What is useState() in React?

Question

I am currently learning hooks concept in React and trying to understand below example.

import { useState } from 'react';

function Example() {
    // Declare a new state variable, which we'll call "count"
    const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

The above example increments the counter on the handler function parameter itself. What if I want to modify count value inside event handler function

Consider below example:

setCount = () => {
  //how can I modify count value here. Not sure if I can use setState to modify its value
  //also I want to modify other state values as well here. How can I do that
}

<button onClick={() => setCount()}>
  Click me
</button>

Answer 1

React hooks are a new way (still being developed) to access the core features of react such as state without having to use classes, in your example if you want to increment a counter directly in the handler function without specifying it directly in the onClick prop, you could do something like:

...
const [count, setCounter] = useState(0);
const [moreStuff, setMoreStuff] = useState(...);
...

const setCount = () => {
    setCounter(count + 1);
    setMoreStuff(...);
    ...
};

and onClick:

<button onClick={setCount}>
    Click me
</button>

Let's quickly explain what is going on in this line:

const [count, setCounter] = useState(0);

useState(0) returns a tuple where the first parameter count is the current state of the counter and setCounter is the method that will allow us to update the counter's state. We can use the setCounter method to update the state of count anywhere - In this case we are using it inside of the setCount function where we can do more things; the idea with hooks is that we are able to keep our code more functional and avoid class based components if not desired/needed.

I wrote a complete article about hooks with multiple examples (including counters) such as this codepen, I made use of useState, useEffect, useContext, and custom hooks. I could get into more details about how hooks work on this answer but the documentation does a very good job explaining the state hook and other hooks in detail, hope it helps.

update: Hooks are not longer a proposal, since version 16.8 they're now available to be used, there is a section in React's site that answers some of the FAQ.