Next.js-håndbogen

Jeg skrev denne tutorial for at hjælpe dig med hurtigt at lære Next.js og blive fortrolig med, hvordan det fungerer.

Det er ideelt for dig, hvis du har nul til lidt kendskab til Next.js, du tidligere har brugt React, og du ser frem til at dykke mere ned i React-økosystemet, især rendering på serversiden.

Jeg finder Next.js et fantastisk værktøj til at oprette webapplikationer, og i slutningen af ​​dette indlæg håber jeg, at du vil være lige så begejstret for det som jeg er. Og jeg håber, det hjælper dig med at lære Next.js!

Bemærk: Du kan downloade en PDF / ePub / Mobi-version af denne vejledning, så du kan læse den offline!

Indeks

  1. Introduktion
  2. De vigtigste funktioner leveret af Next.js
  3. Next.js vs Gatsby vs. create-react-app
  4. Sådan installeres Next.js
  5. Se kilde for at bekræfte, at SSR fungerer
  6. Appbundterne
  7. Hvad er det ikon nederst til højre?
  8. Installer React DevTools
  9. Andre fejlretningsteknikker, du kan bruge
  10. Tilføjelse af en anden side til webstedet
  11. Sammenkædning af de to sider
  12. Dynamisk indhold med routeren
  13. Forhentning
  14. Brug af routeren til at registrere det aktive link
  15. Ved brug af next/router
  16. Foer data til komponenterne ved hjælp af getInitialProps()
  17. CSS
  18. Udfyld hovedmærket med brugerdefinerede tags
  19. Tilføjelse af en indpakningskomponent
  20. API-ruter
  21. Kør kode på serversiden eller på klientsiden
  22. Implementering af produktionsversionen
  23. Implementerer nu
  24. Analysering af appbundterne
  25. Lazy loading moduler
  26. Hvor skal man hen herfra

Introduktion

At arbejde på en moderne JavaScript-applikation drevet af React er fantastisk, indtil du indser, at der er et par problemer relateret til gengivelse af alt indholdet på klientsiden.

For det første tager siden længere tid at blive synlig for brugeren, for inden indholdet indlæses, skal alt JavaScript indlæses, og din applikation skal køre for at bestemme, hvad der skal vises på siden.

For det andet, hvis du bygger et offentligt tilgængeligt websted, har du et SEO-problem med indhold. Søgemaskiner bliver bedre til at køre og indeksere JavaScript-apps, men det er meget bedre, hvis vi kan sende dem indhold i stedet for at lade dem finde ud af det.

Løsningen på begge disse problemer er servergengivelse , også kaldet statisk præ-gengivelse .

Next.js er en React-ramme for at gøre alt dette på en meget enkel måde, men det er ikke begrænset til dette. Det annonceres af dets skabere som en nulkonfigurationsværktøjskæde med enkelt kommando til React-apps .

Det giver en fælles struktur, der giver dig mulighed for nemt at opbygge en frontend React-applikation og håndterer transparent gengivelse på serversiden for dig.

De vigtigste funktioner leveret af Next.js

Her er en ikke-udtømmende liste over de vigtigste Next.js-funktioner:

Genindlæsning af hot code

Next.js genindlæser siden, når den registrerer ændringer, der er gemt på disken.

Automatisk routing

Enhver URL er tilknyttet filsystemet, til filer, der er placeret i pagesmappen, og du har ikke brug for nogen konfiguration (du har selvfølgelig tilpasningsmuligheder).

Komponenter til enkelt fil

Brug styled-jsx, komplet integreret som bygget af det samme team, er det trivielt at tilføje stilarter, der er omfattet af komponenten.

Serverrendering

Du kan gengive React-komponenter på serversiden, før du sender HTML til klienten.

Økosystemkompatibilitet

Next.js spiller godt med resten af ​​JavaScript-, Node- og React-økosystemet.

Automatisk kodedeling

Sider gengives med kun de biblioteker og JavaScript, de har brug for, ikke mere. I stedet for at generere en enkelt JavaScript-fil, der indeholder hele appkoden, opdeles appen automatisk af Next.js i flere forskellige ressourcer.

Indlæsning af en side indlæser kun JavaScript, der er nødvendigt for den pågældende side.

Next.js gør det ved at analysere de importerede ressourcer.

Hvis kun en af ​​dine sider importerer Axios-biblioteket, for eksempel, vil den specifikke side inkludere biblioteket i dets pakke.

Dette sikrer, at din første sideindlæsning er så hurtig som den kan være, og kun fremtidige sidelastninger (hvis de nogensinde vil blive udløst) sender det nødvendige JavaScript til klienten.

Der er en bemærkelsesværdig undtagelse. Ofte anvendte importer flyttes til det vigtigste JavaScript-bundt, hvis de bruges på mindst halvdelen af ​​websidesiderne.

