Question

$('.aClass')[index].addClass('newClass');

This is what I am trying to do, but it isn’t working, moreover its breaking the code after it.
What am I doing wrong ?

Answer 1

When using the index like that, it returns the DOM element, thus no jQuery methods.

Use .eq() instead to return the DOM element at that index while still wrapped by jQuery:

//as a function call
$('.aClass').eq(index).addClass('newClass');

There is also a selector :eq, with slight differences to the method version in terms of form and usage. The index is part of the selector string, so if you use dynamic values, you have to cut the string:

//the selector version:
$('.aClass:eq('+index+')').addClass('newClass');

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Additionally info: what you did was similar to a .get() which returns the DOM element, not wrapped in jQuery, at that index:

//the same thing
$('.aClass').get(index)
$('.aClass')[index]