area-51.user.js

// ==UserScript==
// @name         Alien Trends
// @namespace    https://github.com/ryanml
// @version      0.1
// @description  green
// @author       You
// @match        https://www.facebook.com/*
// @grant        none
// ==/UserScript==
(function() {
  const debounce = (scrollFunc) => {
    let timer = null;

    return () => {
      if (timer) return;

      timer = setTimeout(() => {
        scrollFunc.apply(this, arguments)
        timer = null
      }, 100)
    }
  }

  const findAlienPosts = () => {
    let messages = document.querySelectorAll('[data-testid="post_message"]')

    if (!messages) return;

    for (const message of messages) {
      const text = message.textContent.toLowerCase()
      if (text.match(/area 51/gi)) {
        message.innerHTML = '<p style="color:#39ff14;font-size:30px;">Post about Aliens</p>'
      }
    }
  }

  window.addEventListener('load', findAlienPosts)
  window.addEventListener('scroll', debounce(findAlienPosts))
})()