I'm trying to remove all hidden classes from elements on click with JavaScript. Here is the dummy code I use to try doing this:

    <style>
    .hidden {display:none;}
    </style>
    

    <div>Value 1</div>
    <div class="hidden">Value 2</div>
    <div class="hidden">Value 3</div>
    <div class="hidden">Value 4</div>
    
    
    <button onclick="removeHidden()">Show All</button>
    
    
    <script>
    function removeHidden()
    {
        var hidden = document.getElementsByClassName("hidden");
        for(var i=0; i<hidden.length; i++)
        {
            hidden[i].classList.remove("hidden");
        }
    }
    </script>

I'm trying to remove all hidden classes from elements on click with JavaScript. Here is the dummy code I use to try doing this:

    <style>
    .hidden {display:none;}
    </style>
    

    <div>Value 1</div>
    <div class="hidden">Value 2</div>
    <div class="hidden">Value 3</div>
    <div class="hidden">Value 4</div>
    
    
    <button onclick="removeHidden()">Show All</button>
    
    
    <script>
    function removeHidden()
    {
        var hidden = document.getElementsByClassName("hidden");
        for(var i=0; i<hidden.length; i++)
        {
            hidden[i].classList.remove("hidden");
        }
    }
    </script>

When clicking the button, I would expect all classes 'hidden' to be removed but oddly, it removes the hidden class from the second div and from the fourth but skips the third.

The result I get is:

Value 1
Value 2
Value 4

Any idea why that is because I really don't understand this?

I also tried this code but with the same result:

var els = document.getElementsByClassName("hidden");

Array.prototype.forEach.call(els, function(el) {
    el.ClassList.remove("hidden");
});

• javascript
• arrays

• Possible duplicate of JS: iterating over result of getElementsByClassName using Array.forEach – Dave Everitt Commented Feb 19, 2019 at 20:59

4 Answers 4

The issue is that getElementsByClassName() returns a "live" node list, which is a list that is updated anytime you reference the list. This ensures that you get the most up to date element references at all times. This is an expensive construct and is really only for rare use cases when it is needed.

Every time your code references the hidden variable, the DOM is re-scanned for elements that have the hidden class and after you begin removing that class, the length of the list shrinks by one. It's because of this changing length that one item gets skipped.

To use getElementsByClassName() correctly here, remove the class from the last element first and work your way backward to the first. This ensures that as the length of the node list shrinks, you are not skipping over any nodes.

<style>
    .hidden {display:none;}
</style>
    
    <div>Value 1</div>
    <div class="hidden">Value 2</div>
    <div class="hidden">Value 3</div>
    <div class="hidden">Value 4</div>
    
    
    <button onclick="removeHidden()">Show All</button>
          
    <script>
    function removeHidden()
    {
        var hidden = document.getElementsByClassName("hidden");
        for(var i = hidden.length-1; i > -1; i--)
        {
            hidden[i].classList.remove("hidden");
        }
    }
    </script>

But, because live node lists cause a performance hit, as a general rule, don't use them. Instead, use a static node list, which you get with the more modern and more flexible .querySelectorAll(). Also, if we use the static node list returned by .querySelectorAll(), we can use the Array API to iterate it with .forEach(), which eliminates the need for an indexer.

<style>
    .hidden {display:none;}
</style>
    

    <div>Value 1</div>
    <div class="hidden">Value 2</div>
    <div class="hidden">Value 3</div>
    <div class="hidden">Value 4</div>        
    
    <button onclick="removeHidden()">Show All</button>
            
    <script>
    function removeHidden()
    {
        // Get all the elements that match the selector into a node
        // list that implements the Array.prototype.forEach looping method
        document.querySelectorAll(".hidden").forEach(function(item){
            item.classList.remove("hidden");
        });
    }
    </script>

You can use querySelectorAll. The problem with getElementsByClassName is that the list it generates is dynamic. It means that if some change is made to the DOM it is instantly reflected in the list, because whenever the list is accessed the DOM is scanned to provide the list. So when in the loop the classes are removed one by one the length of the list also grew shorter which was used in the loop(i<hidden.length). While querySelectorAll provides a static list thus gives the correct output.

function removeHidden() {
  var hidden = document.querySelectorAll(".hidden");
  for (var i = 0; i < hidden.length; i++) {
    hidden[i].classList.remove("hidden");
  }

}

.hidden {
  display: none;
}

<div>Value 1</div>
<div class="hidden">Value 2</div>
<div class="hidden">Value 3</div>
<div class="hidden">Value 4</div>


<button onclick="removeHidden()">Show All</button>

The reason is that the list you're iterating is a "live list". This means that it reflects the current state of the DOM at all times. As such, when you remove an element from the DOM with that class, it also gets removed from the list.

After its removal, the list is re-indexed from that point forward, meaning that the current iteration is now pointing to what had been the next element. Upon incrementing i++, you've skipped over that element, and moved on to the next, which had previously been two elements ahead. This continues on as you iterate the list.

To solve it, either iterate from the end of the list to the start, or use a non-live-list for iteration.

use var hidden = document.querySelectorAll(".hidden") instead

EDIT: as Ziggy Wiggy explained this is because your iterating over a list of DOM elements, once you remove one the rest of the elements are essentially "shifted" or re-indexed down one position. So when you iterate through value 2 and remove it, value 3 bees the first element in the list, and you already iterated over it so the loop skips value 3 and goes to value 4. To avoid this, querySelector kind of provides a sort of snapshot of DOM Elements.

working snippet:

<style>
  .hidden {
    display: none;
  }
</style>


<div>Value 1</div>
<div class="hidden">Value 2</div>
<div class="hidden">Value 3</div>
<div class="hidden">Value 4</div>


<button onclick="removeHidden()">Show All</button>


<script>
  function removeHidden() {
    var hidden = document.querySelectorAll(".hidden");
    for (var i = 0; i < hidden.length; i++) {
      hidden[i].classList.remove("hidden");
    }
  }
</script>