With the advent of React and Reduxa common question has emerged:. But this question is actually too simplistic, because there are also two other ways you can store data for use in a component: static and this. When React was first introduced, we were presented with local state.
The important thing to know about local state is that when a state value changes, it triggers a re-render. This state can be passed down to children as propswhich allows you to separate your components between smart data-components and dumb presentational-components if you chose. Your data the value of the counter is stored within the App component, and can be passed down its children. Assuming your counter is important to your app, and is storing data that would be useful to other components, you would not want to use local state to keep this value.
The current best practice is to use local state to handle the state of your user interface UI state rather than data. For example, using a controlled component to fill out a form is a perfectly valid use of local state. Another example of UI data that you could store in local state would be the currently selected tab from a list of options. Takeaway: keep UI state and transitory data such as form inputs in local state.
Then after some time had elapsed and everyone started getting comfortable with the idea of unidirectional data flowwe got Redux. With Redux, we get a global store. This store lives at the highest level of your app and passes data down to all children. You connect to the global store with the connect wrapper and a mapStateToProps function. At first, people put everything in the Redux store. Users, modals, forms, sockets… you name it.
Below is the same counter app, but using Redux. The important thing to note is that counter now comes from this. Now when you click on the button, an action is dispatched and the global store is updated. The data is handled outside of our local component and is passed down.
The Redux store is great for keeping application state rather than UI state. Many of your components will need access to this information, and as soon as the login status changes, all of those components the ones that are rendered, at least will need to be re-rendered with the updated information. Redux is also useful for triggering events for which you need access on multiple components or across multiple routes. An example of this would be a login modal, which can be triggered by a multitude of buttons all across your app.
This is actually an example of how you should not use this. The use case for this is to store values for which a change should not trigger a re-render. For example, sockets are a perfect thing to store on this. The following two code blocks are identical.When state changes, the component responds by re-rendering.
While both hold information that influences the output of render, they are different in one important way: props get passed to the component similar to function parameters whereas state is managed within the component similar to variables declared within a function.
Here are some good resources for further reading on when to use props vs state :.
In React, both this. Pass an updater function instead of an object if you need to compute values based on the current state see below for details. Pass a function instead of an object to setState to ensure the call always uses the most updated version of state see below. Passing an update function allows you to access the current state value inside the updater. Since setState calls are batched, this lets you chain updates and ensure they build on top of each other instead of conflicting:.
This results in significant performance improvements in larger apps. This is an implementation detail so avoid relying on it directly. In the future versions, React will batch updates by default in more cases. This boosts performance by avoiding unnecessary re-renders. This GitHub comment dives deep into the specific examples. You can build quite complex applications using only React.
What does setState do? What is the difference between state and props? State Why is setState giving me the wrong value? Edit this page. Main Concepts. Advanced Guides. API Reference. Concurrent Mode Experimental.The following is an excerpt from React Quicklya hands-on book by Azat Mardan for anyone who wants to learn React. In this article, you will be learning about state in React and how to work with it.
But before we dive into all that, in order to work with state, we need to know how to access the values, update them and how to set the initial values. The state object is an attribute of a component and can be accessed with this reference, e. Similarly, we can render this. This syntax is like accessing properties with this. The goal is to have a self-contained component class anyone can import and use in their application without having to jump through hoops.
To set the initial state, use this. Component syntax. Developers can add other logic when setting the initial state. For example, we can set the value of currentTime using new Date. The value of this. The gist is that, as with other OOP languages, constructor is invoked when an instance of this class is created.
The constructor method name must be constructor. Think about it as an ES6 convention. This way, developers can set state not only in the constructor, but in the body of the class:.Hooks are a new addition in React They let you use state and other React features without writing a class. The introduction page used this example to get familiar with Hooks:. But you can use them instead of writing classes. Our new example starts by importing the useState Hook from React:.
What is a Hook? For example, useState is a Hook that lets you add React state to function components. When would I use a Hook? If you write a function component and realize you need to add some state to it, previously you had to convert it to a class. Now you can use a Hook inside the existing function component.
In a class, we initialize the count state to 0 by setting this. Instead, we call the useState Hook directly inside our component:. What does calling useState do?
State and Lifecycle
Our variable is called count but we could call it anything else, like banana. What do we pass to useState as an argument?
The only argument to the useState Hook is the initial state. In our example, we just want a number for how many times the user clicked, so pass 0 as initial state for our variable. If we wanted to store two different values in state, we would call useState twice. What does useState return? It returns a pair of values: the current state and a function that updates it. This is similar to this.
