SUPERSEDED: jasisk/slack-emoji-randomizer - Slack userscript to replace all your emojis with 🦀

replaceWithCrab.js

(function () {
  function replaceText(msg, replaceWith) {
    emoji.init_colons();
    msg = emoji.replace_emoticons_with_colons(msg);
    return msg.replace(emoji.rx_colons, function (colonEmoji) {
      var emojiKey = colonEmoji.substr(1, colonEmoji.length-2);
      var replacement = emoji.map.colons[emojiKey];
      return replacement ? replaceWith : colonEmoji;
    });
  }
  
  TS.channels.onSendMsg = (function (original) {
    return function (ok, msg) {
      var channel;
      if (ok) {
        channel = TS.channels.getChannelById(msg.SENT_MSG.channel);
        setTimeout(function () {
          var newText = replaceText(msg.text, ':crab:');
          if (newText !== msg.text) {
            TS.msg_edit.commitEdit(msg, channel, newText);
          }
        }, 2000);    
      }
      return original.apply(this, arguments);    
    };
  }(TS.channels.onSendMsg));
}());

Randomly selected replacements with:

var emojis = Object.keys(emoji.map.colons);

function getRandomEmoji() {
  var randomIdx = Math.floor(Math.random() * emojis.length);
  return ':' + emojis[randomIdx] + ':';
}