Twitter entity processing made easy.

twitter-processentities.js

/*global YUI*/
/**
 * Twitter entity processing made easy.
 * @module twitter-processentities
 */
YUI.add("twitter-processentities", function (Y) {
  "use strict";
  var yArrayEach = Y.Array.each,
    yLangSub = Y.Lang.sub,
    startIndexSort = function (a, b) {
      var aVal = a.indices[0],
        bVal = b.indices[0];
      return aVal < bVal ? -1 : aVal > bVal ? 1 : 0;
    },
    hash_tag_template = '<a class="hashtag" href="https://twitter.com/#!/search?q=%23{text}" target="_blank">#{text}</a>',
    mention_template = '<a class="mention" href="https://twitter.com/#!/{screen_name}" target="_blank">{name}</a>',
    url_template = '<a class="url" href="{expanded_url}" target="_blank">{display_url}</a>',
    process = function (entity) {
      var template = entity.text ? hash_tag_template : entity.url ? url_template : mention_template;
      return yLangSub(template, entity);
    };
  /**
   * Process the entities within a tweet to render display text. 
   *
   * Then entities object should contain three properties, all of which are arrays:
   * <dl>
   *   <dt>hashtags</dt>
   *   <dd>An array of objects with a single key: text</dd>
   *   <dt>urls</dt>
   *   <dd>An array of objects with the keys <code>expanded_url</code> and <code>display_url</code></dd>
   *   <dt>mentions</dt>
   *   <dd>An array of objects with the keys <code>screen_name</code> and <code>name</code></dd>
   * </dl>
   *
   * @method processEntities
   * @namespace Twitter
   * @static
   * @param {String} text Text of the tweet
   * @param {Object} entities Twitter entities.
   * @return {String} Tweet HTML for display.
   */
  Y.namespace("Twitter").processEntities = function (text, entities) {
    var all_entities, result_offset, result;
    // combine all entities into a single array
    all_entities = (entities.hashtags || []).concat(entities.urls || [], entities.user_mentions || [], entities.media || []);
    // sort them by their start index
    all_entities.sort(startIndexSort);
    // start with the original text
    result = text;
    // store the difference in length between the original and the new string
    result_offset = 0;
    // loop through all entities
    yArrayEach(all_entities, function (entity) {
        // get the result text
      var entity_result = process(entity),
        // get the text we should replace
        text_to_replace = result.substring(entity.indices[0] + result_offset, entity.indices[1] + result_offset);

      // replace the entity text
      result = result.replace(text_to_replace, entity_result);
      // update the result_offset with the new difference in length
      result_offset += entity_result.length - text_to_replace.length;
    });
    // return the results
    return result;
  };
}, "3.4.1");

I think you could simplify your code by sorting by descending end (rather than ascending start). You wouldn't need the result_offset anymore.