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Document Events JS: DOM API

In addition to events that occur in response to user actions, there are a number of events that are linked to the operation of the browser itself. Such events include page loading and unloading events:

  • DOMContentLoaded – The DOM tree is fully built and ready to work
  • load – all resources are loaded (pictures, styles, scripts, etc.)
  • beforeunload – occurs when the user tries to leave the page

DOMContentLoaded is an important event that's often used in DOM-dependent code. It helps to avoid errors related to the fact that scripts can start executing before the DOM they depend on is ready. In this case, errors will occur.

This event is linked to document:

<script>
  document.addEventListener('DOMContentLoaded', () => {
    // If DOM hadn't been ready, we wouldn't have found anything
    const coll = document.querySelectorAll('.help');
    coll.forEach((el) => el.classList.add('hidden'));
  });
</script>

The speed of building the DOM tree is very dependent on the <script> tags. According to the standard, any <script> encountered in HTML will be executed before the tree is fully built, which means that the speed that code is processed in this <script> block will greatly affect when the user will see the site itself and when the DOMContentLoaded event will be triggered.

The topic of optimizing script loading and fast initialization is quite complicated. This is due not only to a large number of factors affecting the order and speed, but also to the fact that it works differently in different browsers. We won't look at it right now, it's an advanced level topic, and many articles have already been written on it.

Network

Pay attention to the red and blue stripes. The red one shows the moment when load event was triggered, and the blue one shows DOMContentLoaded. At the bottom of the picture, you can see the trigger time of each event, starting from when the page started loading. You can see from the picture that the browser first downloads the page itself (its html), then extracts links to all external resources from it and starts downloading them. For greater efficiency, downloading resources should happen in parallel as often as possible.


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