ReactJS

Transitioning from React class components to function components with hooks

Published Mar 10, 2020

Updated Feb 14, 2023

5 min read

This article was written over 18 months ago and may contain information that is out of date. Some content may be relevant but please refer to the relevant official documentation or available resources for the latest information.

It has been approximately one year since React v16.8 was released, marking the introduction of Hooks. Yet, there are still people used to React class components who still haven't experienced the full potential of this new feature, along with functional components, including myself. The aim of this article is to summarize and encompass the most distinguishable features of the class components, and respectively show their alternatives when using React hooks.

Functional components

Before we start with Hooks examples, we will shortly discuss functional components in case you aren't familiar. They provide an easy way to create new units without the need of creating a new class and extending React.Component.

Note: Keep in mind that functional components have been part of React since it's creation.

Here is a very simple example of a functional component:

const Element = () => (
  <div className="element">
    My Element
  </div>
);

And just like class components, we can access the properties. They are provided as the first argument of the function.

const Element = ({ text }) => (
  <div className="element">
    {text}
  </div>
);

However, these type of components--while very convenient for simple UI elements--used to be very limited in terms of life cycle control, and usage of state. This is the main reason they had been neglected until React v16.8.

Component state

Let's take a look at the familiar way of how we add state to our object-oriented components. The example will represent a component which renders a space scene with stars; they have the same color. We are going to use few utility functions for both functional and class components.

createStars(width: number): Star[] - Creates an array with the star objects that are ready for rendering. The number of stars depends on the window width.
renderStars(stars: Star[], color: string): JSX.Element - Builds and returns the actual stars markup.
logColorChange(color: string) - Logs when the color of the space has been changed.

and some less important like calculateDistancesAmongStars(stars: Star[]): Object.

We won't implement these. Consider them as black boxes. The names should be sufficient enough to understand their purpose.

Note: You may find a lot of demonstrated things unnecesary. The main reason I included this is to showcase the hooks in a single component.

And the example:

Class components

class Space extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      stars: createStars(window.innerWidth)
    };
  }

  render() {
    return (
      <div className="space">
        {renderStars(this.state.stars, this.props.color)}
      </div>
    );
  }
}

Functional components

The same can be achieved with the help of the first React Hook that we are going to introduce--useState. The usage is as follows: const [name, setName] = useState(INITIAL_VALUE). As you can see, it uses array destructuring in order to provide the value and the set function:

const Space = ({ color }) => {
  const [stars, setStars] = useState(createStars(window.innerWidth));

  return (
    <div className="space">
      {renderStars(stars, color)}
    </div>
  );
};

The usage of the property is trivial, while setStars(stars) will be equivalent to this.setState({ stars }).

Component initialization

Another prominent limitation of functional components was the inability to hook to lifecycle events. Unlike class components, where you could simply define the componentDidMount method, if you want to execute code on component creation, you can't hook to lifecycle events. Let's extend our demo by adding a resize listener to window which will change the number of rendered stars in our space when the user changes the width of the browser:

Class components

class Space extends React.Component {
  constructor(props) { ... }

  componentDidMount() {
    window.addEventListener('resize', () => {
      const stars = createStars(window.innerWidth, this.props.color);
      this.setState({ stars });
    });
  }

  render() { ... }
}

Functional components

You may say: "We can attach the listener right above the return statement", and you will be partially correct. However, think about the functional component as the render method of a class component. Would you attach the event listener there? No. Just like render, the function of a functional component could be executed multiple times throughout the the lifecycle of the instance. This is why we are going to use the useEffect hook.

