A First Look at Effects | Lecture 141 | React.JS 🔥

Summary

This video script delves into the concept of side effects in React, distinguishing them from event handler functions. It explains that side effects are interactions between a React component and external systems, such as fetching data from an API. The script highlights the importance of not performing side effects during component rendering and instead using event handlers for event-triggered actions or the useEffect hook for automatic execution during component lifecycle events. The useEffect hook is used to keep components synchronized with external data, and the script emphasizes the preference for event handlers over the useEffect hook whenever possible.

Takeaways

• 🔍 The `useEffect` hook is used to fetch data as a component mounts, creating side effects.
• 🌐 A side effect in React is any interaction between a component and the outside world, such as fetching data from an API.
• ⚠️ Side effects should not occur during component render; they should be separate from render logic.
• 📌 Side effects can be created in two places in React: inside event handlers or using the `useEffect` hook.
• 🎯 Event handlers are functions triggered by specific events, while effects run automatically as the component renders.
• 🔄 Effects can run at different lifecycle moments: on mount, re-render, or unmount.
• 🔄 The timing of an effect's execution depends on its dependency array, which can also trigger it after re-renders.
• 🧹 Each effect can return a cleanup function that runs before the component re-renders or unmounts.
• 🔄 Understanding the component lifecycle (mounting, re-rendering, unmounting) helps grasp how effects work.
• 🔄 The primary purpose of effects is to keep components synchronized with external systems, not just to run code at lifecycle points.
• 🚫 Event handlers are the preferred method for creating side effects; the `useEffect` hook should be used sparingly.

Q & A

What is an effect in the context of React?

An effect in React refers to any interaction between a React component and the world outside that component, such as fetching data from an API. It's a way to make something useful happen.

How is an effect different from an event handler function?

An effect is used to execute code automatically as a component renders, re-renders, or unmounts, while an event handler function is triggered by a specific event happening in the user interface.

Why should side effects not happen during the component render?

Side effects should not occur during the component render to keep the render logic clean and focused on updating the component's state based on its props and state, without external interactions.

When might you want to fetch movie data using an event handler function?

You might want to fetch movie data using an event handler function when the data retrieval is tied to a specific user action or event, such as a button click.

What is the purpose of the useEffect hook?

The useEffect hook is used to create effects, allowing you to execute code at different moments in a component's lifecycle, such as when the component mounts, re-renders, or unmounts.

What are the three parts that an effect can have?

An effect can have three parts: the effect code itself, a dependency array, and a cleanup function that is called before the component re-renders or unmounts.

How does the dependency array in useEffect work?

The dependency array in useEffect determines when the effect should run. If the values in the array change, the effect will re-run after the component re-renders.

What is the main reason for using effects in a React component?

The main reason for using effects is to keep the component synchronized with an external system, such as keeping it up-to-date with data from an external API.

Why are event handlers preferred over the useEffect hook for creating side effects?

Event handlers are preferred because they are more specific and directly tied to user actions or events, which makes the code more predictable and easier to manage. The useEffect hook should be used sparingly and only when necessary.

When is it appropriate to use the useEffect hook to fetch data?

The useEffect hook is appropriate for fetching data when the data retrieval is not directly tied to a user event, such as when the component first mounts and needs to fetch initial data.

What is the cleanup function in an effect used for?

The cleanup function in an effect is used to perform any necessary cleanup before the component re-renders or unmounts, such as canceling a subscription or stopping a timer.

Outlines

00:00

🔍 Understanding Side Effects and Effects

This paragraph introduces the concept of side effects in React, which are interactions between a component and the outside world, such as fetching data from an API. It differentiates side effects from event handler functions, which are triggered by events. The useEffect hook is explained as a way to create effects that run at different lifecycle moments of a component, like mounting, re-rendering, or unmounting. The paragraph emphasizes that side effects should not occur during component render and that effects can be controlled using a dependency array. It also mentions the cleanup function associated with effects.

