React Hooks which were first introduced in React version 16.8 has brought a significant change to the way developers write code in React. They offer a neat and effective approach to reusing stateful behavior across components.
One of the most widely used Hooks in React is the useEffect Hook, which enables developers to manage side effects in functional components, simulating the functionality of the lifecycle methods available in class components.
The React Component Lifecycle defines the different stages that a component goes through, from the time it is mounted on the screen to the time it is removed. Understanding the React Component Lifecycle is critical to writing high-quality React applications.
In this comprehensive blog, we will try to dive deep into the concept of using the useEffect Hook to apply React's Lifecycle Methods in functional components.
We will explore the importance of using useEffect Hook for Lifecycle Methods and how it can make a developer's life easier while using it with functional components. We will learn how to implement it with various examples of Lifecycle methods, including mounting, updating, and unmounting.
So without waiting further let’s dive in…
While the introduction of hooks has made React more approachable and reduced the need for lifecycle methods in many cases, there are still situations where they are useful, such as when working with class components or integrating with external libraries that rely on them.
Therefore, It’s important to understand how React works and the Lifecycle methods, which made it easy to control the different phases of a component's life, such as when it is generated, modified, or destroyed.
By leveraging these methods, developers can create more efficient and robust applications, manage state and props effectively, and optimize their components for performance.
As displayed in the image, React Component Lifecycle refers to the various stages that a component experiences from the point at which it is created until it is removed from the DOM. The component lifetime comprises three main stages: Mounting, Updating, and Unmounting.
Let’s explore each of these stages and their methods with some examples.
The lifecycle of a React component starts with the mounting phase. It occurs when the component is first added to the DOM. The component is initialized, and its state and props are set, during this stage.
The purpose of the mounting phase is to set up the component's initialization and any required infrastructure, such as event listeners or API queries. By the end of the mounting phase, the component is fully set up and ready to be interacted with.
During the mounting phase, the component's state and props are set and the initial DOM structure is generated. Now that we understand the mounting, let's explore the componentDidMount() lifecycle method that is used during the mounting phase in greater detail.
The componentDidMount() is the lifecycle method that is invoked immediately following the mounting of a React component in the DOM, during the mounting phase of the component's lifecycle.
The purpose of this method is to give the component the freedom to perform any setup that may be required, such as setting up event listeners, obtaining data from an API, or communicating with other third-party libraries. Let’s try to take a glimpse of the use case with greater detail.
Fetching Data from an API:
Getting information from an API is one of the most frequent uses of componentDidMount(). A component might need to fetch data to display on the page when it is first mounted. This method can be used to make the API call and set the component's state with the data that is returned.
In this code example we are using the componentDidMount() to fetch data from an API and update the component's state.
The componentDidMount() makes a GET request to an API endpoint using the fetch() method. If the response is successful, the data is parsed as JSON and set as the component's state. If there is an error, the error message is set in the component's state.
There might be some cases where it might be a problem with debugging errors, especially when fetching data from such APIs. To overcome this you can use Zipy which is an intelligent debugging and error-monitoring tool designed for ReactJS applications.
With Zipy you can easily monitor your component's state and quickly identify and resolve errors in your code. It provides detailed stack traces that help you pinpoint errors and resolve them quickly.
The Updating phase is the second phase in the React component lifecycle. This phase begins when the component's state or props change and the component need to be re-rendered.
During this updating phase, React updates the DOM to reflect the changes in the component's state and/or props. The purpose of the Updating phase is to ensure that the component's presentation reflects its current state and props.
By updating the DOM as necessary, React ensures that the component's UI remains responsive and up-to-date. It also allows components to optimize performance by preventing unnecessary re-renders, which can improve the overall performance of the application.
Let’s explore the lifecycle methods that are used during the updating phase in greater detail.
We have two specific methods that can be used with this specific lifecycle of the component, such as the shouldComponentUpdate() and componentDidUpdate().
The shouldComponentUpdate() is a lifecycle method in React that allows a component to decide whether or not it should update based on the changes in props or state.
By default, React re-renders the component every time props or state change. However, if the updated props or state have no effect on the component's UI or output, re-rendering can be prevented by returning false in shouldComponentUpdate().
In the shouldComponentUpdate() method, we're checking if the updated state.count is different from the previous this.state.count. If it is, we're returning true, which allows the component to re-render. If it's not, we're returning false, which prevents the component from re-rendering.
As a result, when the user clicks the button, only the <h1> element displaying the count value will update, rather than the entire component. This optimization can lead to improved performance and a better user experience.
