React Hooks - using useState vs just variables

React Hooks give us useState option, and I always see Hooks vs Class-State comparisons. But what about Hooks and some regular variables?

For example,

function Foo() {
    let a = 0;
    a = 1;
    return <div>{a}</div>;
}

I didn't use Hooks, and it will give me the same results as:

function Foo() {
    const [a, setA] = useState(0);
    if (a != 1) setA(1); // to avoid infinite-loop
    return <div>{a}</div>;
}

So what is the diffrence? Using Hooks even more complex for that case...So why start using it?

The reason is if you useState it rerenders the view. Variables by themselves only change bits in memory and the state of your app can get out of sync with the view.

Compare this examples:

function Foo() {
    const [a, setA] = useState(0);
    return <div onClick={() => setA(a + 1)}>{a}</div>;
}

function Foo() {
    let a = 0;
    return <div onClick={() => a + 1}>{a}</div>;
}

In both cases a changes on click but only when you use useState the view correctly shows a's current value.

Local variables will get reset every render upon mutation whereas state will update:

function App() {
  let a = 0; // reset to 0 on render/re-render
  const [b, setB] = useState(0);

  return (
    <div className="App">
      <div>
        {a}
        <button onClick={() => a++}>local variable a++</button>
      </div>
      <div>
        {b}
        <button onClick={() => setB(prevB => prevB + 1)}>
          state variable b++
        </button>
      </div>
    </div>
  );
}

Your first example only works because the data essentially never changes. The enter point of using setState is to rerender your entire component when the state hanges. So if your example required some sort of state change or management you will quickly realize change values will be necessary and to do update the view with the variable value, you will need the state and rerendering.

Updating state will make the component to re-render again, but local values are not.

In your case, you rendered that value in your component. That means, when the value is changed, the component should be re-rendered to show the updated value.

So it will be better to use useState than normal local value.

function Foo() {
    let a = 0;
    a = 1; // there will be no re-render.
    return <div>{a}</div>;
}

function Foo() {
    const [a, setA] = useState(0);
    if (a != 1) setA(1); // re-render required
    return <div>{a}</div>;
}

function Foo() {
    const [a, setA] = useState(0);
    if (a != 1) setA(1); // to avoid infinite-loop
    return <div>{a}</div>;
}

is equivalent to

class Foo extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            a: 0
        };
    }
    // ...
}

What useState returns are two things:

1. new state variable
2. setter for that variable

if you call setA(1) you would call this.setState({ a: 1 }) and trigger a re-render.