? called ternary operator is shorcut for, "if statement".
// below we make object with key id, and assign it a value from body.id.
// if body.id is null it will return false and we assign it res.id.
{ id: body.id? body.id : res.id }
// we can write it in if statement like this
if(body.id){
return {id:body.id}
}else{
return {id:res.id}
}