Forhentning

The Link component, used to link together different pages, supports a prefetch prop which automatically prefetches page resources (including code missing due to code splitting) in the background.

Dynamic Components

You can import JavaScript modules and React Components dynamically.

Static Exports

Using the next export command, Next.js allows you to export a fully static site from your app.

TypeScript Support

Next.js is written in TypeScript and as such comes with an excellent TypeScript support.

Next.js vs Gatsby vs create-react-app

Next.js, Gatsby, and create-react-app are amazing tools we can use to power our applications.

Let's first say what they have in common. They all have React under the hood, powering the entire development experience. They also abstract webpack and all those low level things that we used to configure manually in the good old days.

create-react-app does not help you generate a server-side-rendered app easily. Anything that comes with it (SEO, speed...) is only provided by tools like Next.js and Gatsby.

When is Next.js better than Gatsby?

They can both help with server-side rendering, but in 2 different ways.

The end result using Gatsby is a static site generator, without a server. You build the site, and then you deploy the result of the build process statically on Netlify or another static hosting site.

Next.js provides a backend that can server side render a response to request, allowing you to create a dynamic website, which means you will deploy it on a platform that can run Node.js.

Next.js can generate a static site too, but I would not say it's its main use case.

If my goal was to build a static site, I'd have a hard time choosing and perhaps Gatsby has a better ecosystem of plugins, including many for blogging in particular.

Gatsby is also heavily based on GraphQL, something you might really like or dislike depending on your opinions and needs.

How to install Next.js?

To install Next.js, you need to have Node.js installed.

Make sure that you have the latest version of Node. Check with running node -v in your terminal, and compare it to the latest LTS version listed on //nodejs.org/.

After you install Node.js, you will have the npm command available into your command line.

If you have any trouble at this stage, I recommend the following tutorials I wrote for you:

  • How to install Node.js
  • How to update Node.js
  • An introduction to the npm package manager
  • Unix Shells Tutorial
  • How to use the macOS terminal
  • The Bash Shell

Now that you have Node, updated to the latest version, and npm, we're set!

We can choose 2 routes now: using create-next-app or the classic approach which involves installing and setting up a Next app manually.

Using create-next-app

If you're familiar with create-react-app, create-next-app is the same thing - except it creates a Next app instead of a React app, as the name implies.

I assume you have already installed Node.js, which, from version 5.2 (2+ years ago at the time of writing), comes with the npx command bundled. This handy tool lets us download and execute a JavaScript command, and we'll use it like this:

npx create-next-app 

The command asks the application name (and creates a new folder for you with that name), then downloads all the packages it needs (react, react-dom, next), sets the package.json to:

and you can immediately run the sample app by running npm run dev:

And here's the result on //localhost:3000:

This is the recommended way to start a Next.js application, as it gives you structure and sample code to play with. There's more than just that default sample application; you can use any of the examples stored at //github.com/zeit/next.js/tree/canary/examples using the --example option. For example try:

npx create-next-app --example blog-starter 

Which gives you an immediately usable blog instance with syntax highlighting too:

Manually create a Next.js app

You can avoid create-next-app if you feel like creating a Next app from scratch. Here's how: create an empty folder anywhere you like, for example in your home folder, and go into it:

mkdir nextjs cd nextjs 

and create your first Next project directory:

mkdir firstproject cd firstproject 

Now use the npm command to initialize it as a Node project:

npm init -y 

The -y option tells npm to use the default settings for a project, populating a sample package.json file.

Now install Next and React:

npm install next react react-dom 

Your project folder should now have 2 files:

  • package.json (see my tutorial on it)
  • package-lock.json (see my tutorial on package-lock)

and the node_modules folder.

Open the project folder using your favorite editor. My favorite editor is VS Code. If you have that installed, you can run code . in your terminal to open the current folder in the editor (if the command does not work for you, see this)

Open package.json, which now has this content:

{ "name": "firstproject", "version": "1.0.0", "description": "", "main": "index.js", "scripts": { "test": "echo \"Error: no test specified\" && exit 1" }, "keywords": [], "author": "", "license": "ISC", "dependencies": { "next": "^9.1.2", "react": "^16.11.0", "react-dom": "^16.11.0" } } 

and replace the scripts section with:

"scripts": { "dev": "next", "build": "next build", "start": "next start" } 

to add the Next.js build commands, which we're going to use soon.

Tip: use "dev": "next -p 3001", to change the port and run, in this example, on port 3001.

Now create a pages folder, and add an index.js file.

In this file, let's create our first React component.

We're going to use it as the default export:

const Index = () => ( 

Home page

) export default Index

Now using the terminal, run npm run dev to start the Next development server.

This will make the app available on port 3000, on localhost.

