The useState hook is a built-in hook in React that allows you to add state to your functional components. With the useState hook, you can create a state variable and a function that can update that variable. Whenever the state variable is updated, React will re-render your component and update the UI accordingly.

To create the useState hook from scratch, we will need to use a few key features of JavaScript: closures and arrays. Here's what the code will look like:

function useState(initialValue) {
	let _val = initialValue; // assign initial value to _val
	const state = () => _val; // define a closure function that returns _val
	const setState = (newVal) => { // define a function that updates _val and triggers re-render
		_val = newVal;
		render(); // re-render the component
	};
	return [state, setState]; // return an array with the state function and the update function
}

Let's break down this code step-by-step.

First, we define the useState function that takes an initial value as its argument. This initial value will be used to set the initial state of the component.

Next, we create a variable _val and assign the initial value to it. This variable will hold the current state value.

Then, we define a closure function state that returns _val. This function will be used to read the current state value.

After that, we define another function setState that takes a new value as its argument. This function will be used to update the state value and trigger a re-render of the component.

Inside setState, we update _val with the new value, and then we call a render function to re-render the component.

Finally, we return an array with the state function and the setState function. This is the same format that the built-in useState hook uses.

Now that we have created our own useState hook, let's see how we can use it in a functional component.

function Counter() {
	const [count, setCount] = useState(0); // use our custom useState hook to add state to the component

	function increment() {
		setCount(count + 1); // call setCount to update the count value
	}

	return (
		<div>
			<h1>{count}</h1>
			<button onClick={increment}>Increment</button>
		</div>
	);
}

In this example, we have defined a functional component called Counter. Inside this component, we use our custom useState hook to add state to the component. We create a state variable count and a function setCount that can update that variable.

In this blog post, we have explored how to create the useState hook in React from scratch using JavaScript. We used closures and arrays to create a custom hook that can add state to a functional component.

While it's not necessary to create your own useState hook in every project, understanding how it works can give you a deeper understanding of how React works behind the scenes.

Will be updating implementations for more React Hooks and functionalities in the near future.