Wait for an element to appear, then when it does, do something with it

waitThenDo.js

const interval = window.setInterval(() => {
  // Search target element
  const target = document.querySelector(...)
  if (target) {
    window.clearInterval(interval)
    // Do something with target element
    target.innerText = 'Meow'
  }                                      
}, 100)