It is a bit different from componentDidMount though--it incorporates componentDidUpdate, and componentDidUnmount as well. In other words, the provided callback to useEffect is executed on every update. Anyhow, you can have certain control with the second argument of useState - it represents an array with the values/dependencies which are monitored for change. If they do, the hook is executed. In case the array is empty, the hook will be executed only once, during initialization, since after that there won't be any values to be observed for change.

const Space = ({ color }) => {
  const [stars, setStars] = useState(createStars(window.innerWidth));

  useEffect(() => {
    window.addEventListener('resize', () => {
      const stars = createStars(window.innerWidth, color);
      setStars(stars);
    });
  }, []); // <-- Note the empty array

  return (
    ...
  );
};

Component destruction

We added an event listener to window, so we will have to remove it on component unmount in order to save us from memory leaks. Respectively, that'll require keeping a reference to the callback:

Class components

class Space extends React.Component {
  constructor(props) { ... }

  componentDidMount() {
    window.addEventListener('resize', this.__resizeListenerCb = () => {
      const stars = createStars(window.innerWidth, this.props.color);
      this.setState({ stars });
    });
  }

  componentDidUnmount() {
    window.removeEventListener('resize', this.__resizeListenerCb);
  }

  render() { ... }
}

Functional component

For the equivalent version of the class component, the useEffect hook will execute the returned function from the provided callback when the component is about to be destroyed. Here's the code:

const Space = ({ color }) => {
  const [stars, setStars] = useState(createStars(window.innerWidth));

  useEffect(() => {
    let resizeListenerCb;

    window.addEventListener('resize', resizeListenerCb = () => {
      const stars = createStars(window.innerWidth, color);
      setStars(stars);
    });

    return () => window.removeEventListener('resize', resizeListenerCb);
  }, []); // <-- Note the empty array

  return (
    ...
  );
};

An important remark

It is worth mentioning that, when you work with event listeners or any other methods that defer the execution in the future of a callback/function, you should take into account that the state provided to them is not mutable.

Taking the window listener we use in our demo as an example; if we used thestars state inside the callback, we would get the exact value at the moment of the definition (callback), which means that, when the callback is executed, we risk having a stale state.

There are various ways to handle that, one of which is to re-register the listener every time the stars are changed, by providing the stars value to the observed dependency array of useEffect.

Changed properties

We already went through useEffect in the sections above. Now, we will briefly show an example of componentDidUpdate. Let's say that we want to log the occurances of color change to the console:

Class components

class Space extends React.Component {
  ...

  componentDidUpdate(prevProps) {
    if (this.props.color !== prevProps.color) {
      logColorChange(this.props.color);
    }
  }

  ...
}

Functional components

We'll introduce another useEffect hook:

const Space = ({ color }) => {
  ...

  useEffect(() => {
    logColorChange(color);
  }, [color]); // <-- Note that this time we add `color` as observed dependency

  ...
};

Simple as that!

Changed properties and memoization

Just as an addition to the example above, we will quickly showcase useMemo; it provides an easy way to optimize your component when you have to perform a heavy calculation only when certain dependencies change:

const result = useMemo(() => expensiveCalculation(), [color]);

References

Due to the nature of functional components, it becomes hard to keep a reference to an object between renders. With class components, we can simply save one with a class property, like:

class Space extends React.Component {
  ...

  methodThatIsCalledOnceInALifetime() {
    this.__distRef = calculateDistancesAmongStars(this.state.stars);
  }

  ...
}

However, here is an example with a functional component that might look correct but it isn't:

const Space = ({ color }) => {
  ...

  let distRef; // Declared on every render.

  function thatIsCalledOnceInALifetime() {
    distRef = caclulateDistancesAmongStars(stars);
  }

  ...
};

As you can see, we won't be able to preserve the output object with a simple variable. In order to do that, we will take a look at yet another hook named useRef, which will solve our problem:

const Space = ({ color }) => {
  ...
  const distRef = useRef();

  function thatIsCalledOnceInALifetime() {
    // `current` keeps the same reference
    // throughout the lifetime of the component instance
    distRef.current = caclulateDistancesAmongStars(stars);
  }

  ...
}

The same hook is used when we want to keep a reference to a DOM element.

Conclusion

Hopefully this should give you a starting point when it comes to using React Hooks for the things that you are already used to doing with class components. Obviously, there are more hooks to explore, including the definition of custom ones. For all of that, you can head to the official docs. Give them a try and experience the potential of functional React!

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Why is My React Reducer Called Twice and What the Heck is a Pure Function?