Open //localhost:3000 in your browser to see it.

View source to confirm SSR is working

Let's now check the application is working as we expect it to work. It's a Next.js app, so it should be server side rendered.

It's one of the main selling points of Next.js: if we create a site using Next.js, the site pages are rendered on the server, which delivers HTML to the browser.

This has 3 major benefits:

  • Klienten behøver ikke at instantiere React for at gengive, hvilket gør webstedet hurtigere for dine brugere.
  • Søgemaskiner indekserer siderne uden at skulle køre JavaScript på klientsiden. Noget, som Google begyndte at gøre, men åbent indrømmede, at det var en langsommere proces (og du bør hjælpe Google så meget som muligt, hvis du vil rangere godt).
  • Du kan have metatags på sociale medier, der er nyttige til at tilføje eksempler på billeder, tilpasse titel og beskrivelse til alle dine sider, der deles på Facebook, Twitter og så videre.

Lad os se kilden til appen.

Ved hjælp af Chrome kan du højreklikke hvor som helst på siden og trykke på Vis sidekilde .

Hvis du ser kilden til siden, kan du se

Home page

uddrag i HTML bodysammen med en masse JavaScript-filer - appbundterne.

We don't need to set up anything, SSR (server-side rendering) is already working for us.

The React app will be launched on the client, and will be the one powering interactions like clicking a link, using client-side rendering. But reloading a page will re-load it from the server. And using Next.js there should be no difference in the result inside the browser - a server-rendered page should look exactly like a client-rendered page.

The app bundles

When we viewed the page source, we saw a bunch of JavaScript files being loaded:

Let's start by putting the code in an HTML formatter to get it formatted better, so we humans can get a better chance at understanding it:

Home page

{"dataManager":"[]","props":{"pageProps":{}},"page":"/","query":{},"buildId":"development","nextExport":true,"autoExport":true}

We have 4 JavaScript files being declared to be preloaded in the head, using rel="preload" as="script":

  • /_next/static/development/pages/index.js (96 LOC)
  • /_next/static/development/pages/_app.js (5900 LOC)
  • /_next/static/runtime/webpack.js (939 LOC)
  • /_next/static/runtime/main.js (12k LOC)

This tells the browser to start loading those files as soon as possible, before the normal rendering flow starts. Without those, scripts would be loaded with an additional delay, and this improves the page loading performance.

Then those 4 files are loaded at the end of the body, along with /_next/static/development/dll/dll_01ec57fc9b90d43b98a8.js (31k LOC), and a JSON snippet that sets some defaults for the page data:

 { "dataManager": "[]", "props": { "pageProps": {} }, "page": "/", "query": {}, "buildId": "development", "nextExport": true, "autoExport": true }  

The 4 bundle files loaded are already implementing one feature called code splitting. The index.js file provides the code needed for the index component, which serves the / route, and if we had more pages we'd have more bundles for each page, which will then only be loaded if needed - to provide a more performant load time for the page.

What's that icon on the bottom right?

Did you see that little icon at the bottom right of the page, which looks like a lightning?

If you hover it, it's going to say "Prerendered Page":

This icon, which is only visible in development mode of course, tells you the page qualifies for automatic static optimization, which basically means that it does not depend on data that needs to be fetched at invokation time, and it can be prerendered and built as a static HTML file at build time (when we run npm run build).

Next can determine this by the absence of the getInitialProps() method attached to the page component.

When this is the case, our page can be even faster because it will be served statically as an HTML file rather than going through the Node.js server that generates the HTML output.

Another useful icon that might appear next to it, or instead of it on non-prerendered pages, is a little animated triangle:

This is a compilation indicator, and appears when you save a page and Next.js is compiling the application before hot code reloading kicks in to reload the code in the application automatically.

It's a really nice way to immediately determine if the app has already been compiled and you can test a part of it you're working on.

Install the React Developer Tools

Next.js is based on React, so one very useful tool we absolutely need to install (if you haven't already) is the React Developer Tools.

Available for both Chrome and Firefox, the React Developer Tools are an essential instrument you can use to inspect a React application.

Now, the React Developer Tools are not specific to Next.js but I want to introduce them because you might not be 100% familiar with all the tools React provides. It's best to go a little into debugging tooling than assuming you already know them.

They provide an inspector that reveals the React components tree that builds your page, and for each component you can go and check the props, the state, hooks, and lots more.

Once you have installed the React Developer Tools, you can open the regular browser devtools (in Chrome, it's right-click in the page, then click Inspect) and you'll find 2 new panels: Components and Profiler.

If you move the mouse over the components, you'll see that in the page, the browser will select the parts that are rendered by that component.

If you select any component in the tree, the right panel will show you a reference to the parent component, and the props passed to it:

You can easily navigate by clicking around the component names.

