You should normally stick to the first one. Calling the setter of useState may lead to undesired re-renders and decreased performance.

In the first block the customer is initialised directly and no re-render happens. The setCustomer method will change the state and rerender the component. In the end the whole function will run twice which you can verify with a console.log.

const [customer, setCustomers] = useState(0);

useEffect(() => {
  setCustomers(15);
}, []);

console.log(customer) // will first output 0 and then 15

Assuming in the second case, you have this as your useState statement:

const [customer, setCustomers] = useState();

The second one sets the value of customer on componentDidMount. So in the initial render, you will not have the appropriate value in your customer variable. But yes, very soon after that the correct value will be set because of the code written in useEffect.

To clear it up, there will be 2 renders here (because the state variable value changes). In the first one, that won't be the case since the state variable has only one value from beginning.

The first one is more effective.

const [customer, setCustomers] = useState(props.location.state);

If you use second one (by using useEffect), your component will be re-rendered again.

That's, your state variable customer will be updated in useEffect after DOM is initially rendered, this leads the 2nd re-render of the component.

But if you want customer to be updated by props.location.state, you need to add useEffect hook like the following.

useEffect(()=> {
    setCustomers(props.location.state);
}, [props.location.state]);