Performance Optimization and Wasted Renders | Lecture 239 | React.JS 🔥
Summary
TLDRThe video script focuses on optimizing React applications by addressing three key areas: preventing wasted renders, improving app speed and responsiveness, and reducing bundle size. To prevent wasted renders, React provides tools like `memo`, `useMemo`, and `useCallback`. These can also enhance perceived speed, along with the `useTransition` hook. Reducing the bundle size is achieved by minimizing third-party packages and implementing code splitting and lazy loading. The script emphasizes that these tools are not always necessary but should be part of a developer's toolkit for when needed. It also clarifies common misconceptions about props and rendering, explaining that changes in props do not directly cause a component to re-render, but rather it's the parent component's re-rendering that triggers it. The concept of a 'wasted render' is introduced, highlighting its inefficiency and the importance of avoiding frequent or slow re-renders to maintain a fluid and responsive user experience.
Takeaways
- 🚦 Optimizing React apps involves preventing wasted renders, improving app speed and responsiveness, and reducing bundle size.
- 🛠️ Use `React.memo`, `useMemo`, and `useCallback` hooks to prevent unnecessary component re-renders and memorize objects and functions.
- 🔁 Passing elements as children or props to other elements can help prevent re-renders, thus optimizing performance.
- 🎢 Utilize `useMemo`, `useCallback`, and the newer `useTransition` hook to enhance the perceived speed of the application.
- 📦 Minimize the use of third-party packages and implement code splitting and lazy loading to reduce the bundle size.
- 🔍 Understand when a component re-renders: state changes, context changes, or parent component re-rendering.
- 🔄 Props change only when the parent component re-renders, which is why child components receiving the props also re-render.
- 🖥️ Rendering a component doesn't necessarily mean the DOM updates; it involves creating a new virtual DOM and performing the reconciliation process.
- 🚫 A 'wasted render' occurs when a render doesn't produce any DOM changes, leading to unnecessary computational overhead.
- 📉 Frequent or slow re-renders can make an application feel sluggish and unresponsive, impacting user experience negatively.
- 🧰 This section aims to provide an arsenal of optimization tools, to be used judiciously based on specific situations and needs.
Q & A
What are the three main areas of optimization in React applications?
-The three main areas of optimization in React applications are: preventing wasted renders, improving overall app speed and responsiveness, and reducing the bundle size.
How can we prevent wasted renders in React components?
-Wasted renders can be prevented by using React's `memo` to memorize components, `useMemo` to memorize objects, and `useCallback` hooks to memorize functions. Additionally, passing elements as children or props to other elements can help prevent unnecessary re-renders.
What is the misconception about props and re-rendering in React?
-A common misconception is that updating props directly causes a component to re-render. However, props only change when the parent component re-renders, which then causes the child components to re-render as well.
What is the role of the `useTransition` hook in optimizing perceived app speed?
-The `useTransition` hook can be used to improve the perceived speed of an application by managing state transitions and reducing visual stutter, providing a smoother user experience.
Why is it important to reduce the bundle size in a React application?
-Reducing the bundle size is important because it can lead to faster load times and better performance, especially for users with slower internet connections or on mobile devices.
How can code splitting and lazy loading help with bundle size optimization?
-Code splitting and lazy loading allow the application to load only the necessary parts of the code at a given time, rather than loading the entire application upfront, thus reducing the initial bundle size.
What is a wasted render in the context of React components?
-A wasted render is a render that does not result in any change to the DOM. It is considered wasteful because all the calculations and operations associated with rendering are performed without leading to an actual update in the user interface.
When does a React component instance get re-rendered?
-A React component instance gets re-rendered when the component's state changes, when there's a change in a context that the component is subscribed to, or when a parent component re-renders.
Why is it important to understand when and why components re-render in React?
-Understanding when and why components re-render is important for optimizing performance. It helps developers identify unnecessary renders, which can lead to performance bottlenecks and a less responsive user interface.
What are some built-in React tools for optimizing performance?
-Built-in React tools for optimizing performance include `memo`, `useMemo`, `useCallback`, and `useTransition`. These tools help in preventing wasted renders, improving app speed, and managing state transitions.
What is the significance of the virtual DOM in React's rendering process?
-The virtual DOM is a lightweight representation of the actual DOM that allows React to compute the minimal number of changes needed to update the real DOM. It helps in optimizing performance by minimizing direct manipulation of the DOM, which can be expensive.
How can third-party packages affect the performance of a React application?
-Third-party packages can affect performance by increasing the bundle size, which can slow down the application load time. It's important to use them judiciously and only when necessary to maintain optimal performance.
Outlines
🔍 Optimizing React Performance: An Overview
This paragraph introduces the concept of optimizing React applications by focusing on three main areas: preventing wasted renders, enhancing app speed and responsiveness, and reducing bundle size. It mentions built-in React tools such as `memo`, `useMemo`, and `useCallback` hooks for optimizing these areas. The paragraph also discusses techniques like passing elements as children or props to prevent unnecessary re-renders. The importance of understanding when a component re-renders is highlighted, including state changes, context updates, and parent component re-renders. It clarifies the misconception about props causing re-renders and emphasizes the distinction between rendering and actual DOM updates. The section aims to equip developers with tools for various optimization scenarios without being exhaustive, and it sets the stage for a deeper dive into wasted renders.
🚫 Understanding Wasted Renders in React
This paragraph delves into the issue of wasted renders, which are re-renders that do not result in any DOM changes, leading to unnecessary computational work. While React is typically fast enough to handle re-renders efficiently, frequent or slow re-renders can degrade the application's performance, making it feel sluggish or unresponsive. The paragraph emphasizes the importance of avoiding such situations for a smooth user experience. It sets the context for subsequent discussions on how to prevent wasted renders and improve the overall performance of React applications.
