instrumental/app/javascript/mastodon/emoji.js

import { unicodeMapping } from './emojione_light';
import Trie from 'substring-trie';

const trie = new Trie(Object.keys(unicodeMapping));

const assetHost = process.env.CDN_HOST || '';

const emojify = (str, customEmojis = {}) => {
  let rtn = '';
  for (;;) {
    let match, i = 0, tag;
    while (i < str.length && (tag = '<&'.indexOf(str[i])) === -1 && str[i] !== ':' && !(match = trie.search(str.slice(i)))) {
      i += str.codePointAt(i) < 65536 ? 1 : 2;
    }
    if (i === str.length)
      break;
    else if (tag >= 0) {
      const tagend = str.indexOf('>;'[tag], i + 1) + 1;
      if (!tagend)
        break;
      rtn += str.slice(0, tagend);
      str = str.slice(tagend);
    } else if (str[i] === ':') {
      try {
        // if replacing :shortname: succeed, exit this block with "continue"
        const closeColon = str.indexOf(':', i + 1) + 1;
        if (!closeColon) throw null; // no pair of ':'
        const lt = str.indexOf('<', i + 1);
        if (!(lt === -1 || lt >= closeColon)) throw null; // tag appeared before closing ':'
        const shortname = str.slice(i, closeColon);
        if (shortname in customEmojis) {
          rtn += str.slice(0, i) + `<img draggable="false" class="emojione" alt="${shortname}" title="${shortname}" src="${customEmojis[shortname]}" />`;
          str = str.slice(closeColon);
          continue;
        }
      } catch (e) {}
      // replacing :shortname: failed
      rtn += str.slice(0, i + 1);
      str = str.slice(i + 1);
    } else {
      const [filename, shortCode] = unicodeMapping[match];
      rtn += str.slice(0, i) + `<img draggable="false" class="emojione" alt="${match}" title=":${shortCode}:" src="${assetHost}/emoji/${filename}.svg" />`;
      str = str.slice(i + match.length);
    }
  }
  return rtn + str;
};

export default emojify;