You can click the eye icon in the Developer Tools toolbar   to inspect the DOM element, and also if you use the first icon, the one with the mouse icon (which conveniently sits under the similar regular DevTools icon), you can hover an element in the browser UI to directly select the React component that renders it.

You can use the bug icon to log a component data to the console.

This is pretty awesome because once you have the data printed there, you can right-click any element and press "Store as a global variable". For example here I did it with the url prop, and I was able to inspect it in the console using the temporary variable assigned to it, temp1:

Using Source Maps, which are loaded by Next.js automatically in development mode, from the Components panel we can click the code and the DevTools will switch to the Source panel, showing us the component source code:

The Profiler tab is even more awesome, if possible. It allows us to record an interaction in the app, and see what happens. I cannot show an example yet, because it needs at least 2 components to create an interaction, and we have just one now. I'll talk about this later.

I showed all screenshots using Chrome, but the React Developer Tools works in the same way in Firefox:

Other debugging techniques you can use

In addition to the React Developer Tools, which are essential to building a Next.js application, I want to emphasize 2 ways to debug Next.js apps.

The first is obviously console.log() and all the other Console API tools. The way Next apps work will make a log statement work in the browser console OR in the terminal where you started Next using npm run dev.

In particular, if the page loads from the server, when you point the URL to it, or you hit the refresh button / cmd/ctrl-R, any console logging happens in the terminal.

Subsequent page transitions that happen by clicking the mouse will make all console logging happen inside the browser.

Just remember if you are surprised by missing logging.

Another tool that is essential is the debugger statement. Adding this statement to a component will pause the browser rendering the page:

Really awesome because now you can use the browser debugger to inspect values and run your app one line at a time.

You can also use the VS Code debugger to debug server-side code. I mention this technique and this tutorial to set this up.

Adding a second page to the site

Now that we have a good grasp of the tools we can use to help us develop Next.js apps, let's continue from where we left our first app:

I want to add a second page to this website, a blog. It's going to be served into /blog, and for the time being it will just contain a simple static page, just like our first index.js component:

After saving the new file, the npm run dev process already running is already capable of rendering the page, without the need to restart it.

When we hit the URL //localhost:3000/blog we have the new page:

and here's what the terminal told us:

Now the fact that the URL is /blog depends on just the filename, and its position under the pages folder.

You can create a pages/hey/ho page, and that page will show up on the URL //localhost:3000/hey/ho.

What does not matter, for the URL purposes, is the component name inside the file.

Try going and viewing the source of the page, when loaded from the server it will list /_next/static/development/pages/blog.js as one of the bundles loaded, and not /_next/static/development/pages/index.js like in the home page. This is because thanks to automatic code splitting we don't need the bundle that serves the home page. Just the bundle that serves the blog page.

We can also just export an anonymous function from blog.js:

export default () => ( 

Blog

)

or if you prefer the non-arrow function syntax:

export default function() { return ( 

Blog

) }

Linking the two pages

Now that we have 2 pages, defined by index.js and blog.js, we can introduce links.

Normal HTML links within pages are done using the a tag:

Blog 

We can't do do that in Next.js.

Why? We technically can, of course, because this is the Web and on the Web things never break (that's why we can still use the tag. But one of the main benefits of using Next is that once a page is loaded, transitions to other page are very fast thanks to client-side rendering.

If you use a plain a link:

const Index = () => ( 

Home page

Blog ) export default Index

Now open the DevTools, and the Network panel in particular. The first time we load //localhost:3000/ we get all the page bundles loaded:

Now if you click the "Preserve log" button (to avoid clearing the Network panel), and click the "Blog" link, this is what happens:

We got all that JavaScript from the server, again! But.. we don't need all that JavaScript if we already got it. We'd just need the blog.js page bundle, the only one that's new to the page.

To fix this problem, we use a component provided by Next, called Link.

We import it:

import Link from 'next/link' 

and then we use it to wrap our link, like this:

import Link from 'next/link' const Index = () => ( 

Home page

Blog ) export default Index

Now if you retry the thing we did previously, you'll be able to see that only the blog.js bundle is loaded when we move to the blog page:

and the page loaded so faster than before, the browser usual spinner on the tab didn't even appear. Yet the URL changed, as you can see. This is working seamlessly with the browser History API.

This is client-side rendering in action.

What if you now press the back button? Nothing is being loaded, because the browser still has the old index.js bundle in place, ready to load the /index route. It's all automatic!

Dynamic content with the router

In the previous chapter we saw how to link the home to the blog page.

A blog is a great use case for Next.js, one we'll continue to explore in this chapter by adding blog posts.

Blog posts have a dynamic URL. For example a post titled "Hello World" might have the URL /blog/hello-world. A post titled "My second post" might have the URL /blog/my-second-post.

