What is that mean ??

Question

Cannot read the property css of undefined?????

Answer 1

It means you are trying to read property from a non existing element, or an element that is not yet loaded in the DOM structure. To improve context clarity you can save and share your code link in the Description of the original question above. Having seen your code people knows better what to suggest.

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Answer 2

Because when you run program JavaScript didn't get Html elements and it show undefined so for this solution you have to put your JavaScript code inside document.ready function.