puyopuyotetr.is/app/javascript/mastodon/emoji.js

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

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

const emojify = str => {
  let rtn = '';
  for (;;) {
    let match, i = 0;
    while (i < str.length && str[i] !== '<' && !(match = trie.search(str.slice(i)))) {
      i += str.codePointAt(i) < 65536 ? 1 : 2;
    }
    if (i === str.length)
      break;
    else if (str[i] === '<') {
      let tagend = str.indexOf('>', i + 1) + 1;
      if (!tagend)
        break;
      rtn += str.slice(0, tagend);
      str = str.slice(tagend);
    } else {
      const [filename, shortCode] = unicodeMapping[match];
      rtn += str.slice(0, i) + `<img draggable="false" class="emojione" alt="${match}" title=":${shortCode}:" src="/emoji/${filename}.svg" />`;
      str = str.slice(i + match.length);
    }
  }
  return rtn + str;
};

export default emojify;