Life Cycle Methods | Mastering React: An In-Depth Zero to Hero Video Series

Summary

In this React JS tutorial, the instructor dives into component lifecycle methods, essential for beginners learning React from scratch. The video explains the three phases of a component's lifecycle: mounting, updating, and unmounting. Key methods like constructor, render, componentDidMount, componentDidUpdate, and componentWillUnmount are covered with examples to illustrate their use. The instructor also touches on best practices, such as avoiding setState in componentWillUnmount and using shouldComponentUpdate to control rendering. The session sets the stage for understanding the useEffect hook in future lessons.

Takeaways

• 📚 This video is part of a React JS tutorial series aimed at beginners learning React from scratch.
• 🔄 The focus of the video is on component lifecycle methods, which are crucial for understanding component behavior in React.
• 🎯 Lifecycle methods are categorized into mounting, updating, and unmounting, corresponding to different stages of a component's existence.
• 🌱 Mounting methods are called when a component is first rendered, in the order of constructor, getDerivedStateFromProps, render, and componentDidMount.
• 🔄 The render method is central to a component's lifecycle, being called on the initial render and subsequent updates.
• 🔁 The updating methods, including getDerivedStateFromProps, shouldComponentUpdate, render, getSnapshotBeforeUpdate, and componentDidUpdate, are involved in subsequent renders triggered by state or props changes.
• 🛑 The componentWillUnmount method is invoked right before a component is removed from the DOM, making it suitable for cleanup activities.
• 🚫 It's important not to call setState inside componentWillUnmount as it won't trigger a re-render.
• 🔍 The video uses a class component example to illustrate the lifecycle methods, emphasizing the render method's purity and the constructor's role in state initialization and method binding.
• 🔄 The video also covers the use of componentDidUpdate for handling updates and the potential pitfalls of causing infinite rerenders if not used carefully.
• 🔄 The shouldComponentUpdate method is highlighted as a way to control whether a component should re-render, with an example showing its interaction with the render and componentDidUpdate methods.

Q & A

What are the three main categories of lifecycle methods in React?

The three main categories of lifecycle methods in React are mounting, updating, and unmounting.

What happens during the mounting phase of a React component?

During the mounting phase, a component renders for the first time, and the following methods are called in this order: constructor, getDerivedStateFromProps, render, and componentDidMount.

Why is the constructor method important in a React class component?

The constructor method is important for initializing local state and binding event handlers. It is also the only place where you should assign this.state directly.

What is the purpose of the componentDidMount lifecycle method?

componentDidMount is invoked immediately after a component is mounted, making it a good place to instantiate network requests or perform any setup that requires DOM nodes.

Why should you avoid calling setState inside the constructor?

You should avoid calling setState inside the constructor because it can lead to an infinite loop of renders and updates. Instead, you should assign state directly to this.state in the constructor.

What is the role of the componentDidUpdate lifecycle method?

componentDidUpdate is called after a component has re-rendered with new props or state. It is used for operations that need to be performed after updates, such as network requests or DOM manipulations, but care should be taken to avoid causing infinite re-renders.

Why is shouldComponentUpdate used and how does it affect componentDidUpdate?

shouldComponentUpdate is used to determine whether a component should re-render when receiving new props or state. If it returns false, the componentDidUpdate method will not be called for that update cycle.

What is the significance of componentWillUnmount in the lifecycle of a React component?

componentWillUnmount is called just before a component is unmounted from the DOM. It is used for cleanup activities like clearing timeouts, canceling network requests, or unsubscribing from subscriptions.

Why should you not call setState in componentWillUnmount?

You should not call setState in componentWillUnmount because the component will never be re-rendered after it is unmounted, and calling setState would be a no-op.

How does the order of lifecycle methods differ between the initial render and subsequent renders?

During the initial render, the methods are called in the order: constructor, getDerivedStateFromProps, render, and componentDidMount. For subsequent renders, the order is: getDerivedStateFromProps, shouldComponentUpdate, render, getSnapshotBeforeUpdate, componentDidUpdate.