Another lifecycle method, which can be used during the updating phase is the componentDidUpdate() method which is called after a component updates and re-renders in the DOM. It is useful for performing additional actions after the component has been updated, such as updating a third-party library or making additional API calls.
The method receives two arguments: prevProps and prevState, which represent the component's previous props and state, respectively. These can be compared to the current props and state to determine what changed and how to update the component accordingly. Let’s understand its use with an example.
In this example, the ZipyComponent component initializes a chart library when it mounts using componentDidMount(). When the component updates, the componentDidUpdate() method is called, which then checks if the component's data prop has changed since the previous render.
If it has, the chart's data is updated and re-rendered using the update() method provided by the library. The main purpose of using this componentDidUpdate() method is to allow components to perform additional actions after they have been updated and re-rendered.
By providing this lifecycle method, React allows components to have fine-grained control over their behavior and can lead to more efficient and performant applications. Now that we know about the mounting and unmounting phases, let’s now finally move on to the final phase of the component lifecycle which is the Unmounting phase.
The unmounting phase in React occurs when a component is removed from the DOM. This can happen when the component is no longer needed, when the user navigates to a different page, or when the entire application is closed.
The purpose of the unmounting phase is to free up resources that are no longer needed, such as event listeners or timers. If these resources are not properly cleaned up, they can lead to memory leaks and degrade the performance of the application.
During this phase, React provides a method called componentWillUnmount() which allows developers to clean up any resources used by the component before it is removed from the DOM. Let’s dive deep into its details and understand how componentWillUnmount() is used in action.
The componentWillUnmount() method is a lifecycle method in React that is called just before a component is removed from the DOM.
This method allows developers to perform any necessary cleanup tasks before the component is unmounted. The purpose of this method is to release any resources that were allocated during the mounting or updating phase of the component's lifecycle.
This could include removing event listeners, canceling timers or network requests, or releasing any other resources that the component may have created. Let’s understand this with a simple example.
In this example, the componentDidMount() method sets up an interval to update the component's state every second. In the componentWillUnmount() method, the interval is cleared using the clearInterval() method to prevent any memory leaks or unnecessary resource usage after the component is removed from the DOM.
Overall, the componentWillUnmount() method is useful for cleaning up any resources that were created during the component's lifecycle. This ensures that the application remains performant and avoids any memory leaks or unnecessary resource usage.
Now that we know how each of these methods works, you can create a small version and deploy your React app by applying these methods to fully understand their implementation.
Over time, React has deprecated several lifecycle methods, meaning that they are no longer supported in the latest version of React or may be removed in future versions. Let’s take a brief understanding of each of the deprecated lifecycle methods:
componentWillMount(): This method was used to perform setup tasks before a component was mounted in the DOM. However, it was prone to causing performance issues because it was called every time the component was updated, not just when it was first mounted. In React 16.3, this method was replaced with getDerivedStateFromProps() and componentDidMount().
componentWillReceiveProps(): This method was used to update the state of a component based on changes to its props. However, it was often misused and caused unnecessary re-rendering of components. In React 16.3, this method was replaced with getDerivedStateFromProps() and componentDidUpdate().
componentWillUpdate(): This method was called before a component was updated with new props or state. However, it was prone to causing performance issues because it was called every time the component was updated, not just when it needed to be updated. In React 16.3, this method was replaced with getSnapshotBeforeUpdate() and componentDidUpdate().
getSnapshotBeforeUpdate(): This method was introduced in React 16.3 to replace componentWillUpdate(). It allows a component to capture some information from the DOM before it is potentially changed by an update.
It's important to note that these methods are now considered legacy and should not be used in new code. Developers should use the replacement methods that were introduced in React 16.3 and later versions to ensure that their code is compatible with future versions of React.
So far we’ve understood all about React Component Lifecycle and its various methods that are mainly used in each of its phases. Let’s now move on to explore our next topic which is the modern useEffect hook for handling side effects.
The React useEffect Hook is a way to add lifecycle methods to a functional component. It allows us to run side effects.
Side effects are anything that affects the state of the application outside of the component, (e.g. fetching data from an API, setting up event listeners, updating the DOM) in a declarative way, without needing to use class components and the traditional lifecycle methods.
The purpose of the useEffect hook is to allow functional components to have access to these side effects in a way that is similar to lifecycle methods in class components. The basic syntax of the useEffect hook:
The first argument to useEffect is a function that contains the side effect code we want to run. The second argument is an optional array of dependencies, which tell React when the effect should run. If any of the dependencies change, the effect will run again.