This content is dynamic, and might be taken from a database, markdown files or more.

Next.js can serve dynamic content based on a dynamic URL.

We create a dynamic URL by creating a dynamic page with the [] syntax.

How? We add a pages/blog/[id].js file. This file will handle all the dynamic URLs under the /blog/ route, like the ones we mentioned above: /blog/hello-world, /blog/my-second-post and more.

In the file name, [id] inside the square brackets means that anything that's dynamic will be put inside the id parameter of the query property of the router.

Ok, that's a bit too many things at once.

What's the router?

The router is a library provided by Next.js.

We import it from next/router:

import { useRouter } from 'next/router' 

and once we have useRouter, we instantiate the router object using:

const router = useRouter() 

Once we have this router object, we can extract information from it.

In particular we can get the dynamic part of the URL in the [id].js file by accessing router.query.id.

The dynamic part can also just be a portion of the URL, like post-[id].js.

So let's go on and apply all those things in practice.

Create the file pages/blog/[id].js:

import { useRouter } from 'next/router' export default () => { const router = useRouter() return (  

Blog post

Post id: {router.query.id}

) }

Now if you go to the //localhost:3000/blog/test router, you should see this:

We can use this id parameter to gather the post from a list of posts. From a database, for example. To keep things simple we'll add a posts.json file in the project root folder:

{ "test": { "title": "test post", "content": "Hey some post content" }, "second": { "title": "second post", "content": "Hey this is the second post content" } } 

Now we can import it and lookup the post from the id key:

import { useRouter } from 'next/router' import posts from '../../posts.json' export default () => { const router = useRouter() const post = posts[router.query.id] return (  

{post.title}

{post.content}

) }

Reloading the page should show us this result:

But it's not! Instead, we get an error in the console, and an error in the browser, too:

Why? Because.. during rendering, when the component is initialized, the data is not there yet. We'll see how to provide the data to the component with getInitialProps in the next lesson.

For now, add a little if (!post) return check before returning the JSX:

import { useRouter } from 'next/router' import posts from '../../posts.json' export default () => { const router = useRouter() const post = posts[router.query.id] if (!post) return  return (  

{post.title}

{post.content}

) }

Now things should work. Initially the component is rendered without the dynamic router.query.id information. After rendering, Next.js triggers an update with the query value and the page displays the correct information.

And if you view source, there is that empty

We'll soon fix this issue that fails to implement SSR and this harms both loading times for our users, SEO and social sharing as we already discussed.

We can complete the blog example by listing those posts in pages/blog.js:

import posts from '../posts.json' const Blog = () => ( 

Blog

    {Object.entries(posts).map((value, index) => { return
  • {value[1].title}
  • })}
) export default Blog

And we can link them to the individual post pages, by importing Link from next/link and using it inside the posts loop:

import Link from 'next/link' import posts from '../posts.json' const Blog = () => ( 

Blog

    {Object.entries(posts).map((value, index) => { return (
  • {value[1].title}
  • ) })}
) export default Blog

Prefetching

I mentioned previously how the Link Next.js component can be used to create links between 2 pages, and when you use it, Next.js transparently handles frontend routing for us, so when a user clicks a link, frontend takes care of showing the new page without triggering a new client/server request and response cycle, as it normally happens with web pages.

There's another thing that Next.js does for you when you use Link.

As soon as an element wrapped within appears in the viewport (which means it's visible to the website user), Next.js prefetches the URL it points to, as long as it's a local link (on your website), making the application super fast to the viewer.

This behavior is only being triggered in production mode (we'll talk about this in-depth later), which means you have to stop the application if you are running it with npm run dev, compile your production bundle with npm run build and run it with  npm run start instead.

Using the Network inspector in the DevTools you'll notice that any links above the fold, at page load, start the prefetching as soon as the load event has been fired on your page (triggered when the page is fully loaded, and happens after the DOMContentLoaded event).

Any other Link tag not in the viewport will be prefetched when the user scrolls and it

Prefetching is automatic on high speed connections (Wifi and 3g+ connections, unless the browser sends the Save-Data HTTP Header.

You can opt out from prefetching individual Link instances by setting the prefetch prop to false:

 A link 

Using the router to detect the active link

One very important feature when working with links is determining what is the current URL, and in particular assigning a class to the active link, so we can style it differently from the other ones.

This is especially useful in your site header, for example.

The Next.js default Link component offered in next/link does not do this automatically for us.

We can create a Link component ourselves, and we store it in a file Link.js in the Components folder, and import that instead of the default next/link.

In this component, we'll first import React from react, Link from next/link and the useRouter hook from next/router.

Inside the component we determine if the current path name matches the href prop of the component, and if so we append the selected class to the children.