Mindmap
Keywords
💡Optimization
💡Wasted Renders
💡memo
💡useMemo
💡useCallback
💡useTransition
💡Bundle Size
💡Code Splitting
💡Lazy Loading
💡Re-renders
💡Virtual DOM
Highlights
Three main areas to optimize performance in React applications: preventing wasted renders, improving app speed and responsiveness, and reducing bundle size.
Memoization can prevent wasted renders using React's memo function for components and useMemo and useCallback hooks for objects and functions.
Passing elements as children or props can prevent unnecessary re-renders.
UseMemo and useCallback hooks, along with the useTransition hook, can improve perceived app speed.
Redistributing the bundle size can be achieved by minimizing third-party packages and implementing code splitting and lazy loading.
Optimization tools and techniques should be used judiciously based on the situation.
The section aims to provide developers with a toolbox of optimization tools to be used as needed.
Understanding when a component re-renders is crucial for optimizing wasted renders.
Components re-render when their state changes, when there's a change in a subscribed context, or when a parent component re-renders.
A common misconception is that prop changes cause re-renders, but it's actually the parent component's re-render that triggers it.
Rendering a component does not necessarily mean the DOM is updated, which can lead to wasted renders.
Wasted renders occur when a component function is called but no DOM changes result, leading to unnecessary calculations.
React's speed usually prevents wasted renders from being a problem, but frequent re-renders or slow rendering can impact performance.
Avoiding situations where the application feels sluggish or unresponsive is a key goal in optimizing React applications.
The next couple of videos will focus on dealing with wasted renders to maintain a fluid and responsive user interface.
There are many optimization best practices not exhaustively covered in this list, such as not defining components inside other components.
The list of tools and techniques provided is primarily for discussion in this and future sections.
Transcripts
We're gonna start this section
by getting a quick overview
of what can actually be optimized
in React applications and how we can do it.
So, there are three main areas that we can focus on
when we need to optimize performance of React apps.
First, we can try to prevent wasted renders.
Second, we can improve the overall app speed
and responsiveness to make sure
that the app is 100% fluid and without delays.
And third, we can reduce the bundle size.
So, let's quickly look at some built
in React tools for each of these areas.
So in order to prevent wasted renders,
we can memorize components with memo
or we can memorize objects
and functions with useMemo and useCallback hooks.
And of course, you don't know what any
of this means at this point
but we'll get to it over the next few lectures.
We can also use a technique where we pass elements
into other elements as children or as a normal prop
in order to prevent them from being re-rendered.
Now to improve the perceived up speed,
we can again use the use memo and use callback hooks
and we can also use the modern useTransition hook net.
Again, more about these later.
Finally, we can reduce the bundle size simply
by using fewer third-party packages in our code base
and we can also implement code splitting and lazy loading.
Now there is no need to use all these tools
and techniques all the time.
So, the main goal
of this section is to basically just give you access
to these tools so that you can have them
in your toolbox whenever a situation calls for one of them.
And throughout this section,
you will learn exactly when that is.
So in which situations you need each of these tools.
Also, keep in mind that this list
of tools and techniques is by no means exhaustive.
So there are many other optimization best practices
that I already told you
about before and that you are already using, for example,
not defining components inside other components.
And so I didn't include all of these in this list.
So, these are mainly tools that we will talk about
in this or in future sections.
Now the first part
of this section is gonna be about optimizing wasted renders.
However, to understand what wasted renders actually are,
we first need to review when exactly a component instance
gets re-rendered.
So, this will serve as a nice reminder
of how rendering works in practice.
So in React, a component instance only gets re-rendered
in three different situations.
First, when the component state changes.
Second, a component instance gets re-rendered
whenever there's a change in a context
that the component is subscribed to.
And this is exactly what we've learned
in the previous section.
And finally, as we learned in the section
on how React works behind the scenes, whenever
a component re-renders, all its child components
will automatically be re-rendered as well.
And so the third reason for a component
to re-render is a parent component re-rendering.
But now, you might be wondering what about prop changes?
Doesn't updating props also re-render a component?
Well, actually, that's technically not the case.
So, that's a common misconception.
And actually even I told you this
at the beginning of the course, but it was only
because you didn't know yet how rendering works.
Because it is true that it does look
as if components re-render when their props change.
But what actually happens is that props only change
when the parent re-renders
but when the parent re-renders,
the children who receives the prop will re-render anyway.
And so, the real reason why a component re-renders
when props change is that the parent has re-rendered.
Okay.
But anyway, it's important to also remember
that rendering a component does not mean
that the DOM actually gets updated, right?
So all that rendering means is
that the component function gets called
and that React will create a new virtual DOM
and to all the differing
and reconciliation that we have already talked about before.
And this can be an expensive and wasteful operation
which brings us to an important topic
of this video, wasted renders.
So, a wasted render is basically a render
that didn't produce any change in the DOM.
So, it's a waste because all the different calculations
still had to be done
but it didn't result in any new DOM.
And so therefore, all the calculations were for nothing.
Now, most of the time, this is actually no problem at all
because React is very fast.
However, it can become a problem when re-renders
happen way too frequently
or when the component is very slow in rendering.
And so, this can then make the application feel leggy
and unresponsive.
For example, not updating the UI fast enough
after the user performs a certain action.
So, we want to avoid situations like that at all costs.
So, that's basically what we're gonna be dealing with
in the next couple of videos.
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