twitter-markdown.js

// ==UserScript==
// @name         Twitter Markdown
// @namespace    twitter.cm
// @version      0.1
// @description  Display basic markdown in twitter
// @author       jaywick
// @match        https://twitter.com/*
// @license MIT
// @grant        none
// ==/UserScript==

var applyStyle = function(css) {
    var head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    var style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
};

(function() {
    'use strict';
    applyStyle(`
code {
font-family: monospace,monospace;
font-size: 1em;
font-size: 13px;
padding: 1px 6px;
background: #F3E5F5;
color: #9C27B0;
border-radius: 2px;
{
`);

    Object.values(document.querySelectorAll('.tweet-text'))
    .forEach(x => x.innerHTML = x.innerHTML
             .replace(/`(.+)`/g, '<code>$1</code>')
             .replace(/\b_(.+)_\b/g, '<em>$1</em>')
             .replace(/\b\*(.+)*\b/g, '<strong>$1</strong>'));
})();