Rules of hooks – react

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. There are 10 in-built hooks that was shipped with React These are the 4 basic hooks that are commonly used by React developers that have adopted React Hooks into their codebases. The useState hook allows React developers to update, handle and manipulate state inside functional components without needing to convert it to a class component.

The useState hook receives an initial state as an argument and then returns, by making use of array destructuring in JavaScript, the two variables in the array can be named what. 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. 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.

Also, if you are already familiar with how React lifecycle methods work, you can also think of useEffect hook as component mountingupdating and unmounting — all combined in one function. It lets us replicate the lifecycle methods in functional components. We will use the code snippets below to explain the most basic way that we can by using the useEffect hook. Just like we discussed in the previous section on how to use the useState hook to handle state inside functional components, we used it in our code snippet to set the state for our app that renders my full name.

We have now imported the useEffect hook and also made use of the useEffect function to set the state of our the name and surname property which is pretty neat and concise. You may have noticed the useEffect hook in the second argument which is an empty array; this is because it contains a call to the setFullName which does not have a list of dependencies.

We can also use change the title property of our application inside the useEffect function by calling the setTitle function, like so:. The useContext hook accepts a context object, i. This hook gives functional components easy access to your React app context. Basically, the useContext hook works with the React Context API which is a way to share data deeply throughout your app without the need to manually pass your app props down through various levels.

Now, the useContext makes using Context a little easier. Below, we are creating a context called NumberContext. Then we use the Provider value that was returned from the NumberContext we created to make a particular value available to all the children.

With that, we can use the Consumer value that was returned from the NumberContext we created to get the value we made available to all children. If you have noticed, this component did not get any props.

Note how we were able to get the value from the App component into the Display component by wrapping our content in a NumberContext. Consumer and using the render props method to retrieve the value and render it.I build … More about Adeneye …. Every second Tuesday, we send a newsletter with useful techniques on front-end and UX. React Hooks are a new addition in React They do not work inside class components. React provides a few built-in Hooks like useState.

You can also create your own Hooks to reuse stateful behavior between different components. The example below shows a counter whose state is managed using the useState hook. Each time you click on the button, we make use of setCount to update the value of count by 1. This example renders a counter with a value of 0. When you click the button, it increments the value by 1. The initial value of the component is defined using useState. As you can see, we set that to be 0.

Then we use the onClick method to call setCount when we want to increment the value. Before we dive deep into the best practices, we need to understand the rules of React Hooks which are also some of the fundamental concepts of the practices presented in this article.

Note : These two rules were introduced in React Hooks, as opposed to being part of JavaScript itself. Always use Hooks at the top level of your React function.

By following this rule, you ensure that Hooks are called in the same order each time a component renders. Now, this component will be maybe to successfully manage its state if it remains the same between multiple calls of useState and useEffect. If the order of our Hooks changes which can be possible when they are called in loops or conditionalsReact will have a hard time figuring out how to preserve the state of our component.

Since the order remains the same, it will be able to preserve the state of our component. But what happens if we put a Hook call inside a condition?You can build all components as functional components with React Hooks but neither you have to so, nor you have to convert all your existing, class-based components to functional ones with Hooks. This course will take you from zero to advanced React developer!

Learned enough about React? Want to build a native mobile app? React Native can do that! And you can, with this course! When you set a new state with React Hooks i. Hence, if you have multiple, independent state slices, you want to use multiple useState calls:. Each state will be independent of the other states and updating one state will therefore have no impact on the others. State can be anything when using React Hooks: An array, an object, a number, a string or a boolean.

You will always overwrite the old state and hence any merging that might be required has to be done by you!

Another important reason for using classes was that you could add lifecycle methods to them - for example to load a list of products when your component gets rendered. You might lose your old state if a user selected a product or a product got deleted locally in the meantime. But in addition, you introduced an infinite loop because calling setProducts And so on…. The function it takes will be executed for you after every render cycle.

Don't call a React function component

And that is really important: After the render cycle. And after every render cycle. Both is important! Thankfully, useEffect has a solution! It takes a second argument which allows us to control when the function that was passed in as the first argument will be executed. This leads React to only execute the function passed as the first argument when the component is rendered for the first time.