Why is My React Reducer Called Twice and What the Heck is a Pure Function? In a recent project, we encountered an interesting issue: our React reducer was dispatching twice, producing incorrect values, such as incrementing a number in increments of two. We hopped on a pairing session and started debugging. Eventually, we got to the root of the problem and learned the importance of pure functions in functional programming. This article will explain why our reducer was being dispatched twice, what pure functions are, and how React's strict mode helped us identify a bug in our code. The Issue We noticed that our useReducer hook was causing the reducer function to be called twice for every action dispatched. Initially, we were confused about this behavior and thought it might be a bug in React. Additionally, we had one of the dispatches inside a useEffect, which caused it to be called twice due to React strict mode, effectively firing the reducer four times and further complicating our debugging process. However, we knew that React's strict mode caused useEffect to be called twice, so it didn't take very long to realize that the issue was not with React but with how we had implemented our reducer function. React Strict Mode React's strict mode is a tool for highlighting potential problems in an application. It intentionally double-invokes specific lifecycle methods and hooks (like useReducer and useEffect) to help developers identify side effects. This behavior exposed our issue, as we had reducers that were not pure functions. What is a Pure Function? A pure function is a function that: - Is deterministic: Given the same input, always returns the same output. - Does Not Have Side Effects: Does not alter any external state or have observable interactions with the outside world. In the context of a reducer, this means the function should not: - Modify its arguments - Perform any I/O operations (like network requests or logging) - Generate random numbers - Depend on any external state Pure functions are predictable and testable. They help prevent bugs and make code easier to reason about. In the context of React, pure functions are essential for reducers because they ensure that the state transitions are predictable and consistent. The Root Cause: Impure Reducers Our reducers were not pure functions. They were altering external state and had side effects, which caused inconsistent behavior when React's strict mode double-invoked them. This led to unexpected results and made debugging more difficult. The Solution: Make Reducers Pure To resolve this issue, we refactored our reducers to ensure they were pure functions. Here's an extended example of how we transformed an impure reducer into a pure one in a more complex scenario involving a task management application. Let's start with the initial state and action types: ` And here's the impure reducer similar to what we had initially: ` This reducer is impure because it directly modifies the state object, which is a side effect. To make it pure, we must create a new state object for every action and return it without modifying the original state. Here's the refactored pure reducer: ` Key Changes: - Direct State Modification: In the impure reducer, the state is directly modified (e.g., state.tasks.push(action.payload)). This causes side effects and violates the principles of pure functions. - Side Effects: The impure reducer included side effects such as logging and direct state changes. The pure reducer eliminates these side effects, ensuring consistent and predictable behavior. I've created an interactive example to demonstrate the difference between impure and pure reducers in a React application. Despite the RESET_TASKS action being implemented similarly in both reducers, you'll notice that the impure reducer does not reset the tasks correctly. This problem happens because the impure reducer directly modifies the state, leading to unexpected behavior. Check out the embedded StackBlitz example below: Conclusion Our experience with the reducer dispatching twice was a valuable lesson in the importance of pure functions in React. Thanks to React's strict mode, we identified and fixed impure reducers, leading to more predictable and maintainable code. If you encounter similar issues, ensure your reducers are pure functions and leverage React strict mode to catch potential problems early in development. By embracing functional programming principles, you can write cleaner, more reliable code that is easier to debug and maintain....

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As part of the Concurrent Mode, React introduced several lifecycle methods that allow you to prioritize the rendering of certain parts of your UI, such as: - useTransition hook that allows you to update the state without blocking the UI, - useDeferredValue hook that allows you to defer the rendering of certain parts of your UI, - startTransition API that, similarly to the useTransition hook lets you mark a state update as non-blocking. It lacks, however, an indication of whether it's still pending. Automatic Batching Introduced in React 18, Automatic Batching reduces the number of re-renders that happen on state changes even when they happen outside of event handlers, e.g. in a setTimeout or Promise callback. This feature comes out of the box and you don't have to do anything to enable it, and it makes a great argument for upgrading to React 18. Selective Hydration Selective Hydration allows you to take hydration off the main thread by wrapping your components in a Suspense boundary. 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To investigate your hook dependencies and re-renders, you can leverage React Developer Tools or use this handy helper hook I found on the internet to trace your re-renders: ` Conclusion Improving INP in React and Next.js is not easy and can require much investigation and fine-tuning. Still, it's worth doing to avoid being penalized by Google in its search results and provide a better experience for your users. Adopting React 18's new features, leveraging SSR and SSG in Next.js, utilizing Web Workers, and optimizing hooks and state management can significantly boost your INP score and deliver a faster application to your users. Remember, INP is just one among many performance metrics emphasizing the need for a comprehensive approach to performance optimization...

