How do I transition color or opacity when mouseover using code?

Hello all,

I am seeking to adjust the color or opacity of an object solely with code, without using any keyframes.

How would I enter the proper code to shift the opacity of an object to be 50% on mouseover, and 100% on mouseout? It would be amazing if I could even apply transition timing.

I tried using:

{-webkit-transition: opacity 0.5;}

but had no luck.

Thank you in advance for any help!

Eric