React Mount Vs Render: Understanding Component Life Cycle

In React development, two fundamental processes that every software engineer encounters are mounting and rendering. These processes are the backbone of how React applications dynamically display content on the web. While they may seem similar at first glance, understanding the nuances between mounting and rendering is pivotal for crafting efficient and scalable React applications.

This blog will demystify mount vs render, guiding you through their differences, how they interact with the React lifecycle, and their impact on your application’s performance. Additionally, it is essential to import React for defining component behavior and lifecycle methods.

What is Mounting and Rendering?

Mounting and rendering are two essential concepts in React that determine how components are displayed and updated in the Document Object Model (DOM). Mounting refers to the process of rendering a component to the DOM for the first time, marking the beginning of the component’s lifecycle. This is when React creates a new instance of the component, initializes its state and props, and integrates it into the DOM tree.

On the other hand, rendering is the process of updating a component’s UI to reflect changes in its state or props. This can happen multiple times throughout the component’s lifecycle, ensuring that the UI stays in sync with the underlying data.

What is Mounting in React?

Mounting in React is akin to the first chapter of a book or the opening scene of a movie. It’s the process where a React component makes its debut in the Document Object Model (DOM), marking the beginning of the component’s life. This phase is crucial as React creates a new instance of the component, initializing its state and props , and integrating it into the DOM tree. It’s a one-time event per component instance, setting the stage for everything that follows.

Copy
1class Welcome extends React.Component {
2  constructor(props) {
3    super(props);
4    this.state = { hello: "world" };
5  }
6
7  render() {
8    return <h1>Hello, {this.state.hello}!</h1>;
9  }
10}

In the above example, when the Welcome component is mounted, React calls the constructor, initializing the state before the component is added to the DOM. An already mounted component maintains its state unless it is mounted again, distinguishing between mounting and re-rendering during the React lifecycle.

The Mounting Process

The mounting process involves several key steps that prepare a React component for its debut in the DOM:

1. Constructor Method: The constructor method is the first to be called when a component is initiated. It sets up the initial state and any initial values needed for the component.
2. Render Method: Next, the render method is called to generate the component’s UI. This method returns a React element that represents the component’s structure and appearance.
3. React Creates DOM: During the mounting phase, React creates the corresponding DOM nodes for the component for the first time, integrating it into the DOM tree.
4. Local State: The local state of the component is set to its default values during mounting, ensuring that the component starts with a clean slate.

These steps are crucial for setting up the component correctly, allowing it to function as intended within the application.

The Render Method

At the heart of every React component is the render method. It’s the artist of the component, painting the UI onto the web page. Whether you’re working with a function component or a class component, the render method is called both during the mounting phase and whenever the component needs to update its appearance due to changes in state or props. It’s a pure function, meaning it should not produce any side effects but simply return the React elements that describe the component’s UI.

Copy
1function Welcome(props) {
2  return <h1>Hello, {props.name}</h1>;
3}

This function component’s render logic is straightforward, showcasing how React elements are created and returned to depict the UI. Having the same components in different tabs can lead to unexpected re-rendering behaviors due to the alignment of components within the Virtual DOM hierarchy during tab transitions.

Key Differences Between Mounting and Rendering

While mounting and rendering are both integral to the way React displays components, they serve distinct purposes. Mounting is the process of introducing a new component to the DOM, a one-time setup phase for any given component. On the other hand, rendering is an ongoing process that can happen multiple times throughout a component's lifecycle, triggered by state or props changes. The main difference lies in their frequency and purpose: mounting occurs once to set up the initial state, while rendering happens repeatedly to reflect state or props updates in the UI.

Re-Mounting vs. Re-Rendering

Understanding the distinction between re-mounting and re-rendering is crucial for managing component states effectively. Re-mounting implies that a component is removed from the DOM and added back, causing it to lose its state and start afresh. Conversely, re-rendering updates the component's UI in response to state or props changes without resetting its state. This distinction highlights the importance of managing state wisely to avoid unnecessary re-mounts that could lead to performance issues.

React State Updates

React state updates are a fundamental part of how React components stay responsive to user interactions and data changes. When a component’s state or props change, it triggers the updating phase, which involves several lifecycle methods:

1. getDerivedStateFromProps : This method is called when the component’s props change, allowing the state to be updated based on the new props.
2. shouldComponentUpdate : Before the component is re-rendered, this method determines whether the update is necessary, helping to optimize performance.
3. Render: The render method is called again to update the component’s UI based on the new state or props.
4. getSnapshotBeforeUpdate : This method captures some information from the DOM before it is updated, which can be used in the next phase.
5. componentDidUpdate: After the component’s state is updated, this method is called to perform any post-update operations.

These methods work together to ensure that the component’s UI is always up-to-date, reflecting the latest state and props.

Unmounting

Unmounting is the process of removing a component from the DOM, typically when it is no longer needed or when the user navigates away from the page. The componentWillUnmount method is called just before the component is unmounted, providing an opportunity to perform any necessary cleanup, such as cancelling network requests, clearing timers, or removing event listeners. This ensures that the application remains efficient and free of memory leaks, even as components come and go.

By understanding and managing the mounting, updating, and unmounting phases, developers can create React applications that are both robust and performant.

Best Practices for Child Component Rendering

When dealing with child components, it's essential to follow best practices to ensure efficient rendering. React provides utility methods like React.createElement, React.cloneElement, and React.Children to work with child components effectively. By understanding and utilizing these methods, developers can avoid common pitfalls such as unnecessary renders, ensuring that child components update only when needed.

Optimizing Rendering for Performance

To keep your React application running smoothly, minimizing unnecessary renders is key. Techniques such as implementing shouldComponentUpdate, extending React.PureComponent, or using the React.memo function for function components can significantly reduce the number of renders. These optimizations help prevent performance bottlenecks, especially in large applications with complex component trees.

Common Pitfalls to Avoid

A common mistake in React development is over-rendering components, which can lead to sluggish performance. Avoid using methods that cause unnecessary renders, such as rendering components in methods that get called frequently. Additionally, be cautious with utility methods; while powerful, they should be used judiciously to prevent performance issues.

Final Thoughts!

Mounting and rendering are cornerstone concepts in React that, when understood and applied correctly, can greatly enhance the performance and scalability of your applications. By distinguishing between mount vs render, optimizing component renders, and adhering to best practices, you can build React applications that are both powerful and efficient. As you continue to develop your skills as a React developer, keep these distinctions and strategies in mind to elevate your projects to the next level.