Why is the size of my array shrinking with every iteration of the for-loop in JavaScript?

Question

Why is the size of my array shrinking with every iteration of the for-loop in JavaScript?

I am struggling to change the classname of three elements that share the same classname. Unfortunately, as I loop through my array, it seems to decrease in size with each iteration, preventing me from successfully changing all three elements. Any advice or guidance would be greatly appreciated as I am currently feeling quite lost.

javascript

 var i;

 for(i=0; i < toAssignArray.length; i++){
        console.log('size of aaray: '+ toAssignArray.length);
        console.log('id in array ['+ i +']: ' + toAssignArray[i].id);

        toAssignArray[i].className = 'toAssignOff';

        console.log('className of ['+i+']' + toAssignArray[i].className);

            }

HTML

 <div id="toAssign_thanhphan_618" class="toAssign" onclick="pcoment.assignThisAuthor('thanhphan', 'reply_618', '740')" style="display: inline;">Assign Comment</div>

 <div id="toAssign_jimmywhite_618" class="toAssign" onclick="pcoment.assignThisAuthor('jimmywhite', 'reply_618', '740')">Assign Comment</div>

         <div id="toAssign_anquoc_618" class="toAssign" onclick="pcoment.assignThisAuthor('anquoc', 'reply_618', '740')">Assign Comment</div>

console

[Log] size of aaray: 3 (pub_comments.js, line 604)
[Log] id in array [0]: toAssign_thanhphan_618 (pub_comments.js, line 606)
[Log] className of [0]toAssign (pub_comments.js, line 610)
[Log] size of aaray: 2 (pub_comments.js, line 604)
[Log] id in array [1]: toAssign_anquoc_618 (pub_comments.js, line 606)

javascript html arrays

Answer 1

Answer №1

An array is not what you have here, rather a NodeList. NodeList instances from various DOM APIs are considered to be live, meaning they change dynamically as the elements within them change. For instance, if you use .getElementsByClassName() and alter an element in the list so it no longer has the specified class name, it will immediately disappear from the list.

There are two methods to address this issue. Firstly, you can convert the NodeList into a traditional array. In modern JavaScript (ES2015), this process is quite simple:

var realArray = Array.of(nodeList);

In older ES5, the approach is slightly more complex but achieves the same result:

var realArray = Array.prototype.slice.call(nodeList, 0);

The second option involves iterating through the list differently. Instead of using a for loop with an index variable, one can utilize a while loop and only perform operations on the first element:

while (nodeList.length) {
  nodeList[0].className = ""; // or any necessary action
}

This method works best when consistently removing elements from the list. Otherwise, utilizing the "real array" method is recommended.

Another alternative would be to utilize a different API that does not return a live NodeList. The .querySelectorAll() function is a versatile API for selecting DOM nodes without returning a live list. Therefore, instead of employing .getElementsByClassName(), consider using:

var nodeList = document.querySelectorAll(".the-class-name");

Answer 2

An array is not what you have here, rather a NodeList. NodeList instances from various DOM APIs are considered to be live, meaning they change dynamically as the elements within them change. For instance, if you use .getElementsByClassName() and alter an element in the list so it no longer has the specified class name, it will immediately disappear from the list.

There are two methods to address this issue. Firstly, you can convert the NodeList into a traditional array. In modern JavaScript (ES2015), this process is quite simple:

var realArray = Array.of(nodeList);

In older ES5, the approach is slightly more complex but achieves the same result:

var realArray = Array.prototype.slice.call(nodeList, 0);

The second option involves iterating through the list differently. Instead of using a for loop with an index variable, one can utilize a while loop and only perform operations on the first element:

while (nodeList.length) {
  nodeList[0].className = ""; // or any necessary action
}

This method works best when consistently removing elements from the list. Otherwise, utilizing the "real array" method is recommended.

Another alternative would be to utilize a different API that does not return a live NodeList. The .querySelectorAll() function is a versatile API for selecting DOM nodes without returning a live list. Therefore, instead of employing .getElementsByClassName(), consider using:

var nodeList = document.querySelectorAll(".the-class-name");

Answer 3

Answer №2

To efficiently iterate through a dynamically changing node list, another common method is to reverse the loop direction:

  for(i=toAssignArray.length-1 ; i >=0; i--){...

Answer 4

To efficiently iterate through a dynamically changing node list, another common method is to reverse the loop direction:

  for(i=toAssignArray.length-1 ; i >=0; i--){...

Why is the size of my array shrinking with every iteration of the for-loop in JavaScript?

Answer №1

Answer №2

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