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Whats wrong with my javascript?

I'm trying to add a class to an element to trigger an animation on a button click.

  document.getElementsByClassName('box')[0].className = 'active';

errors I get are:

Uncaught TypeError: Cannot set property 'className' of null

Uncaught TypeError: Cannot set property 'className' of undefined at HTMLDivElement.<anonymous>

1 Answer

Here you are overwriting the class name 'box' with the class name 'active'.

document.getElementsByClassName('box')[0].className = 'active';

On subsequent clicks the element 0 with class name 'box' no longer exists which is why you receive this error:

Cannot set property 'className' of undefined at HTMLDivElement.<anonymous>

So instead of setting the class name you could try to add an 'active' class to the element as follows:


The first part of your answer is what I suspected, since the element I was wanting to change the class of literally disappeared when I clicked the button. That makes sense seeing how you worded it, thank you.

The second part of your answer was awesome, thanks! That's the way I should have done it. I could use .classList.toggle and even gotten more use out of the button too. :)