Merging two variables/fields into one

I feel like this should be so obvious, but...wow, am I failing spectacularly.

I have two fields in my DB I'd like to merge: first name, last name. I want to create a new output called fullName. I was thinking an Add On would be appropriate, but I'm stuck.

I just want to combine the first and last name into one name, and send it to my front end. So...

firstName: Josh
lastname: Chambers
...becomes...
fullName: Josh Chambers

I need all three fields in the output.

Thanks!