Beginner's guide to useState hook in React

A key component of a web application is the state. Taken directly from the beta(new) version of the ReactJS Docs:

Think of state as the minimal set of changing data that your app needs to remember. For example, if you’re building a shopping list, you can store the items as an array in state.

Now, how do we manage state inside a React Component? So, in this tutorial we'll be talking just about that!

Prerequisites

• Basic knowledge of ReactJS

What are React Hooks?

First, let's start with what is a React Hook. The beta docs of ReactJS says the following:

Functions starting with use are called Hooks. useState is a built-in Hook provided by React. You can find other built-in Hooks in the React API reference. You can also write your own Hooks by combining the existing ones. Hooks are more restrictive than regular functions. You can only call Hooks at the top level of your components (or other Hooks). If you want to useState in a condition or a loop, extract a new component and put it there.

So basically, if you know a little history about React, you would know that React used to be mainly comprised of something called Class-Based Components. But now, the community is starting to move to Function-Based Components. Now, React Hooks allow the functional components to have access to state and other React Features.

Setting up the app

Create a project using the following command

npx create-react-app react-hooks-tutorial

Then edit your src/App.js to look like this

function App() {
  return (
    <div>
      <h1>Hello, World</h1>
      <p>1</p> // this will update based on the state after this tutorial
      <button>Increase counter</button>
    </div>
  );
}

export default App;

useState hook

Now, the way the useState hook is used in React is very interesting. In a conventional, class based component you'd do something like this

state = {
    color: "red"
}

and then access it via this.state. But using the useState hook you'd do something like this

+ import { useState } from "react"
  function App() {
+ const [count, setCount] = useState(1);
    return (

Now the useState hook returns two things:

1. The first variable is the actual state variable. For eg. count in this case is 1.
2. setCount is a function which you can use to change the value of the count state.

Using the count state variable to show data

First, update the hardcoded value of 1 with the count variable like so

       <h1>Hello, World</h1>
- <p>1</p>
+ <p>{count}</p>
       <button>Increase counter</button>

Using setCount to update state

Now to make this work, let's make the click of the button increase the count variable by 1.

       <p>{count}</p>
- <button>Increase counter</button>
+ <button onClick={() => setCount(count + 1)}>Increase counter</button>

Using previous state to update state

Now, let's try something. Change the button line to the following

- <button onClick={() => setCount(count + 1)}>Increase counter</button>
+ <button onClick={incrementCount}>Increase counter</button>


  const [count, setCount] = useState(1);
+ function incrementCount() {
+ setCount(count + 1);
+ setCount(count + 1);
+ setCount(count + 1);
+ }

Now this will give you an issue. Straight from the docs again (little modified according to our app):

This is because calling the set function does not update the count state variable in the already running code. So each setCount(count + 1) call becomes setCount(2).

To fix this issue, you can reference the previous state and then update the current state using that. So change your code like this

- function incrementCount() {
- setCount(count + 1);
- setCount(count + 1);
- setCount(count + 1);
- }
+ function incrementCount() {
+ setCount((prevCount) => prevCount + 1);
+ setCount((prevCount) => prevCount + 1);
+ setCount((prevCount) => prevCount + 1);
+ }

So, you get the prevCount argument and increment the current state according to that and this should work!

Note: This is obviously not a real life scenario. To increment by 3, you'd probably do something like setCount(count+3) but it's good to know when you can use the previous state variable as a reference.

That is it for this tutorial! You final code should look like this:

import { useState } from "react";

function App() {
  const [count, setCount] = useState(1);
  function incrementCount() {
    setCount((prevCount) => prevCount + 1);
    setCount((prevCount) => prevCount + 1);
    setCount((prevCount) => prevCount + 1);
  }
  return (
    <div>
      <h1>Hello, World</h1>
      <p>{count}</p>
      <button onClick={incrementCount}>Increase counter</button>
    </div>
  );
}

export default App;