We finally return this children with the updated class, using React.cloneElement():

import React from 'react' import Link from 'next/link' import { useRouter } from 'next/router' export default ({ href, children }) => { const router = useRouter() let className = children.props.className || '' if (router.pathname === href) { className = `${className} selected` } return {React.cloneElement(children, { className })} } 

Using next/router

We already saw how to use the Link component to declaratively handle routing in Next.js apps.

It's really handy to manage routing in JSX, but sometimes you need to trigger a routing change programmatically.

In this case, you can access the Next.js Router directly, provided in the next/router package, and call its push() method.

Here's an example of accessing the router:

import { useRouter } from 'next/router' export default () => { const router = useRouter() //... } 

Once we get the router object by invoking useRouter(), we can use its methods.

This is the client side router, so methods should only be used in frontend facing code. The easiest way to ensure this is to wrap calls in the useEffect() React hook, or inside componentDidMount() in React stateful components.

The ones you'll likely use the most are push() and prefetch().

push() allows us to programmatically trigger a URL change, in the frontend:

router.push('/login') 

prefetch() allows us to programmatically prefetch a URL, useful when we don't have a Link tag which automatically handles prefetching for us:

router.prefetch('/login') 

Full example:

import { useRouter } from 'next/router' export default () => { const router = useRouter() useEffect(() => { router.prefetch('/login') }) } 

You can also use the router to listen for route change events.

Feed data to the components using getInitialProps

In the previous chapter we had an issue with dynamically generating the post page, because the component required some data up front, and when we tried to get the data from the JSON file:

import { useRouter } from 'next/router' import posts from '../../posts.json' export default () => { const router = useRouter() const post = posts[router.query.id] return ( 

{post.title}

{post.content}

) }

we got this error:

How do we solve this? And how do we make SSR work for dynamic routes?

We must provide the component with props, using a special function called getInitialProps() which is attached to the component.

To do so, first we name the component:

const Post = () => { //... } export default Post 

then we add the function to it:

const Post = () => { //... } Post.getInitialProps = () => { //... } export default Post 

This function gets an object as its argument, which contains several properties. In particular, the thing we are interested into now is that we get the query object, the one we used previously to get the post id.

So we can get it using the object destructuring syntax:

Post.getInitialProps = ({ query }) => { //... } 

Now we can return the post from this function:

Post.getInitialProps = ({ query }) => { return { post: posts[query.id] } } 

And we can also remove the import of useRouter, and we get the post from the props property passed to the Post component:

import posts from '../../posts.json' const Post = props => { return ( 

{props.post.title}

{props.post.content}

) } Post.getInitialProps = ({ query }) => { return { post: posts[query.id] } } export default Post

Now there will be no error, and SSR will be working as expected, as you can see checking view source:

The getInitialProps function will be executed on the server side, but also on the client side, when we navigate to a new page using the Link component as we did.

It's important to note that getInitialProps gets, in the context object it receives, in addition to the query object these other properties:

  • pathname: the path section of URL
  • asPath - String of the actual path (including the query) shows in the browser

which in the case of calling //localhost:3000/blog/test will respectively result to:

  • /blog/[id]
  • /blog/test

And in the case of server side rendering, it will also receive:

  • req: the HTTP request object
  • res: the HTTP response object
  • err: an error object

req and res will be familiar to you if you've done any Node.js coding.

CSS

How do we style React components in Next.js?

We have a lot of freedom, because we can use whatever library we prefer.

But Next.js comes with styled-jsx built-in, because that's a library built by the same people working on Next.js.

And it's a pretty cool library that provides us scoped CSS, which is great for maintainability because the CSS is only affecting the component it's applied to.

I think this is a great approach at writing CSS, without the need to apply additional libraries or preprocessors that add complexity.

To add CSS to a React component in Next.js we insert it inside a snippet in the JSX, which start with

{` 

and ends with

`} 

Inside this weird blocks we write plain CSS, as we'd do in a .css file:

{` h1 { font-size: 3rem; } `} 

You write it inside the JSX, like this:

const Index = () => ( 

Home page

{` h1 { font-size: 3rem; } `} ) export default Index

Inside the block we can use interpolation to dynamically change the values. For example here we assume a size prop is being passed by the parent component, and we use it in the styled-jsx block:

const Index = props => ( 

Home page

{` h1 { font-size: ${props.size}rem; } `} )

If you want to apply some CSS globally, not scoped to a component, you add the global keyword to the style tag:

{` body { margin: 0; } `} 

If you want to import an external CSS file in a Next.js component, you have to first install @zeit/next-css:

npm install @zeit/next-css 

and then create a configuration file in the root of the project, called next.config.js, with this content:

const withCSS = require('@zeit/next-css') module.exports = withCSS() 

After restarting the Next app, you can now import CSS like you normally do with JavaScript libraries or components:

import '../style.css' 

You can also import a SASS file directly, using the @zeit/next-sass library instead.

