useEffect hook

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Overview

React is a JavaScript library for building user interfaces. It is used for creating reusable UI components that can be rendered on the front end or server side. React allows developers to build complex applications with declarative, reusable components and enables efficient updates and rendering of those components. useEffect hook can be used to handle lifecycle events, such as componentDidMount, componentDidUpdate, and componentWillUnmount, in a functional component.

What is useEffect Hook in React?

Hooks is a brand-new, interesting feature that has been added to React since version 16.8. Numerous common built-in hooks are available in React to manage states, isolate side effects, create references, boost performance, etc.

The useEffect hook is a function in React that allows developers to perform side effects in a functional component. This can include things like data fetching, setting up subscriptions, responding to the component's lifecycle events, or updating the DOM in response to changes in state or props.

The useEffect react hook is called after every render and takes a callback function as an argument, which contains the code for the side effect. This allows for a cleaner and more declarative approach to managing side effects in functional components.

Why Is It Called useEffect?

Many developers were perplexed by the name of this hook, "useEffect," when the core React Hooks (useState, useEffect, and so forth) were added to the library in 2018.

This hook is called "useEffect" as it is used to perform side effects in a functional component. The term "effect" refers to any changes to the applicable state or behavior that happen as a result of the code in the hook. This can include updating the DOM, fetching data, or subscribing to a data source.

The useEffect react hook allows developers to declaratively specify the side effects that should happen in response to changes in state or props, which makes it easier to understand and manage the behavior of the application.

How do I Use useEffect in React?

The following is the proper technique to implement the side effect in our User component:

  • We import useEffect from “react.”
  • In our component, we call it above the returning JSX.
  • We supply an array and a function as its two arguments.

Here is the basic syntax:

Basic usage of useEffect

We are saving a message in the code sample above by utilizing React useState. Then we take our message state variable and print it to the screen.

useeffect in react

Now, though, we want to useEffect to change the message a split second after mounting the component.

useeffect in react

We have added our effect under the useState line after importing useEffect from the React framework.

The first parameter to useEffect react hook is a function. When this function handler is executed, it will take care of any unwanted side effects. The function is a callback function that is executed when a React component's lifecycle has been triggered.

What does React UseEffect Do?

useEffect react hook allows you to perform side effects in function components. This is similar to lifecycle methods in class-based components. useEffect is called after every render, including the first render. It is used for things like data fetching, setting up subscriptions, and manually changing the DOM in response to component state changes.

Here is an example of how to use useEffect to fetch data from an API and set it in the component state:

In this example, the useEffect react hook is called after every render, and it will fetch data from the specified API and update the component state with the new data.

It's important to note that you can specify a second argument to useEffect, which is an array of dependencies. If any of the dependencies in the array change, the useEffect hook will be called again. This is useful for optimizing performance by skipping effects when unnecessary.

Here is an example of using the dependencies array to only fetch data when a specific prop changes:

In this example, the useEffect hook will only be called when the id prop changes. This means that if the id prop stays the same between renders, the data will not be prefetched.

Side Effect Runs and Different Ways to Control it

A side effect run is when a side effect is executed or triggered as a result of some other action or event. In programming, a side effect can refer to any change in state that occurs as a result of a function or program. This can include modifying a global variable, modifying one of the function's arguments, modifying a data structure in memory, or even just printing to the screen.

When a function or program has side effects, it means that it is capable of causing state changes that may not be immediately apparent and that can have unintended consequences elsewhere in the program. This can make it difficult to understand and predict the behavior of the program and can also make it harder to maintain and debug.

To avoid these problems, it is often necessary to control side effects in programming, using techniques such as isolation, referential transparency, pure functions, immutability, and lazy evaluation. These techniques can help to make programs more predictable and easier to understand and can also make it easier to write and maintain complex programs.

The second parameter, which takes an array as input, must always be used. Dependencies can be included in this array and optionally passed to useEffect.

No Dependency Passed

In the useEffect hook in React, the dependency array is an optional argument that you can pass to the hook. This array should contain a list of values that the hook depends on, and the hook will only be executed if one of these values changes.

If you do not pass a dependency array to the useEffect react hook, then the hook will be executed every time the component renders. This can lead to unwanted side effects, such as making unnecessary network requests or causing unnecessary re-renders of the component.

To avoid these problems, it is recommended to always provide a dependency array to the useEffect hook, so that it only runs when necessary.

For example:

In the example above, the useEffect hook will only be executed if the values of value1, value2, etc. change. This can help to avoid unnecessary side effects and improve the performance of your React application.

An Empty Array

In the useEffect hook in React, an empty array can be passed as the second argument to indicate that the hook does not have any dependencies. This means that the hook will only be executed once when the component is initially rendered.

For example:

In the example above, the useEffect react hook will be executed only when the component is first rendered and not on subsequent renders. This can be useful in cases where the hook only needs to be executed once, such as for setting up event listeners or performing one-time initialization tasks.

It is important to note that passing an empty array to the useEffect hook is not the same as not passing a dependency array at all. If you do not pass a dependency array, then the hook will be executed on every render, which can lead to unwanted side effects and performance issues. Therefore, it is recommended to always provide a dependency array, even if it is empty, to avoid these problems.

State Values of Props

In the useEffect hook in React, the state values of a component can be accessed through the state property of the component, and the props values can be accessed through the props property. These properties can be used inside the useEffect react hook to perform tasks based on the current state and props of the component.

