Observing DOM

Background

It is a common case to operate on elements that are created dynamically, which may not be ready even on document-end.

In earlier userscripts we can see authors using timers to detect DOM elements periodically, which works in most browsers but is less effective, and there is likely obvious lag after the elements appear and before the operations are performed.

A better way to do stuff when certain element is ready is to use MutationObserver instead of timers. MutationObserver is well supported in modern browsers, including those Violentmonkey works on, as defined here.

You can stop reading this now if you are familiar with MutationObserver and prefer native APIs.

Introducing @violentmonkey/dom

If you are looking for an easy and friendly way to observe elements, @violentmonkey/dom might be what you want.

Once the library is required, we can use its methods under the VM namespace. See the documentation for more details.

Requirements

Add @violentmonkey/dom to the meta block of your script:

// ==UserScript==
// ...
// @require https://cdn.jsdelivr.net/npm/@violentmonkey/dom@2
// ==/UserScript==

If the project is initiated from our generator, it's likely that the dependency is already included.

Observing

After preparing the requirements, we can observe elements by VM.observe (doc), which utilizes MutationObserver under the hood.

For example, prepend <h1>Profile</h1> to the dynamically created <div class="profile">:

const disconnect = VM.observe(document.body, () => {
  // Find the target node
  const node = document.querySelector('.profile');

  if (node) {
    const h1 = document.createElement('h1');
    h1.textContent = 'Profile';
    node.prepend(h1);

    // disconnect observer
    return true;
  }
});

// You can also disconnect the observer explicitly when it's not used any more
disconnect();

Note that return true in the end is needed to disconnect the observer once we find the target node. Otherwise the detection continues and makes useless callbacks.

To observe document.body we must make sure document.body exists. This should not be a problem if @run-at is omitted or set to a value other than document-start.

(Optionally) Using jQuery

This is not recommended though, we put it here just in case some people are big fans of jQuery.

It is quite simple to integrate jQuery with the observer. Before we begin we must include jQuery as a dependency:

// ==UserScript==
// ...
// @require https://cdn.jsdelivr.net/npm/@violentmonkey/dom@1
// @require https://cdn.jsdelivr.net/npm/jquery@3/dist/jquery.min.js
// ==/UserScript==

Then we can wrap the node with jQuery and operate it the jQuery way:

VM.observe(document.body, () => {
  // Find the target node
  const $node = $('.profile');

  if ($node.length) {
    $node.prepend('<h1>Profile</h1>');

    // disconnect observer
    return true;
  }
});