Outlines

00:00

🛠️ Understanding React Component Lifecycle Methods

This segment of the React JS tutorial focuses on the lifecycle methods of React components, which are crucial for managing the lifecycle of components from creation to destruction. The instructor begins by explaining the concept of lifecycle methods and categorizes them into three phases: mounting, updating, and unmounting. Mounting occurs when a component is rendered for the first time, with methods like 'constructor', 'getDerivedStateFromProps', 'render', and 'componentDidMount' being called in a specific order. The 'render' method is highlighted as the only required method in a class component and should remain pure, not modifying the component state. The instructor also discusses the 'constructor' method, emphasizing the importance of calling 'super' and the appropriate use of 'this.state' versus 'this.setState'. 'componentDidMount' is detailed as the place to perform actions after the component is mounted, such as making network requests or setting state that triggers re-renders.

05:02

🔄 Exploring Update and Cleanup Lifecycle Methods in React

The second paragraph delves into the update phase of a React component's lifecycle, where 'componentDidUpdate' is introduced as a method called after every re-render except the initial one. The instructor cautions against setting state inside 'componentDidUpdate' to prevent infinite re-renders and suggests using it for network requests when props have changed. The 'shouldComponentUpdate' method is also discussed, explaining its role in determining whether a component should re-render based on prop or state changes. The cleanup phase is covered with 'componentWillUnmount', which is called right before a component is removed from the DOM, making it the ideal place for cleanup activities like clearing timeouts or canceling network requests. The instructor demonstrates these concepts with a practical example involving a parent and child component, showing how lifecycle methods are called during rendering, updating, and unmounting processes.

10:03

📢 Conclusion and Call to Action for React JS Series

In the final paragraph, the instructor wraps up the session on React component lifecycle methods and transitions to the next topic, the 'useEffect' hook, which is dependent on understanding these lifecycle methods. The instructor encourages viewers to like the video and subscribe to the channel for continued support and learning. The segment ends with a friendly sign-off, reinforcing the educational value of the series and the instructor's commitment to helping learners master React JS from scratch.

Keywords

💡React JS

React JS is a popular open-source JavaScript library for building user interfaces, particularly single-page applications. It is maintained by Facebook and a community of individual developers and companies. In the video, React JS serves as the central theme, with the series aimed at teaching beginners how to learn React JS from scratch, focusing on various aspects such as conditional rendering and component lifecycle methods.

💡Component Lifecycle Methods

Component lifecycle methods in React are a set of functions that are called at different stages of a component's life. These methods are categorized into mounting, updating, and unmounting. In the video, the instructor explains that understanding these lifecycle methods is crucial for grasping more advanced concepts like the `useEffect` hook, and they are used to manage the component's behavior at different stages of its existence.

💡Mounting

Mounting refers to the process of inserting a React component into the DOM for the first time. The script mentions that the mounting lifecycle methods, such as `constructor`, `getDerivedStateFromProps`, `render`, and `componentDidMount`, are called in a specific order when a component mounts. These methods are essential for setting up the initial state and performing any necessary side effects before the component is displayed to the user.

💡Updating

Updating in React occurs when a component re-renders due to changes in its props or state. The video script explains that the updating lifecycle methods, including `getDerivedStateFromProps`, `shouldComponentUpdate`, `render`, `getSnapshotBeforeUpdate`, and `componentDidUpdate`, are called in a specific sequence during this process. These methods allow developers to manage state changes and optimize performance by controlling when a component should update.

💡Unmounting

Unmounting is the process of a React component being removed from the DOM. The script highlights that the `componentWillUnmount` method is called during this phase, which is the right time to perform cleanup activities such as canceling network requests or clearing timeouts. This ensures that no memory leaks occur and resources are properly freed when the component is no longer needed.

💡Constructor

The constructor in a React class component is a special method that is called before the component mounts. As explained in the video, it is used for initializing local state and binding event handlers. The script emphasizes that the constructor should call `super` first, and it is the only place where `this.state` can be directly assigned, rather than using `this.setState`.

