This post examines all the ways multiple DOM elements can be manipulated. That can be done with a while loop or with recursion when using a live HTMLCollection.

A common issue for the command-line coder is manipulating multiple elements. It requires usually a loop, but not necessary if going the purely functional way by using recursion.

Here's the manipulated HTML:

<div class="something active">foo</div>
<div class="something active">bar</div>
<div class="something active">baz</div>
<div class="something active">fooz</div>
 
<button class="button">Deactivate</button>

We want to remove those .active classes when the "Deactivate" button is clicked.

For loop

First let's use the good old for loop as a starting point.

var els = document.querySelectorAll('.something.active')
var button = document.querySelector('.button')
 
button.addEventListener('click', _removeClasses)
 
function _removeClasses() {
  for (var i = 0; i < els.length; i++) {
    els[i].classList.remove('active')
  }
}

That's not too bad, but we can do better with Document.getElementsByClassName() to fetch the elements, it returns an array-like object that is a live HTMLCollection.

An HTMLCollection in the HTML DOM is live; it is automatically updated when the underlying document is changed.

The below while loop will illustrate the live nature of HTML collections better.

While loop

// This gives us a live an `HTMLCollection`
var els = document.getElementsByClassName('something active')
var button = document.querySelector('.button')
 
button.addEventListener('click', _removeClasses)
 
function _removeClasses() {
  while (els[0]) {
    els[0].classList.remove('active')
  }
}

Breakdown:

document.getElementsByClassName()

Returns a live HTMLCollection rather than a nodeList.

while (els[0])

Goes on as long as there’s juice left in the collection, the collection updates "live".

els[0].classList.remove('active')

When a class name is removed, the element is removed from the collection, hence it's "live".

Whenever there is a while loop, recursion can be used instead.

Recursive function

As a reminder: recursion happens when a function calls itself. Here the _removeClasses function calls itself after the if (els[0]) stop condition:

var els = document.getElementsByClassName('something active')
var button = document.querySelector('.button')
 
button.addEventListener('click', _removeClasses)
 
function _removeClasses() {
  els[0].classList.remove('active')
  if (els[0]) _removeClasses()
}

Recursive function can be broken into 3 parts:

1. The business logic: els[0].classList.remove('active')
2. The stop condition: if (els[0])
3. The recursion: _removeClasses()

Demo

JS Bin on jsbin.com

Conclusions

For me this was an eye opener in HTMLCollections. Hope it also helped you, drop a comment if you have something to add.