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We learned that Next.js pre-renders

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our components on the server.

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We can see that with a simple log message:

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when we request the HomePage in the browser,

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the "rendering" message is printed

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only in the server logs,

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and not in the browser console.

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This is also different from how Next.js

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works with the older Pages Router.

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The new App Router by default

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uses React Server Components.

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This is a feature recently added to React

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that allows us to create components that

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are rendered only on the server,

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without sending any JavaScript

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code to the browser.

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That's what "Zero-Bundle-Size" means:

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they're components that add zero

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code to the JavaScript bundle.

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Let me show you how this is different

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from the previous Next.js approach.

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I have prepared another project here,

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that is still based on Next.js,

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but using the Pages Router.

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Note how, rather than having an "app" folder,

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in this project there is a "pages" folder.

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But the dependencies are exactly the

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same as our first Next.js project.

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Using the Pages Router there

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is a file called "_app.jsx",

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that plays a similar role to the

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RootLayout for the App Router.

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And the HomePage is in "index.jsx".

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It's very similar to our other HomePage.

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Now, we can start the server

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with "npm run dev" as usual,

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and it's running on port 3001.

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You can see that the page is almost the same.

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Now, let's see what happens if we clear the logs

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and reload the page in the browser.

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You can see that the HomePage component

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is rendered on the server,

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just like with the App Router,

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but then it's also rendered in the browser.

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This is in fact the main difference

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between the old Pages Router

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and the new App Router.

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The Pages Router always renders all components

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both on the server and in the browser.

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While the App Router by default only

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renders them on the server.

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But why does the Pages Router render

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them in the browser as well?

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The reason is that in a React component we

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may use some client-side functionality.

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Just as a quick example,

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suppose we want to display a pop up window

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as soon as the page is loaded.

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We could do that by calling the "useEffect" hook,

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that lets us execute a function after

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the component is rendered.

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If we pass an empty array as the dependency list,

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that's the second argument,

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our effect function will only run once,

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after the component is rendered

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for the first time.

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Now, to keep this simple I'll

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call "window.alert" here,

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and show a message saying "Welcome to my site!!!".

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If we save this change,

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you can see that we get this alert straight away.

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And this will show up any time we load the page.

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Obviously, this is a rather silly example.

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Displaying a popup as soon as the page

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loads is a horrible user experience.

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But sometimes you do need to display

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a cookie consent banner or similar.

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Anyway, the point is that the code that

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shows the alert must run in the client,

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it cannot run on the server, because

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there's no browser window there.

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That's why Next.js re-renders the HomePage

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component in the browser.

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Let me show you in more detail how this works.

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When we load the page, the browser requests

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the HTML document as usual.

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And, even when using the Pages Router,

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you can see that Next.js pre-renders the HTML.

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That's why it renders the component on the server.

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But then it will also send some JavaScript

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files to the browser,

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including our code that triggers

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the "window.alert".

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Which is part of our HomePage component function.

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So, Next.js first renders our

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component on the server

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to generate the pre-rendered HTML

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returned in the response.

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But then it also sends the same component

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code in a JavaScript bundle

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so that we can run client-side functionality,

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like displaying an annoying popup to the user.

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The limitation of the old Pages Router

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is that it always sends to the browser

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all the code for all our components.

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If you remember, it was re-rendering

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the HomePage in the browser

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even before we added the popup window,

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when the HomePage was entirely static.

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That was unnecessary.

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As we've seen, the new App Router uses

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React Server Components instead

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that are only rendered on the server,

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without sending any JavaScript

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code to the browser.

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And that's a lot more efficient.

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But, it also begs the question:

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what happens if we need some

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client-side functionality

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in a project that uses the App Router?

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Let's try, and we'll find out.

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I just need to import "useEffect" here.

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And if we save,

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it doesn't work!

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We get a "ReactServerComponentError".

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There are some more details in the console.

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It says "You're importing a component

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that needs useEffect."

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"It only works in a Client Component"

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"but none of its parents are

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marked with use client"

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"so they're Server Components by default".

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Let's unpack this message.

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It contains three useful pieces of information.

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First: our HomePage is currently

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a Server Component,

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because that's the default.

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Second: to use client-side functionality

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(like useEffect)

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it needs to be a Client Component instead.

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And third: we can make it a Client Component

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by marking it with "use client",

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that is a special directive we

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can add at the top of a file.

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With this simple change, our

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page is working again,

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and it's displaying the popup window,

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so the client-side functionality also works.

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So, simply by adding "use client" at the top,

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this is now a Client Component rather

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than a Server Component.

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But, what does this mean exactly?

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Let's clear the logs, and see how

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a Client Component is rendered.

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You can see that it's rendered both

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on the server and in the browser.

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So it works pretty like with the old Pages Router.

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The difference is that with the new App Router

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we need to explicitly declare if we

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need client-side functionality

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by adding the "use client" directive at the top.

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Without it, it will be treated

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as a Server Component,

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meaning that it will only be

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rendered on the server,

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without sending unnecessary JavaScript

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code to the browser.

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To recap, in Next.js with the App Router

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there are two different kinds of

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components: Server and Client.

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Server Components are the default,

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which means that all our components

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will be Server Components,

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unless we specify otherwise.

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If we want a Client Component instead,

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we need to put the "use client" directive

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at the top of the file.

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Server Components are only rendered on the server,

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so their code is not sent to the browser,

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making them more efficient, which

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is why they're the default.

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Client Components on the other hand are

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still pre-rendered on the server

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but then they are also rendered in the browser.

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This allows them to include

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client-side functionality,

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something that's not possible

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with Server Components.

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So in a Server Component we cannot use React hooks,

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like useEffect,

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nor event handlers, like the

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"onClick" on a button,

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nor any browser APIs, like accessing

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the "window" object.

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On the other hand, as we'll

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see later in the course,

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Server Components can potentially

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use server-side functionality,

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like Node.js built-in modules for example.

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Something that's obviously not

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possible in the browser.