Effectively, it now behaves like componentDidMount. Both problems from above are now solved, we only fetch data when the component renders for the first time and we introduce no infinite loop! It should run whenever selectedId changes. As a first step, we can remove the second argument we passed to useEffect. This will cause the function to run whenever the component re-renders. That certainly solves the problem of only one Http request being sent but it re-introduces another one: We now have an infinite loop again because setProducts leads to a re-render cycle.

Of course, we only want to send a new Http request when selectedId changed, not when products changed as set via setProducts. This second argument which always has to be an array actually is simply a list of dependencies of your useEffect function.

You can add any input to that array which should trigger the useEffect function to run again. When that changes, you should run the function again. Ignore any other changes to any other variable or constant. So please re-execute whenever any of these dependencies change. Thus far, we can replicate componentDidMount and componentDidUpdate - the two most important lifecycle methods we used in class-based components.

Cleaning up event listeners, Websocket connections or timers comes to mind. In class-based components, you would typically use componentWillUnmount for that.

The function passed as a first argument to useEffect can return another function or nothing, as we did in the previous examples. If you return a function, that function will be executed right before the function passed to useEffect runs.Hooks are the new feature introduced in the React It allows you to use state and other React features without writing a class.

Hooks are the functions which "hook into" React state and lifecycle features from function components. It does not work inside classes. Hooks are backward-compatible, which means it does not contain any breaking changes. Also, it does not replace your knowledge of React concepts. If you write a function component, and then you want to add some state to it, previously you do this by converting it to a class. But, now you can do it by using a Hook inside the existing function component.

Hooks are similar to JavaScript functions, but you need to follow these two rules when using them. Hooks rule ensures that all the stateful logic in a component is visible in its source code.

These rules are:. Do not call Hooks inside loops, conditions, or nested functions. Hooks should always be used at the top level of the React functions. This rule ensures that Hooks are called in the same order each time a components renders.

You cannot call Hooks from regular JavaScript functions. Instead, you can call Hooks from React function components. Hooks can also be called from custom Hooks. Make sure the package. Hook state is the new way of declaring a state in React app.

rules of hooks – react

Hook uses useState functional component for setting and retrieving state. Let us understand Hook state with the following example. In the above example, useState is the Hook which needs to call inside a function component to add some local state to it. After that, we will call this function from an event handler or somewhere else. The useState is similar to this. The equivalent code without Hooks looks like as below.

The Effect Hook allows us to perform side effects an action in the function components. It does not use components lifecycle methods which are available in class components. The above code is based on the previous example with a new feature which we set the document title to a custom message, including the number of clicks.

It is used in useEffect which does not block the browser from updating the screen. It makes the app more responsive. The most common example of effects which don't require a cleanup are manual DOM mutations, Network requests, Logging, etc. Some effects require cleanup after DOM updation. For example, if we want to set up a subscription to some external data source, it is important to clean up memory so that we don't introduce a memory leak.Since then, people have been rapidly learning and adopting hooks in their production codebases because hooks drastically simplify the management of state and side effects in applications.

It definitely took its rightful place as "the new hotness. So let's look a bit at what pitfalls you could come across and how you can change your thinking so you avoid them. So give yourself a good hour or two and just read the docs without touching your keyboard.

It'll give you a great overview of hooks conceptually and save you a lot of time. Around the time Hooks was released, the eslint-plugin-react-hooks package was built and released. It has two rules: "rules of hooks" and "exhaustive deps.

I strongly advise that you install, use, and follow these rules. It will not only catch real bugs that you can easily miss, but it will also teach you things about your code and hooks in the process not to mention the awesome auto-fix feature. I've talked with a lot of people who are annoyed by the exhaustive deps plugin, so let me quickly demonstrate a scenario where ignoring that plugin can lead to a bug:.

Imagine you have one screen that shows a list of dogs, and when you click on a dog, it takes you to another page that shows the information for that dog. Something like this:.

Ok, so for the details page we make a DogInfo component which accepts a dogId prop and fetches the dog's information:. We feel comfortable omitting dependencies in the array there because this request should only be made on mount. And with the way things are today we'll be ok. But now let's imagine that the UI changes a bit and we start listing a "related dogs" UI on this page. We're going to have a bug and clicking on the related dog won't update the dog information, even though the component is re-rendered!

Observe click "Poodle" then under "Related Dogs" click "Bernedoodle" and notice no change occurs :. So it's triggering a re-render of our DogInfo with a new dogIdbut because we provided an empty array of dependencies, our effect is not re-running.

rules of hooks – react

Here's the key takeaway from this example: If it really will never change, then there's no harm in including it anyway. Also, if you think it will never change and it does, including it will help you avoid bugs.