Populating the head tag with custom tags

From any Next.js page component, you can add information to the page header.

This is handy when:

  • you want to customize the page title
  • you want to change a meta tag

How can you do so?

Inside every component you can import the Head component from next/head and include it in your component JSX output:

import Head from 'next/head' const House = props => ( The page title  {/* the rest of the JSX */} ) export default House 

You can add any HTML tag you'd like to appear in the section of the page.

When mounting the component, Next.js will make sure the tags inside Head are added to the heading of the page. Same when unmounting the component, Next.js will take care of removing those tags.

Adding a wrapper component

All the pages on your site look more or less the same. There's a chrome window, a common base layer, and you just want to change what's inside.

There's a nav bar, a sidebar, and then the actual content.

How do you build such system in Next.js?

There are 2 ways. One is using a Higher Order Component, by creating a components/Layout.js component:

export default Page => { return () => ( 
     
    ....
) }

In there we can import separate components for heading and/or sidebar, and we can also add all the CSS we need.

And you use it in every page like this:

import withLayout from '../components/Layout.js' const Page = () =>

Here's a page!

export default withLayout(Page)

But I found this works only for simple cases, where you don't need to call getInitialProps() on a page.

Why?

Because getInitialProps() gets only called on the page component. But if we export the Higher Order Component withLayout() from a page, Page.getInitialProps() is not called. withLayout.getInitialProps() would.

To avoid unnecessarily complicating our codebase, the alternative approach is to use props:

export default props => ( 
     
    ....
{props.content} )

and in our pages now we use it like this:

import Layout from '../components/Layout.js' const Page = () => ( 
     
      Here's a page!
      

)} /> )

This approach lets us use getInitialProps() from within our page component, with the only downside of having to write the component JSX inside the content prop:

import Layout from '../components/Layout.js' const Page = () => ( 
     
      Here's a page!
      

)} /> ) Page.getInitialProps = ({ query }) => { //... }

API Routes

In addition to creating page routes, which means pages are served to the browser as Web pages, Next.js can create API routes.

This is a very interesting feature because it means that Next.js can be used to create a frontend for data that is stored and retrieved by Next.js itself, transferring JSON via fetch requests.

API routes live under the /pages/api/ folder and are mapped to the /api endpoint.

This feature is very useful when creating applications.

In those routes, we write Node.js code (rather than React code). It's a paradigm shift, you move from the frontend to the backend, but very seamlessly.

Say you have a /pages/api/comments.js file, whose goal is to return the comments of a blog post as JSON.

Say you have a list of comments stored in a comments.json file:

[ { "comment": "First" }, { "comment": "Nice post" } ] 

Here's a sample code, which returns to the client the list of comments:

import comments from './comments.json' export default (req, res) => { res.status(200).json(comments) } 

It will listen on the /api/comments URL for GET requests, and you can try calling it using your browser:

API routes can also use dynamic routing like pages, use the [] syntax to create a dynamic API route, like /pages/api/comments/[id].js which will retrieve the comments specific to a post id.

Inside the [id].js you can retrieve the id value by looking it up inside the req.query object:

import comments from '../comments.json' export default (req, res) => { res.status(200).json({ post: req.query.id, comments }) } 

Heres you can see the above code in action:

In dynamic pages, you'd need to import useRouter from next/router, then get the router object using const router = useRouter(), and then we'd be able to get the id value using router.query.id.

In the server-side it's all easier, as the query is attached to the request object.

If you do a POST request, all works in the same way - it all goes through that default export.

To separate POST from GET and other HTTP methods (PUT, DELETE), lookup the req.method value:

export default (req, res) => { switch (req.method) { case 'GET': //... break case 'POST': //... break default: res.status(405).end() //Method Not Allowed break } } 

In addition to req.query and req.method we already saw, we have access to cookies by referencing req.cookies, the request body in req.body.

Under the hoods, this is all powered by Micro, a library that powers asynchronous HTTP microservices, made by the same team that built Next.js.

You can make use of any Micro middleware in our API routes to add more functionality.

Run code only on the server side or client side

In your page components, you can execute code only in the server-side or on the client-side, by checking the window property.

This property is only existing inside the browser, so you can check

if (typeof window === 'undefined') { } 

and add the server-side code in that block.

Similarly, you can execute client-side code only by checking

if (typeof window !== 'undefined') { } 

JS Tip: We use the typeof operator here because we can't detect a value to be undefined in other ways. We can't do if (window === undefined) because we'd get a "window is not defined" runtime error

Next.js, as a build-time optimization, also removes the code that uses those checks from bundles. A client-side bundle will not include the content wrapped into a if (typeof window === 'undefined') {} block.

