Strange behavior when iterating over HTMLCollection from getElementsByClassName

Question

I wrote a function to change the class of elements to change their properties. For some reason, only some of the elements have changed. It took me a few hours to find a solution, but it seems odd to me. Perhaps you can explain this to me.

This isn’t working:

function replace(){
  var elements = document.getElementsByClassName('classOne');

  for (var i = 0; i < elements.length; i++) {
    elements[i].className = 'classTwo';               
  }
}

See the JSFiddle: only every second item is affected; only every second red element changes color to blue.

So I changed the final expression of the for loop to not increment i anymore:

function replace(){
  var elements = document.getElementsByClassName('classOne');

  for (var i = 0; i < elements.length; i) { // Here’s the difference
    elements[i].className = 'classTwo';               
  }
}

This works well! It seems as though push is called and no increment is needed. Is this normal? It is different from the examples I’ve seen.

Abysm

I have never heard the term “loop coefficient”. JavaScript calls the last part of a for loop the “final expression” (or “increment” in certain contexts of the spec) and i++ an “increment” (or, generalized, “UpdateExpression” in the spec). – Herniorrhaphy

Related: for...in loop not looping through all properties?. – Herniorrhaphy

Answer 1

What's going on is an odd side effect. When you reassign className for each element of elements, the element gets removed from the array! (Actually, as @ user2428118 points out, elements is an array-like object, not an array. See this thread for the difference.) This is because it no longer has the classOne class name. When your loop exits (in the second case), the elements array will be empty.

You could write your loop as:

while (elements.length) {
    elements[0].className = 'classTwo'; // removes elements[0] from elements!
}

In your first case, by incrementing i, you are skipping half of the (original) elements that have class classOne.

Excellent question, by the way. Well-researched and clear.

Airburst

Thank you! sounds right.. but... it is sense programming? "elements" is an exist object, not dynamically object that can change without you? – Abysm

@Abysm - While the elements variable is local to your function, the array that it references is also accessible to (and maintained by) the DOM. The className property of each element actually triggers a setter function within the DOM machinery. One of the side effects of that function is to remove the element from the array that elements references. So it doesn't change "without you"; it changes because you changed the className attribute of one of its elements. To quote the sages of old: "That's not a bug; that's a feature!" – Airburst

@TedHopp - Thank you again! now all is clearly. (PS, thanks about the 'while()', it's a nicely and elegant solution.) – Abysm

You should probably mention that elements isn't actually an array, but merely an array-like datastructure – Phlegm

Answer 2

getElementsByClassName returns a NodeList. A NodeList collection is a live collection, which means that the modification of the document affects the collection. more

Preoccupancy

Tnx. Indeed the 'live collection' term explains the point well. – Abysm

NOT all NodeList collection is live. Some "fetch" methods such as querySelectorAll(), returns STATIC NodeList which are not affected by subsequent DOM changes. – Stivers

It returns an live HTMLCollection – Ardeth

Answer 3

Or revert the loop, beginning by length-1 and step down to 0

Creation

Yes, that would work as well, but the question begs for an explanation rather than a solution. – Quant