Just trust me, every time I thought "oh, I don't need to follow the rule this time" I later regretted disabling it because I was wrong. Please do note that sometimes the rule is incapable of performing static analysis on your code properly thanks to limitations of static analysis tools like ESLint. I believe this is why it's recommended to set the exhaustive deps rule to "warn" instead of "error. I recommend that you try to restructure your code a bit to avoid that warning please favor explicitness over cleverness.

If that won't work, then disabling the plugin is your escape hatch so you can keep working. For as long as React has been popular before hookswe had a nice and clear component API that made it easy for us to tell React when it should do certain things:. Writing components like this still works and will for the foreseeable futureand it worked really well for many years. Hooks come with a lot of benefits, but one of my favorites is that it makes your components more declarative in that it allows you to stop thinking about "when things should happen in the lifecycle of the component" which doesn't matter all that much and more about "when things should happen in relation to state changes" which matters much more.

The reason I love this so much is because it naturally helps me avoid bugs. So often I have found I had a bug in my code because I forgot to handle a prop or state update in componentDidUpdateand when I did remember, I would often forget to cleanup the previous side-effect before starting up the new one for example, if you make an HTTP request, but a prop changes before that request completes, you should cancel the previous request.

With React Hooks, you do still kinda think about when side effects should run, but you're not thinking about component Lifecycles, you're thinking about synchronizing the state of the side-effects with the state of the application.

Grasping that requires a little bit of unlearning, but it's such a powerful idea that once you wrap your head around it, you will naturally experience fewer bugs in your apps thanks to the design of the API.This means that within a given component, its hooks should never be called in such a way that the order in which those hooks are called might ever be different between different renders.

Read that slowly, twice. Hooks should not be placed inside loops, conditions, or nested functions. At first glance this rule may seem unintuitive. Why should they need to be called in the same order every render? Take a look at a simple example with two hook calls. Suppose we render this component. React is keeping track of two pieces of state for us, and needs a way to know what to return for each useState call when this component is re-rendered. It then does the same thing for the second call to useState but with the second element of the array, and so on and so forth for any other hooks calls that might be in the component.

React maps the order of hook calls to array elements to know which data is associated to which hook call in a given render. Notice there are now four hooks being used. Suppose now that the title was set to an empty string, triggering a re-render of the component. Keep in mind that React still has that 4-element array in its memory. As you can see, this effect will cascade down through the array and cause all manner of bugs.

To fix this specific issue, simply move the condition into the body of the function passed to the useEffect call. So this is why hooks must always be called at the top level of a component. The best way to enforce this rule is to install the ESLint plugin I mentioned at the top of this article!

Let me know in the comments!

React - The Complete Guide (includes Hooks)

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rules of hooks – react

Full Stack JavaScript Expert. Written by Shawn Reisner Follow. Write the first response. More From Medium. Creating a Custom Library with Angular.Hooks are a new addition in React They let you use state and other React features without writing a class. Hooks are backwards-compatible. This page provides an overview of Hooks for experienced React users. This is a fast-paced overview. If you get confused, look for a yellow box like this:. They link to detailed explanations. We call it inside a function component to add some local state to it.

React will preserve this state between re-renders. You can call this function from an event handler or somewhere else.

The only argument to useState is the initial state. In the example above, it is 0 because our counter starts from zero. Note that unlike this. The initial state argument is only used during the first render. The array destructuring syntax lets us give different names to the state variables we declared by calling useState. Instead, React assumes that if you call useState many times, you do it in the same order during every render.

Invalid Hook Call Warning

React provides a few built-in Hooks like useState. You can also create your own Hooks to reuse stateful behavior between different components. The Effect Hook, useEffectadds the ability to perform side effects from a function component.

Effects are declared inside the component so they have access to its props and state. By default, React runs the effects after every render — including the first render. In this example, React would unsubscribe from our ChatAPI when the component unmounts, as well as before re-running the effect due to a subsequent render. Just like with useStateyou can use more than a single effect in a component:.

Hooks let you organize side effects in a component by what pieces are related such as adding and removing a subscriptionrather than forcing a split based on lifecycle methods. You can learn more about useEffect on a dedicated page: Using the Effect Hook. We provide a linter plugin to enforce these rules automatically.

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