Let’s try to understand the usage of the useEffect hook with some quick examples:
Fetching data from an API Example:
In this example, we’re using the useEffect hook to fetch data from an API and update the state of a component in a functional component. The useState hook is also used to initialize the data state to null.
The useEffect hook is called with a function that fetches the data using async/await and updates the state with setData. The dependency array  ensures that the effect only runs once when the component mounts.
If the data state is null, the component will render a "Loading..." message. Once the data is fetched, the component will render a list of items with the map function. The key attribute is used to identify each item in the list. This example demonstrates how the useEffect hook can be used to fetch and update data in a functional component.
Setting up & cleaning up event listeners:
We’re using useEffect hook here to add and remove an event listener. In this example, the event listener listens for the keydown event and increments the count state when the up arrow is pressed.
The useEffect hook is used with an empty dependency array so that the effect is only run once when the component mounts. The hook also returns a cleanup function that removes the event listener when the component unmounts. This ensures that the event listener is only active when the component is mounted and avoids memory leaks.
Updating the DOM:
In this example, we're using useState to store the count state, and useEffect to update the document title with the current count whenever it changes. We pass [count] as the second argument to useEffect to ensure that the effect runs whenever the count state changes.
When the user clicks the "Increment" button, the count state is updated, causing the useEffect hook to run again. The hook updates the document title with the new count, which is reflected in the browser tab. This demonstrates how useEffect can be used to update the DOM in response to changes in state.
As we saw useEffect makes it really easier to handle side effects in functional components. In traditional class components, side effects were handled using lifecycle methods, which could make code harder to understand and maintain.
It provides a more declarative and intuitive way of handling side effects, making it easier to reason about the behavior of a component. Overall, it simplifies the process of handling side effects in React, making it more accessible for developers to write and understand complex applications.
As we learned before, in React's useEffect Hook, dependencies are used to determine when the effect should be executed. If any of the dependencies have changed since the last render, the effect will be executed again.
The dependencies are passed as an array to the useEffect Hook and can include state variables, props, and functions. Let’s see this in action:
In this example, we are using the count state variable as a dependency for the useEffect Hook. Whenever the count variable changes, the effect will be executed, and the message "Count changed:" along with the new count value will be logged to the console.
To handle dependencies in useEffect, there are a few things to keep in mind:
In functional components, the useEffect Hook can be used to replicate the functionality of the traditional lifecycle methods. The useEffect Hook is called after every render of the component and can perform any side effect, such as modifying the DOM or fetching data from an API.
By using the second argument of the useEffect Hook, we can control when it should be called based on specific dependencies. Let’s understand this in greater detail with some examples:
In class components, the componentDidMount() method is called after the component has been mounted in the DOM.
To replicate this behavior in functional components, we can use the useEffect Hook with an empty array as the second argument. This will ensure that the effect is only run once after the component has been mounted.
In this example, useEffect will only run once when the component mounts because the dependency array is empty. fetchData is an async function that fetches data from an API and updates the state using setData.
In class components, the componentDidUpdate method is called after the component has been updated with new props or states.
To replicate this behavior in functional components, we can use the useEffect Hook with the appropriate dependencies listed as the second argument. This will ensure that the effect is only run when the listed dependencies have changed.
In this example, useEffect will run every time count changes because the count is listed as a dependency in the array. The effect updates the document title to include the current count.
In class components, the componentWillUnmount method is called before the component is unmounted from the DOM.
To replicate this behavior in functional components, we can return a cleanup function from the useEffect Hook. This function will be called before the component is unmounted.
In this example, useEffect sets up a mousemove event listener when the component mounts. The return function removes the event listener when the component unmounts. This ensures that the event listener is only active when the component is mounted and avoids potential memory leaks.
Overall, using the useEffect Hook with lifecycle methods allows us to manage state and side effects in functional components in a similar way to class components, without having to write boilerplate code. It can help to make the code more concise and easier to understand.
React's useEffect hook is a powerful and flexible way to add side effects to the functional components. It allows us to manage state and perform side effects in a more concise and declarative way, without the need for class components and traditional lifecycle methods.
With useEffect, we have fine-grained control over when side effects are executed and when they are cleaned up. But with this control comes the problem with debugging errors, especially when fetching data from APIs. To overcome this you can use Zipy which is an intelligent debugging and error-monitoring tool designed for ReactJS applications.
With Zipy, you can easily monitor your component's state and quickly identify and resolve errors in your code. It provides detailed stack traces that help you pinpoint errors and resolve them quickly.
Overall, useEffect provides a simple and elegant solution to the problem of managing side effects in React. By understanding how to use lifecycle methods in functional components, we can write more maintainable, readable, and scalable code for React applications.