For example:

In the example above, the useEffect react hook will be executed whenever the value1 state value or the value2 prop value changes. Inside the hook, the current values of value1 and value2 can be accessed and used as needed.

It is important to note that the state and props properties are only available inside the useEffect to react hook, and cannot be accessed directly from other parts of the component. To use the values of state and props in other parts of the component, you can either pass them as arguments to other functions or store them in the local state using the useState hook.

What is Cleanup?

In the useEffect hook in React, the cleanup function is a function that is executed before the hook is executed again or the component is unmounted. This function can be used to perform tasks such as canceling network requests, removing event listeners, or cleaning up any other resources that were created by the hook.

To define a cleanup function for the useEffect react hook, you can return the function from the hook itself.

For example:

In the example above, the cleanup function is defined by returning a function from the useEffect react hook. This function will be executed before the hook is executed again or the component is unmounted. Inside the cleanup function, you can perform any necessary cleanup tasks, such as canceling network requests or removing event listeners.

It is important to note that the cleanup function will only be executed if the hook has dependencies and one of those dependencies changes. If the hook does not have any dependencies or if an empty array is passed as the dependencies, then the cleanup function will not be executed.

How to Replicate the ComponentDidMount Lifecycle?

To replicate the behavior of the componentDidMount lifecycle method in a functional component, you can use the useEffect hook in React. The useEffect hook allows you to perform tasks such as making network requests or setting up event listeners, and it is executed after the component is initially rendered.

To use the useEffect react hook to replicate the behavior of componentDidMount, you can pass an empty array as the second argument to the hook. This tells React that the hook does not have any dependencies, so it will only be executed once when the component is initially rendered.

For example:

In the example above, the useEffect react hook will be executed only when the component is first rendered, which is the same behavior as componentDidMount. Inside the hook, you can perform any tasks that you would normally do in componentDidMount, such as making network requests or setting up event listeners.

It is important to note that the useEffect hook will not be executed on subsequent renders of the component, unlike componentDidMount, which is executed every time the component is rendered. If you want the hook to be executed on every render, then you can omit the second argument or pass an array with one or more dependencies.

How to Replicate the componentWillUnmount Lifecycle?

To use the useEffect react hook with a cleanup function to replicate the behavior of componentWillUnmount, you can define the cleanup function by returning a function from the hook itself. This function will be executed before the hook is executed again or the component is unmounted, and it can be used to perform any necessary cleanup tasks, such as canceling network requests or removing event listeners.

For example:

In this example, the code inside the useEffect callback will be run after the component is mounted. This is equivalent to the behavior of the componentDidMount lifecycle method.

The useEffect react hooks also allow you to return a cleanup function from the callback. This function will be run before the component is unmounted or before the effect is re-run. This is where you can perform any necessary cleanup, such as removing event listeners or canceling network requests.

This behavior is similar to the componentWillUnmount lifecycle method, which is called just before a component is unmounted and destroyed.

How to Create Self-sufficient Code for componentDidUpdate Lifecycle?

To create self-sufficient code for the componentDidUpdate lifecycle method in a functional component, you can use the useEffect hook in React. The useEffect hook allows you to perform tasks such as making network requests or updating the state of the component, and it is executed after the component is rendered.

To use the useEffect react hook to replicate the behavior of componentDidUpdate, you can pass an array of dependencies as the second argument to the hook. This tells React which values the hook depends on, and the hook will be executed whenever one of these values changes.

In this example, the code inside the useEffect callback will be run after the component is updated. This is equivalent to the behavior of the componentDidUpdate lifecycle method.

Some Tips for Using UseEffects in React

Using the useEffect in React is a powerful way to add side effects to your components. It allows you to perform operations in response to changes in your component's state or props without having to write complex lifecycle methods.

Here are some tips for effectively using useEffect in React components:

  • Make sure to include a dependency array as the second argument to useEffect. This tells React which values to watch for changes and will prevent the effect from running on every render.
  • Use useEffect for things that don't require a lot of computation, such as setting up event listeners or making network requests. For complex calculations, use useMemo instead.
  • To clean up an effect, return a function from the useEffect callback. This function will be called when the component is unmounted, or the effect is re-run.
  • Use the useDebugValue hook to display a custom label for your effect in the React DevTools. This can make it easier to debug your application.
  • If you need to perform an effect only once (on mount), pass an empty array ([]) as the second argument to useEffect. This tells React that the effect does not depend on any values, so it will only be run once.

Note: useEffect is a useful tool for adding side effects to your React components. By following these tips, you can use them effectively to improve the behavior of your application.

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Conclusion

  • You can perform side effects in your components with the useEffect Hook.

  • useEffect is a straightforward hook that is activated by any one of three React component lifecycles:

    1. componentDidMount
    2. componentDidUpdate
    3. componentWillUnmount
  • After initial mounting and on subsequent renderings, useEffect(callback, dependencies) calls the callback if any value in the dependencies has changed.

  • Callback argument is a function to put the side-effect logic.

  • Dependencies is a list of your side-effects dependencies, whether they are state values or props.

  • The useEffect react hook will not be executed on subsequent renders of the component, unlike componentDidMount, which is executed every time the component is rendered.

  • If you began using React before the beginning of 2019, you would need to unlearn your natural tendency to think in terms of lifecycle methods and adopt an effects-focused mindset.