React Hooks useState() with Object

What is the correct way of updating state, in a nested object, in React with Hooks?

export Example = () => {
  const [exampleState, setExampleState] = useState(
  {masterField: {
        fieldOne: "a",
        fieldTwo: {
           fieldTwoOne: "b"
           fieldTwoTwo: "c"
           }
        }
   })

How would one use setExampleState to update exampleState to a (appending an field)?

const a = {
masterField: {
        fieldOne: "a",
        fieldTwo: {
           fieldTwoOne: "b",
           fieldTwoTwo: "c"
           }
        },
  masterField2: {
        fieldOne: "c",
        fieldTwo: {
           fieldTwoOne: "d",
           fieldTwoTwo: "e"
           }
        },
   }
}

b (Changing values)?

const b = {masterField: {
        fieldOne: "e",
        fieldTwo: {
           fieldTwoOne: "f"
           fieldTwoTwo: "g"
           }
        }
   })

You can pass new value like this:

  setExampleState({...exampleState,  masterField2: {
        fieldOne: "a",
        fieldTwo: {
           fieldTwoOne: "b",
           fieldTwoTwo: "c"
           }
        },
   })

If anyone is searching for useState() hooks update for object

- Through Input

        const [state, setState] = useState({ fName: "", lName: "" });
        const handleChange = e => {
            const { name, value } = e.target;
            setState(prevState => ({
                ...prevState,
                [name]: value
            }));
        };

        <input
            value={state.fName}
            type="text"
            onChange={handleChange}
            name="fName"
        />
        <input
            value={state.lName}
            type="text"
            onChange={handleChange}
            name="lName"
        />
   ***************************

 - Through onSubmit or button click
    
        setState(prevState => ({
            ...prevState,
            fName: 'your updated value here'
         }));

Generally you should watch out for deeply nested objects in React state. To avoid unexpected behavior, the state should be updated immutably. When you have deep objects, you end up deep cloning them for immutability, which can be quite expensive in React. Why?

Once you deep clone the state, React will recalculate and re-render everything that depends on the variables, even though they haven't changed!

So, before trying to solve your issue, think how you can flatten the state first. As soon as you do that, you will find handy tools that will help dealing with large states, such as useReducer().

In case you thought about it, but are still convinced you need to use a deeply nested state tree, you can still use useState() with libraries like immutable.js and Immutability-helper. They make it simple to update or clone deep objects without having to worry about mutability.