 */

var usersIndex = users.reduce(function(object, user) {
  object[user.id] = user;
  return object;
}, {});

/**
 * 2. Finding a specific object in the array (one-line):
 */
var batman = users.reduce(function(result, user) {
  return result || user.id === 'wbruc' ? user : result;
}, null);

/**
 * In this use-case, I search in the whole array without breaking, when using
 * Array.prototype.some could avoid that. But since "some" casts the finally
 * returned value to a boolean, I cannot use it as a one-liner.
 *
 * So obviously, I use this last case only when my array is short and the
 * operation is not repeated too often.
 *
 * I often transform "some" to "reduce" because I need a one-liner. And in that
 * case, forgetting the "null" initial value is really bad...
 *
 * P.S.: And of course I would definitely enjoy Array.prototype.some to return
 * the final falsy or truthy value returned by the last evalutation of the
 * callback.
 */