Why Does Changing `className` Only Affect Every Other Element in an HTMLCollection?

className Only Changing Every Other Class

In this code snippet, an issue is encountered where the className property is only changing for every other element in a collection. The goal is to understand the reason behind this behavior and find a solution.

The provided code utilizes the getElementsByClassName() method to retrieve HTML elements with the class "block-default". It then proceeds to modify the className property of each element in the collection to "block-selected". However, the result is that only alternate elements are updated, leaving others with the original "block-default" class.

The culprit lies in the nature of HTMLCollections. These collections are live and reflect the current state of the DOM. When the className property of an element is modified, the collection itself is also affected. Specifically, the size of the collection decreases since the modified element is removed from the list.

To rectify this issue, it's crucial to remember that any subsequent changes to the collection's elements will affect the indices of the remaining elements. The solution is to consistently change only the first element's className.

One method involves iterating through the collection and modifying the first element repeatedly.

for (var i = 0; i < blockSetLength; i++) {
  blockSet[0].className = "block-selected";
}

This approach ensures that the first element is consistently updated, regardless of changes to the collection.

Alternatively, using the spread operator can convert the HTMLCollection into an array, allowing for more flexibility in modifying the elements.

var blockArray = [...blockSet];

for (var i = 0; i < blockArray.length; i++) {
  blockArray[0].className = "block-selected";
}

In either case, by modifying only the first element's className, the desired result of changing the class of every element in the collection can be achieved.

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