Calling a function after state is set

Question

I have noticed that hooks set functions are asynchonous but I need to call a function once my new value has been set.

This is how my state looks:

const[agenda, setAgenda] = useState([{ id: "0", date:new Date(2022,0,21,13,25,0), titulo: "Test 1", obs:'Lorem ipsum dolor sit amet', prioridade:1,local:'-',lugar:'', diatodo:false  },
{ id: "1", date:new Date(2022,0,21,13,25,0), titulo: "Test 2", obs:'Lorem ipsum dolor sit amet', prioridade:1,local:'-',lugar:'', diatodo:false  }])

It's an array so I don't know how to do it, I have tried doing this:

setAgenda({[...agenda,obj]},testFunction);

Visual Studio Code gives me the following error though: Error

How should I do it? Thanks in advance