Underscore.js indexBy mixin

indexBy.js

/**
 * Underscore function that combines _.find and _.indexOf
 * Source: https://github.com/documentcloud/underscore/issues/413
 *
 * _.indexBy([2, 6, 4, 7, 9, 0], function(num) {
 *     return num % 2 === 1;
 * });
 * => 3
 */

_.mixin({

  indexBy: function(list, func) {
    for (var i = 0, l = list.length; i < l; i++) {
      if (func(list[i])) return i;
    }
    return -1;
  }

});