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ReactJS componentDidMount() Method

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5th Jun, 2023
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ReactJS componentDidMount() Method

What is the ‘componentDidMount()’ Method in ReactJS?

The componentDidMount() method is invoked after rendering all the page elements accurately, marking the culmination of the first render cycle. This step, commonly called “post mounting,” signifies the final stage of the mounting process. It entails calling the method once all children elements and components have been successfully mounted in the Document Object Model (DOM).

In the mounting phase of React’s lifecycle, one can establish network requests or subscriptions within the componentDidMount() method. However, to prevent any potential performance complications, it is crucial to unsubscribe from these requests in the componentWillUnmount() method.

In the React constituent lifecycle, the componentDidMount() method is a crucial parting of the mounting phase. It is a lifecycle method called automatically by React after the component has been rendered for the first clip and inserted into the DOM.

The componentDidMount() method’s main purpose is to perform any setup tasks or face effects required for the element to work decently. Since this method is called after the initial render, it is a suitable property to make AJAX requests, bring information from APIs, initialise third-party libraries, or set up event listeners. 

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Understanding the Lifecycle Methods in ReactJS

Developers using ReactJS can leverage a series of lifecycle methods to facilitate their ability to engage with various stages of a component’s lifecycle. These methods empower developers to execute specific tasks at distinct junctures within the component’s lifespan, encompassing initialisation, rendering, updating, and unmounting. 

The three phases in React component lifecycle:

Mounting:

  • Initialisation and background-up default props.
  • Rendering the component on the DOM.
  • Executing code after rendering componentDidMount().

Updating:

  • Receiving new props and performing necessary actions componentWillReceiveProps().
  • Deciding whether to re-render shouldComponentUpdate().
  • Executing code after re-rendering componentDidUpdate().

Unmounting:

  • Executing cleanup operations before removing the component from the DOM componentWillUnmount().

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What Happens When the ‘componentDidMount()’ Method is Called?

When the componentDidMount in functional component is called, it signifies that the component has been rendered and effectively mounted onto the Document Object Model (DOM). This moment presents an opportunity to execute tasks requiring access to the DOM, initialise timers or network requests, and engage with external APIs or libraries.

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How to Use the ‘componentDidMount()’ Method in ReactJS?

To use the componentDidMount in React, you must define it within your component class or functional component. In a class component, you write it as follows:

class MyComponent extends React.Component {
  'componentDidMount()' {
    // Perform tasks after the component is mounted
  }
  render() {
    // Component rendering logic
  }
}

In a functional component, you can use the ‘useEffect()’ hook to achieve the same behaviour:

import React, { useEffect } from 'react';
function MyComponent() {
  useEffect(() => {
    // Perform tasks after the component is mounted
  }, []);
  // Component rendering logic
}

In both cases, the code inside the componentDidMount() or useEffect() will be executed once the component has been mounted.

Examples of the ‘componentDidMount()’ Method in ReactJS

List of a few examples to demonstrate the usage of the React componentDidMount() method:

Example 1: Fetching data from an API

class UserList extends React.Component {
  state = {
    users: [],
  };
  'componentDidMount()' {
    fetch('https://api.example.com/users')
      .then(response => response.json())
      .then(data => {
        this.setState({ users: data });
      });
  }
  render() {
    // Render user list using this.state.users
  }
}

Example 2: Initialising a third-party library

class MyComponent extends React.Component {
  'componentDidMount()' {
    // Initialize a third-party library
    ThirdPartyLibrary.init();
  }
  render() {
    // Component rendering logic
  }
}

The Difference Between ‘componentDidMount()’ and ‘componentWillMount()’

The componentDidMount() and componentWillMount() methods have a key difference. In earlier versions of React, developers used componentWillMount() before mounting the component. However, React 16.3 deprecated this method and recommends using componentDidMount() for actions requiring DOM access or initialisation.