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Web PerformanceReactNextJSJavaScript

Introduction to Zod for Data Validation

As web developers, we're often working with data from external sources like APIs we don't control or user inputs submitted to our backends. We can't always rely on this data to take the form we expect, and we can encounter unexpected errors when it deviates from expectations. But with the Zod library, we can define what our data ought to look like and parse the incoming data against those defined schemas. This lets us work with that data confidently, or to quickly throw an error when it isn't correct. Why use Zod? TypeScript is great for letting us define the shape of our data in our code. It helps us write more correct code the first time around by warning us if we are doing something we shouldn't. But TypeScript can't do everything for us. For example, we can define a variable as a string or a number, but we can't say "a string that starts with user_id_ and is 20 characters long" or "an integer between 1 and 5". There are limits to how much TypeScript can narrow down our data for us. Also, TypeScript is a tool for us developers. When we compile our code, our types are not available to the vanilla JavaScript. JavaScript can't validate that the data we actually use in our code matches what we thought we'd get when we wrote our TypeScript types unless you're willing to manually write code to perform those checks. This is where we can reach for a tool like Zod. With Zod, we can write data schemas. These schemas, in the simplest scenarios, look very much like TypeScript types. But we can do more with Zod than we can with TypeScript alone. Zod schemas let us create additional rules for data parsing and validation. A 20-character string that starts with user_id_? It's z.string().startsWith('user_id_').length(20). An integer between 1 and 5 inclusive? It's z.number().int().gte(1).lte(5). Zod's primitives give us many extra functions to be more specific about *exactly* what data we expect. 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The main difference is that parse will throw an error if validation fails, while safeParse will return an object with a success property of either true or false, and either a data property with your parsed data or an error property with the details of a ZodError explaining why the parsing failed. In the case of our example data, userInput2 will parse just fine and return the data for you to use. But userInput1 will create a ZodError listing all of the ways it has failed validation. ` ` We can use these error messages to communicate to the user how they need to fix their form inputs if validation fails. Each error in the list describes the validation failure and gives us a human readable message to go with it. You'll notice that the validation errors for checking for a valid email and for checking password length have a lot of details, but we've got three items at the end of the error list that don't really tell us anything useful: just a custom error of Invalid input. The first is from our refine checking if the passwords match, and the second two are from our refine functions checking for password complexity (numbers and special characters). Let's modify our refine functions so that these errors are useful! We'll add our own error parameters to customize the message we get back and the path to the data that failed validation. ` Now, our error messages from failures in refine are informative! You can figure out which form fields aren't validating from the path, and then display the messages next to form fields to let the user know how to remedy the error. ` By giving our refine checks a custom path and message, we can make better use of the returned errors. In this case, we can highlight specific problem form fields for the user and give them the message about what is wrong. Integrating with TypeScript Integrating Zod with TypeScript is very easy. Using z.infer<typeof YourSchema> will allow you to avoid writing extra TypeScript types that merely reflect the intent of your Zod schemas. You can create a type from any Zod schema like so: ` Using a TypeScript type derived from a Zod schema does *not* give you any extra level of data validation at the type level beyond what TypeScript is capable of. If you create a type from z.string.min(3).max(20), the TypeScript type will still just be string. And when compiled to JavaScript, even that will be gone! That's why you still need to use parse/safeParse on incoming data to validate it before proceeding as if it really does match your requirements. A common pattern with inferring types from Zod schemas is to use the same name for both. Because the schema is a variable, there's no name conflict if the type uses the same name. However, I find that this can lead to confusing situations when trying to import one or the other—my personal preference is to name the Zod schema with Schema at the end to make it clear which is which. Conclusion Zod is an excellent tool for easily and confidently asserting that the data you're working with is exactly the sort of data you were expecting. It gives us the ability to assert at runtime that we've got what we wanted, and allows us to then craft strategies to handle what happens if that data is wrong. Combined with the ability to infer TypeScript types from Zod schemas, it lets us write and run more reliable code with greater confidence....

