1let markers = [ ...this.state.markers ];
2markers[index] = {...markers[index], key: value};
3this.setState({ markers });

1const initialState = [
2    { name: "foo", counter: 0 },
3    { name: "far", counter: 0 },
4    { name: "faz", counter: 0 }
5  ];
6
7const [state, setState] = useState(initialState);
8
9const clickButton = () => {
10	// 1. Make a shallow copy of the array
11	let temp_state = [...state];
12	
13	// 2. Make a shallow copy of the element you want to mutate
14	let temp_element = { ...temp_state[0] };
15	
16	// 3. Update the property you're interested in
17	temp_element.counter = temp_element.counter+1;
18	
19	// 4. Put it back into our array. N.B. we *are* mutating the array here, but that's why we made a copy first
20	temp_state[0] = temp_element;
21	
22	// 5. Set the state to our new copy
23	setState( temp_state );
24}

1const [array, setArray] = useState([1, 2, 3])
2
3// Add
4setArray(prevArray => [...prevArray, 4]); // -> [1, 2, 3, 4]
5
6// Remove last entry
7setArray(prevArray => prevArray.splice(0, prevArray.length - 1)); // -> [1, 2, 3]