05:03

🔄 Synchronization vs. Lifecycle Events

The second paragraph delves into the purpose of effects, which is to keep a component synchronized with external systems, like an API for movie data. It clarifies that effects are not just for running code at different lifecycle points but for maintaining consistency with external data. The paragraph suggests that effects should be used sparingly and that event handlers are the preferred method for creating side effects when possible. It concludes by hinting at further practical use of the useEffect hook to better understand its concepts.

Keywords

💡effect hook

The effect hook, specifically `useEffect`, is a React hook used to handle side effects in a component. It allows you to specify a function that should run after a component renders, which is particularly useful for operations like fetching data from an API. In the video, it's used to fetch movie data when the component mounts, distinct from event handlers that respond to user actions.

💡side effect

A side effect in React refers to any interaction between a component and the world outside of it, such as fetching data from an API or updating the DOM. Side effects are essential for making components interact with external systems, but they should not occur during the component's render phase to avoid unexpected behavior.

💡event handler

An event handler in React is a function that is triggered in response to a specific event, such as a user clicking a button or typing in a text field. Event handlers are used to react to user interactions and are preferred over the `useEffect` hook for side effects whenever possible.

💡component lifecycle

The component lifecycle in React refers to the various stages a component goes through from its creation (mounting) to its destruction (unmounting). Understanding these stages helps developers manage side effects and ensure that components are kept in sync with external data.

💡dependency array

The dependency array in the `useEffect` hook is an array of values that the effect depends on. If any of these values change, the effect will re-run after the component re-renders. This feature allows for fine control over when and how effects are executed.

💡cleanup function

A cleanup function is a function returned by an effect in the `useEffect` hook. It is called before the component re-renders or unmounts, allowing for the cleanup of any side effects, such as canceling a network request or removing event listeners.

💡synchronization

Synchronization in the context of React refers to keeping the component's state in sync with external data or systems. Effects are used to ensure that the component reflects the most up-to-date information from external sources, like APIs.

💡render logic

Render logic is the code within a React component that determines what to display based on the component's state and props. Side effects should not be included in the render logic because they can lead to performance issues and unpredictable behavior.

💡re-render

A re-render in React occurs when a component's state or props change, causing the component to update its output. Effects can be triggered by re-renders, and the dependency array can influence when this happens.

💡user interface

The user interface (UI) is the part of a software application that allows users to interact with it. Event handlers in React are used to respond to user interactions within the UI, such as clicks, mouse movements, or keyboard inputs.

💡overuse

Overuse refers to using a tool or method more than is necessary or effective. In the context of React, it means using the `useEffect` hook for side effects when an event handler would suffice, which can lead to unnecessary complexity.

Highlights

The use of the useEffect hook for fetching movie data as a component mounts.

A side effect in React is any interaction between a component and the outside world.

Side effects should not occur during component render, they should be separate from render logic.

Side effects can be created in two places in React: inside event handlers and using the useEffect hook.

Event handlers are functions triggered by specific events.

The useEffect hook allows for code execution at different moments in a component's lifecycle.

Effects can run on component mount, re-render, or unmount.

The dependency array in useEffect determines when the effect runs.

Each effect can return a cleanup function that runs before component re-render or unmount.

Effects are used to keep components synchronized with external systems.

Event handlers are preferred for creating side effects when possible.

The useEffect hook is used to fetch data immediately after component mount.

Event handlers execute when an event happens, while effects execute at specific lifecycle moments.

Understanding the component lifecycle and synchronization is key to grasping how effects work.

The useEffect hook is a practical tool for managing side effects in React applications.

The distinction between event handlers and effects is crucial for efficient React development.

Effects are not just for running code at lifecycle points, but for maintaining component-state consistency.

Transcripts

‫So we just used the used effect hook

‫for the very first time in order to fetch movie data

‫as the component mounts.

‫But what actually is an effect

‫and how is it different from an event handler function?