The main distinction lies in the invocation timing of componentDidMount() and the deprecated componentWillMount(). After rendering and mounting the component onto the DOM, componentDidMount() gets called. On the other hand, componentWillMount() is called before the component is mounted. Therefore, using componentDidMount() for actions that rely on the DOM or external data is generally safer.

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The Benefits of Using ‘componentDidMount()’ in ReactJS

Some advantages of using the componentDidMount() method in ReactJS:

  • DOM interaction: The componentDidMount() method is the preferred way to interact with the DOM because it is called once the component has finished rendering and has been successfully inserted into the DOM. This way, you can safely control DOM elements or access their properties, such as adding or removing classes, ever-changing styles, or attaching case listeners. 

For example, if you demand to initialise a third-party library that requires a consultation with a DOM element, you can do so in componentDidMount() and ensure that the DOM is ready before performing any actions.

  • Asynchronous tasks: Components often need to bring data from an API or perform other asynchronous operations. The componentDidMount() method provides a convenient location to savant these tasks. For instance, you can make AJAX requests using JavaScript’s bring or axios libraries to retrieve data from a server. 

Once the information is fetched, you can update the component’s tell or trigger a re-render to exhibit the retrieved data. Placing these asynchronous tasks in componentDidMount() ensures they are executed after the ingredient has been rendered and the DOM is ready.

  • Side effects: React encourages a declaratory programming style, where components deliver based on their props and state. However, you may need to perform side effects in some situations, such as scope-up subscriptions, timers, or other long-lived operations. 

The componentDidMount() method provides a suitable place to address such side effects. For instance, if your component needs to subscribe to real-time updates from a WebSocket or event emitter, you can establish the subscription in componentDidMount(). This way, the subscription is initialised only once, and you can start receiving updates and react accordingly. Remember to strip up these subscriptions or side effects in the componentWillUnmount method to avoid memory leaks or unnecessary operations when the constituent is unmounted.

The componentDidMount() method plays a critical role in the React element lifecycle by providing a designated location to perform tasks that require access to the DOM, induct asynchronous operations, or handle face effects. It helps ensure that your components are of decent setup and ready to interact with the user port, making it a profound method for edifice dynamic and interactive React applications.

Common Mistakes to Avoid

When using the componentDidMount() method, it’s essential to avoid the following common mistakes:

  • Making blocking calls: Avoid making synchronous or long-running operations inside componentDidMount(), as it could block the rendering and negatively impact the user experience. Prefer asynchronous operations instead.
  • Modifying the state directly: Do not modify the component’s state directly inside componentDidMount(). Instead, use this.setState() to update the state and trigger a re-render.

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Tips For Debugging Issues

When encountering issues related to the componentDidMount() method, consider these debugging tips:

  • Check for errors: Review the browser console for error messages that might provide insights into what went wrong.
  • Verify dependencies: Ensure that all the dependencies required for your code inside componentDidMount() are correctly installed and imported.
  • Review the code logic: Double-check your code inside componentDidMount() for any logical errors, missing dependencies, or incorrect assumptions.

Best practices

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Follow these best practices when using the componentDidMount() method in ReactJS:

  • Perform clean-up: If you initialise any resources or subscriptions inside the componentDidMount(), remember to clean them up in the componentWillUnmount() method to prevent memory leaks.
  • Limit network requests: Although componentDidMount() is a suitable place to make API calls, be mindful of excessive network requests. Consider caching data or using state management libraries to avoid unnecessary requests.
  • Keep it focused: Stick to performing only necessary operations inside componentDidMount(). If your component is too complex, consider breaking it into smaller, more manageable components.

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Conclusion

The componentDidMount() method in ReactJS plays a crucial role in the lifecycle of a component. It allows you to perform actions after successfully mounting the component onto the DOM. It offers flexibility for interacting with the DOM, making network requests, and initialising external resources. By understanding its usage and implementing best practices, you can enhance the functionality and performance of your ReactJS applications.

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Pavan Vadapalli

Blog Author
Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.