Now that we know what the useState Hook does, our example should make more sense:. We declare a state variable called countand set it to 0. React will remember its current value between re-renders, and provide the most recent one to our function.
If we want to update the current countwe can call setCount. You might be wondering: why is useState not named createState instead? During the next renders, useState gives us the current state. When we want to display the current count in a class, we read this. In a class, we need to call this. This might seem like a lot to take in at first.
It is equivalent to this code:.This page introduces the concept of state and lifecycle in a React component. You can find a detailed component API reference here. Consider the ticking clock example from one of the previous sections. In Rendering Elementswe have only learned one way to update the UI.
We call ReactDOM. Try it on CodePen. In this section, we will learn how to make the Clock component truly reusable and encapsulated. It will set up its own timer and update itself every second. However, it misses a crucial requirement: the fact that the Clock sets up a timer and updates the UI every second should be an implementation detail of the Clock. Ideally we want to write this once and have the Clock update itself:.
You can convert a function component like Clock to a class in five steps:. This lets us use additional features such as local state and lifecycle methods. Class components should always call the base constructor with props. We want to set up a timer whenever the Clock is rendered to the DOM for the first time.
We also want to clear that timer whenever the DOM produced by the Clock is removed. We can declare special methods on the component class to run some code when a component mounts and unmounts:.
This is a good place to set up a timer:. Note how we save the timer ID right on this this. While this. We will tear down the timer in the componentWillUnmount lifecycle method:. Finally, we will implement a method called tick that the Clock component will run every second. It will use this. The only place where you can assign this. React may batch multiple setState calls into a single update for performance. Because this.We are going to build a sign up form like the one below:. At the time of writing this post, v Whilst we are in App.
The useState function is the new React function we are going to use a little later to manage our state. So, the consumer of the component will interact with the web server and do the sign up submission.
Now we have reached the interesting bit. How are we going to store and set the state for the first name and email address?
Well, we can leverage that useState function we imported earlier on:. The useState function takes in the default value of the state and returns a two element array containing the state variable and a function to set the value of the variable. We need some more state for the validation error messages. So, we double check that the first name and email address are okay before invoking the onSignUp function prop.
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We want to inform the user whether the submission was successful or not, so, we need some more state for this. Firstly, we need state to indicate the form has been submitted:. Our final job is to reference our SignUp component in our App component:. If we hit the Sign Up button without filling in the form, we correctly get the validation errors rendered:. If we properly fill out the form and hit the Sign Up button, we get confirmation that the form has been submitted okay:.
I thought function-based components were supposed to be stateless more or less by definition. Thanks for the question Max. Yes, before React React This will allow us to use function-based components for the majority of our components if we want rather than having a mix of function-based and class-based components.
Want to learn more about React and TypeScript? Check out my book.Shedrack Akintayo is a software engineer from Lagos, Nigeria, who has a love for community building, open source and creating content and tech for the next … More about Shedrack ….
Every second Tuesday, we send a newsletter with useful techniques on front-end and UX. When React Some curious developers have been making use of the Hooks API even before it was released officially, but back then it was not stable and was only an experimental feature.
Now it is stable and recommended for React developers to use. React Hooks are in-built functions that allow React developers to use state and lifecycle methods inside functional components, they also work together with existing code, so they can easily be adopted into a codebase. The way Hooks were pitched to the public was that they allow developers to use state in functional components but under the hood, Hooks are much more powerful than that. They allow React Developers to enjoy the following benefits:.
With React Hooks, developers get the power to use functional components for almost everything they need to do from just rendering UI to also handling state and also logic — which is pretty neat. According to the ReactJS official documentationthe following are the motivation behind the release of React Hooks:. There are two main rules that are strictly to be adhered to as stated by the React core team in which they outlined in the hooks proposal documentation.
The first variable is the actual state, while the second variable is a function that is meant for updating the state by providing a new state. This is how our component should look when it is rendered in our React application.
Getting Started With The React Hooks API
The useEffect hook accepts a function that would contain effectual code. In functional components, effects like mutations, subscriptions, timers, logging, and other effects are not allowed to be placed inside a functional component because doing so would lead to a lot of inconsistencies when the UI is rendered and also confusing bugs.
In using the useEffect hook, the effectual function passed into it will execute right after the render has been displayed on the screen. By default, effects are executed mainly after the render has been completed, but you have the option to also fire them when certain values change.