‫Well, let's find out in this video.

‫And just so we're all on the same page,

‫let's start by reviewing what a side effect is.

‫So basically in React,

‫a side effect is any interaction between a React component

‫and a world outside that component.

‫And we can think of a side effect

‫as some code that actually makes something useful happen.

‫For example, fetching data from some API.

‫So what this means is that we actually need

‫side effects all the time when we build React apps.

‫Now, we already know that side effects should not happen

‫during the component render, or in other words

‫side effects should not be in render logic.

‫Instead, we can create side effects

‫in two different places in React.

‫And the first one is inside event handlers.

‫And remember that event handlers are simply

‫functions that are triggered whenever the event

‫that they are listening to happens.

‫However, simply reacting to events is sometimes not enough

‫for what an application needs.

‫Instead, in some situations, we need to

‫write some code that will be executed automatically

‫as the component renders.

‫And so this is when we can create a so-called effect

‫by using the useEffect hook.

‫So by creating an effect

‫we can basically write code that will run

‫at different moments of a component instance life cycle.

‫So when the component mounts, when it re-renders,

‫or even when it unmounts.

‫And this is really great

‫because it opens up a whole new door of possibilities.

‫Okay, but let's now get just a bit deeper

‫into how effects work by comparing event handlers

‫to effects created with the useEffect hook.

‫And let's go back to the example

‫of fetching movie data that we have been using.

‫So fetching movie data is very clearly a side effect

‫because it's clearly an interaction

‫with the world outside the component.

‫Now, there are two different possibilities

‫of when we might want to create this side effect.

‫The first possibility is that we might want to

‫fetch movie data only when a certain event happens.

‫So in that case, we will

‫of course just use an event handler function.

‫So just like we have been doing up until this point

‫I mean we haven't been using event handlers to fetch data

‫but we have used them for other stuff.

‫Now the other possibility

‫of when to fetch the data would be to do so immediately

‫after the component mounts,

‫so after it is first rendered.

‫And so this is exactly what we did

‫in the previous lecture when we first used the use event

‫to specify an effect that was executed right

‫after the component was painted to the screen.

‫So we can say that these two pieces

‫of code produce the exact same result.

‫So they both fetch data about a movie

‫but they do so at different moments in time.

‫So the event handler executes when an event happens

‫and the effect executes whenever the component first renders

‫at least in this situation because the exact moment

‫at which the effect is executed actually depends

‫on its dependency array

‫which I shortly mentioned in the last video.

‫So we can basically use this dependency array to

‫tell the effect to also run after a component re-renders.

‫But I won't go deep into this right now

‫because that's easier to explain with code.

‫But speaking of the dependency array, this array is just one

‫of three parts that any effect can have.

‫So besides the dependency array

‫we have of course the effect code itself.

‫And also each effect can return a so-called

‫cleanup function, which is a function that will be called

‫before the component re-renders or unmounts.

‫Now thinking about different moments

‫of the component lifecycle,

‫so mounting, re-rendering and unmounting, can be very

‫helpful to understand how effects work.

‫However, we should actually not think about life cycles,

‫but about synchronization.

‫So the real reason why effects exist is not to run code

‫at different points of the life cycle, but to

‫keep a component synchronized with some external system.

‫So in this example, that would be to keep the component

‫in sync with the movie data that comes

‫from some external API.

‫And if that sounds super confusing, keep in mind

‫that this is just a first introduction to effects.

‫We will come back to all this after having used

‫the useEffect hook a bit more in practice.

‫But anyway, to finish our comparison here,

‫as we just learned, we use effects to keep a component

‫in sync with the external world.

‫While on the other hand we use event handlers

‫to React to a certain event

‫that happened in the user interface.

‫Now, what's very important to note here is

‫that event handlers are always the preferred way

‫of creating side effects.

‫So whenever possible

‫we should not overuse the useEffect hook.

‫So basically everything that can be handled

‫inside event handlers should be handled there.