💡componentDidMount

The `componentDidMount` method is called immediately after a component is mounted, making it an ideal place to perform network requests or other side effects that require DOM access. The video script illustrates this by showing an example where `componentDidMount` is used to trigger an action that results in an additional rendering of the component.

💡componentDidUpdate

The `componentDidUpdate` method is invoked after a component updates its output. The video script clarifies that this method is not called after the initial render but after every subsequent update. It is used to perform actions after updates, such as network requests, but caution is advised to avoid causing infinite re-renders by improperly setting state within this method.

💡shouldComponentUpdate

The `shouldComponentUpdate` method is a lifecycle method that allows developers to control whether a component should re-render when its props or state change. The video script explains that if this method returns false, the component will not update, and therefore, the `componentDidUpdate` method will not be called. This can be used as an optimization technique to prevent unnecessary renders.

💡useEffect Hook

Although not explicitly detailed in the script, the `useEffect` hook is mentioned as a concept that requires understanding of component lifecycle methods. Introduced in React 16.8, the `useEffect` hook allows developers to perform side effects in function components, which were previously only possible in class components using lifecycle methods. The hook is used for data fetching, subscriptions, or manually manipulating the DOM, and it is explained as a key concept to understand after grasping lifecycle methods.

Highlights

Introduction to React JS lifecycle methods for beginners.

Explanation of the importance of lifecycle methods for understanding hooks.

Lifecycle methods categorized into mounting, updating, and unmounting.

Order of mounting lifecycle methods: constructor, getDerivedStateFromProps, render, and componentDidMount.

Render method is called twice initially due to strict mode, then once after adjustment.

The constructor's role in initializing state and binding methods.

Why setState should not be called inside the constructor.

componentDidMount is used for data fetching and calling setState.

componentDidUpdate is called after every re-render except the initial one.

Avoiding setState in componentDidUpdate to prevent infinite rerenders.

shouldComponentUpdate to control rendering based on prop or state changes.

componentWillUnmount for cleanup activities like clearing timeouts or canceling network requests.

Demonstration of componentWillUnmount using a child component.

Explanation of how componentDidUpdate and componentWillUnmount work together during component updates and unmounting.

The significance of lifecycle methods in understanding the useEffect hook.

Call to action for viewers to like, subscribe, and support the channel.

Transcripts

Hi Friends

Welcome back to React JS - Zero to Hero series.

This series is for beginners who wants to learn React JS from Scratch.

In the last video I have explained about conditional rendering in react.

In this video I am going to explain about component lifecycle methods.

Let's start.

Actually, I have already explained about useState hook.

And, I thought of explaining about the other hooks.

But to understand useEffect hook, we need to understand about component lifecycle methods.

And so, in this video, let's see about component lifecycle methods.

What are component lifecycle methods in react?

Lifecycle methods are series of events that happen throughout the birth, growth, and death

of a React component.

There are three main categories of lifecycle methods: mounting, updating, and unmounting.

A component "mounts" when it renders for the first time.

This is when mounting lifecycle methods get called.

And, these methods are called in the following order when an instance of a component is being

created and inserted into the DOM:

constructor, getDerivedStateFromProps, render and componentDidMount.

The first time that a component instance renders, it does not update.

Starting with the second render, a component updates every time that it renders.

An update can be caused by changes to props or state.

These methods are called in the following order when a component is being re-rendered:

getDerivedStateFromProps, shouldComponentUpdate, render, getSnapshotBeforeUpdate and componentDidUpdate.

A component's unmounting period occurs when the component is removed from the DOM.

And, this method is called when a component is being removed from the DOM:

componentWillUnmount.

Among these, all the highlighted methods are the most commonly used lifecycle methods and

so let me explain all these methods using an example.

I have created a new react application, in which I have created a class component.

In this component, I just have a render method which returns a JSX element.

The render method is the only required method in a class component.

Other methods are optional.

The render method should be pure always.

That means, render method should not modify component state.

