TIL: HTMLCollection is alive

TIL: HTMLCollections are live-collection. What does it mean? Look. Imagine you have HTML like this:

<div>
  <br/>
  <br/>
</div>

Then you want to find all its <br/>s:

const collection  = $0.getElementsByTagName('br');

Thus far, everything is obvious, right? Ok, let’s add one more <br/>:

$0.append(document.createElement('br'));

So, now have three of them. Let’s read the same collection element again:

Whoa… It was updated too. Why? HTMLCollection is a “live” collection:

An HTMLCollection in the HTML DOM is live; it is automatically updated when the underlying document is changed. For this reason it is a good idea to make a copy (e.g., using Array.from) to iterate over if adding, moving, or removing nodes.

• Mozilla