Appending with and without the option to replace repeated values [closed]

I've seen so many libraries that contain JSON appending but they don't really satisfy my needs. What I need is something like:

foo = { color: red, size: 16, } bar = { shape: triangle, size: 30 }

What I want to do is if the prop doesn't exist append it to foo, but if it does exist to have an option to replace the value or don't do anything.