Let me put a console log and so, if I refresh the page, we can see

the render method is called.

It is called twice because our app is in strict mode.

Let me change that, otherwise it will confuse us.

Now we can see it is called only once.

Next let's see about constructor.

If we don't have any state or we don't want to bind any methods, we don't need a constructor.

But even in that scenario, we can have a constructor.

And, if we have a constructor, first we need to call super.

Otherwise it will throw error.

And, if we need to have a state or we need to bind any methods, a constructor is needed.

Even in that case, first we need to call super.

If we are going to have any code before that, it won't work.

The constructor for a React component is called before it is mounted.

And so, if I have a console log, we can see it is called even before the render method.

Typically in React, constructors are only used for two purposes:

For initializing local state and for binding event handlers.

Let me create a state and let me bind a method.

We should not call setState inside a constructor.

Instead, we need to assign it directly to this dot state.

Constructor is the only place where we should assign, this dot state directly.

In all other methods, we need to use, this dot setState instead.

Ok, next let's see about componentDidMount.

componentDidMount is invoked immediately after a component is mounted.

Let me have a console log here.

And so, we can see first the constructor is called and then the render method is called

and finally the componentDidMount is called.

If we need to load data from a remote end point, this is a good place to instantiate

the network request.

We can call setState inside componentDidMount.

But this will trigger an extra rendering.

Ok, let's see about componentDidUpdate.

This method will be called for every rerender.

This will not be called for initial render.

And so, if I have a console log, we will not see it during the first render.

Let me have a button and on clicking this button let me call this function.

In this function, we are updating the state and so this will trigger render again.

Now, let's see.

If I refresh, we will not see componentDidUpdate.

But if I update, we can see the render is triggered again and componentDidUpdate is

called.

And, it will be called for every rerender even though the state is not changed.

So, we need to be little careful.

We should avoid setting state inside componentDidUpdate.

In case we are going to set that, we need to make sure we are setting only if the previous

state is different from the current state, otherwise it will lead to infinite rerender.

This is also a good place to do network requests as long as we compare the current props to

previous props because a network request may not be necessary if the props have not changed.

In case, if we are implementing shouldComponentUpdate also, then if it returns false, the componentDidUpdate

will not be called.

So, if I have a console log, we can see shouldComponentUpdate is getting called, but as we are returning

false componentDidUpdate is not called, in case if we are going to return true, we can

see shouldComponentUpdate is called first and then the render method is called and finally

componentDidUpdate is called.

So, we can use shouldComponentUpdate to decide whether to trigger the render or not.

And now, let's see about componentWillUnmount.

This method will be called just before the component is unmounted from the DOM.

So, this will be useful to do some clean up activities.

For example, if we need to clear the timeout, or to cancel any network requests or to unsubscribe

any subscriptions, this is the right place.

And, We should not call setState() in componentWillUnmount() because the component will never be re-rendered.

And to show you how componentWillUnmount works, let me create a child component, in which

let me implement both componentDidMount and componentWillUnmount.

In render method, let me return a simple JSX.

So, in this component, let me introduce a state, and let me create two functions, one

to set this state to true and another one to set this to false.

Let me bind these functions.

Please note that, if we are using normal function, we need to bind it and if we are using arrow

function, we don't want to bind it.

Ok, and then let me have two more buttons and let me call the respective methods.

Finally, let me conditionally render this component based on the state.

And so, if I refresh, we can see Parent constructor is called, parent render method is called

and then parent componentDidMount is called.

Let me clear this and let me click the show button.

Now we can see, should component update in parent is called and then parent render method

is called and then child render method is called and then child component is mounted

and finally parent component is updated.

Let me clear this and click hide button.

Now again, should component update in parent is called and then parent render method is

called.

After that, child component is unmounted and finally parent component is updated.

Hope you understand how these lifecycle methods are working.

This will help you to understand useEffect hook, which is coming next.

For this concept, we don't have any task.

That's all for today.

Please like this video.

Subscribe to my channel and support me.

Thank you.

Bye.