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Building a Stripe App: A Step-by-Step Guide to QR Code Generation Why Build a Stripe App? I recently participated in an audio space with the Stripe team, and something they said really stuck with me: the Stripe app store is a growing area that isn't overly saturated yet. There's a lot of potential for new apps, and companies can use this opportunity to grow. I work at a company called This Dot Labs, and we've created several Stripe apps and even own one. After looking at the data, I can confirm that the Stripe team was right! Creating a QR Code Generator App For this tutorial, we'll build a QR code app that can take a URL and generate a code for it. This is a good use case to help you understand the ins and outs of Stripe's developer tools. Why QR Codes? QR codes are useful tools that have become common in e-commerce, restaurants, and other industries. While Stripe already has a QR code tool, we'll make our own to familiarize ourselves with their syntax and problem-solving approaches. Project Structure Before we dive into the implementation, let's look at the structure of our Stripe App QR Code project: * .vscode: Contains settings for Visual Studio Code * source/views: Holds the main application views * .gitignore: Specifies files to ignore in version control * stripe-app.json: Defines the Stripe app configuration * ui-extensions.d.ts: TypeScript declaration file for UI extensions * .build: this is where the built Stripe app gets placed. Step-by-Step Implementation 1\. Install Stripe Locally First, you need to install Stripe on your local machine. The documentation provides great instructions for this: * For Mac users: Use Brew to install * For Windows users: Download the package and add it to your environment variables You can find the details here in the stripe docs to install the Stripe CLI https://docs.stripe.com/stripe-cli When using Windows, you must do stripe login from Powershell, NOT from Git bash or any other tool. After the server is up, then you can continue using git bash for everything else. After stripe login, you need to enter stripe apps start. Once you do that, the server is up and running and you can go back to using git bash or any other tool. 2\. Install Dependencies We'll be using an extra package for QR code generation. Install it using npm: ` 3\. Set Up the Main Component Let's look at the home.tsx file, where we'll use Stripe's UI components: ` These components are similar to other UI libraries like Bootstrap or Tailwind CSS. 4\. Create the UI Structure Our app will have: * An input field for the URL * Validation using a regex pattern * Error handling for invalid URLs * QR code generation and display Here is the Home.tsx file that is located in the src/views folder ` * ContextView` is at the top level of the app where we see the Title and the link to the Stripe Docs that we placed in our Context View. * Box is how you use Divs. * Banners can be used to show notification errors or any other item you wish to display. * Textfields are input fields. * Everything else is pretty self-explanatory. 5\. Handle Content Security Policy One problem I personally ran into was when I tried to redirect users, the Stripe policies would block it since I did not express that I knew what it was doing. I had to go into the stripe-app.json file and mention the specific security policies. For this particular exercise, I kept these as null. This is my stripe-app.json file. ` 6\. Configure App Views As you can see here, the stripe-app.json file shows the views for each file I have. The Home.tsx file and the Invoice.tsx are also included This is our way of saying that for each view we have, show the app functionality on that page. Our stripe-app.json file will show it but also, the manifest.js file in our .build folder will also show the same. Any view that doesn't have a file will not show the application's functionality. So, if I were to go to transactions, the app would not show the same logic as the home or invoices page. By following these steps, you'll have a fully functional QR code generator app for Stripe. This is just a simple example, but the potential for Stripe apps is massive, especially for businesses serving e-commerce customers. If you need help or get stuck, don't hesitate to reach out, danny.thompson@thisdot.co. The Stripe team is also very active in answering questions, so leverage them as a resource. Happy coding!...

Dec 18, 2024

4 mins

stripe

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Transitioning from React class components to function components with hooks

Functional components

Component state

Class components

Functional components

Component initialization

Class components

Functional components

Component destruction

Class components

Functional component

An important remark

Changed properties

Class components

Functional components

Changed properties and memoization

References

Conclusion

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