Deploying the production version

Deploying an app is always left last in tutorials.

Here I want to introduce it early, just because it's so easy to deploy a Next.js app that we can dive into it now, and then move on to other more complex topics later on.

Remember in the "How to install Next.js" chapter I told you to add those 3 lines to the package.jsonscript section:

"scripts": { "dev": "next", "build": "next build", "start": "next start" } 

We used npm run dev up to now, to call the next command installed locally in node_modules/next/dist/bin/next. This started the development server, which provided us source maps and hot code reloading, two very useful features while debugging.

The same command can be invoked to build the website passing the build flag, by running npm run build. Then, the same command can be used to start the production app passing the start flag, by running npm run start.

Those 2 commands are the ones we must invoke to successfully deploy the production version of our site locally. The production version is highly optimized and does not come with source maps and other things like hot code reloading that would not be beneficial to our end users.

So, let's create a production deploy of our app. Build it using:

npm run build 

The output of the command tells us that some routes (/ and /blog are now prerendered as static HTML, while /blog/[id] will be served by the Node.js backend.

Then you can run npm run start to start the production server locally:

npm run start 

Visiting //localhost:3000 will show us the production version of the app, locally.

Deploying on Now

In the previous chapter we deployed the Next.js application locally.

How do we deploy it to a real web server, so other people can access it?

One of the most simple ways to deploy a Next application is through the Now platform created by Zeit,  the same company that created the Open Source project Next.js. You can use Now to deploy Node.js apps, Static Websites, and much more.

Now makes the deployment and distribution step of an app very, very simple and fast, and in addition to Node.js apps, they also support deploying Go, PHP, Python and other languages.

You can think of it as the "cloud", as you don't really know where your app will be deployed, but you know that you will have a URL where you can reach it.

Now is free to start using, with generous free plan that currently includes 100GB of hosting, 1000 serverless functions invocations per day, 1000 builds per month, 100GB of bandwidth per month, and one CDN location. The pricing page helps get an idea of the costs if you need more.

The best way to start using Now is by using the official Now CLI:

npm install -g now 

Once the command is available, run

now login 

and the app will ask you for your email.

If you haven't registered already, create an account on //zeit.co/signup before continuing, then add your email to the CLI client.

Once this is done, from the Next.js project root folder run

now 

and the app will be instantly deployed to the Now cloud, and you'll be given the unique app URL:

Once you run the now program, the app is deployed to a random URL under the now.sh domain.

We can see 3 different URLs in the output given in the image:

  • //firstproject-2pv7khwwr.now.sh
  • //firstproject-sepia-ten.now.sh
  • //firstproject.flaviocopes.now.sh

Why so many?

The first is the URL identifying the deploy. Every time we deploy the app, this URL will change.

You can test immediately by changing something in the project code, and running now again:

The other 2 URLs will not change. The first is a random one, the second is your project name (which defaults to the current project folder, your account name and then now.sh.

If you visit the URL, you will see the app deployed to production.

You can configure Now to serve the site to your own custom domain or subdomain, but I will not dive into that right now.

The now.sh subdomain is enough for our testing purposes.

Analyzing the app bundles

Next provides us a way to analyze the code bundles that are generated.

Open the package.json file of the app and in the scripts section add those 3 new commands:

"analyze": "cross-env ANALYZE=true next build", "analyze:server": "cross-env BUNDLE_ANALYZE=server next build", "analyze:browser": "cross-env BUNDLE_ANALYZE=browser next build" 

Like this:

{ "name": "firstproject", "version": "1.0.0", "description": "", "main": "index.js", "scripts": { "dev": "next", "build": "next build", "start": "next start", "analyze": "cross-env ANALYZE=true next build", "analyze:server": "cross-env BUNDLE_ANALYZE=server next build", "analyze:browser": "cross-env BUNDLE_ANALYZE=browser next build" }, "keywords": [], "author": "", "license": "ISC", "dependencies": { "next": "^9.1.2", "react": "^16.11.0", "react-dom": "^16.11.0" } } 

then install those 2 packages:

npm install --dev cross-env @next/bundle-analyzer 

Create a next.config.js file in the project root, with this content:

const withBundleAnalyzer = require('@next/bundle-analyzer')({ enabled: process.env.ANALYZE === 'true' }) module.exports = withBundleAnalyzer({}) 

Now run the command

npm run analyze 

This should open 2 pages in the browser. One for the client bundles, and one for the server bundles:

This is incredibly useful. You can inspect what's taking the most space in the bundles, and you can also use the sidebar to exclude bundles, for an easier visualization of the smaller ones:

Lazy loading modules

Being able to visually analyze a bundle is great because we can optimize our application very easily.

Say we need to load the Moment library in our blog posts. Run:

npm install moment 

to include it in the project.

Now let's simulate the fact we need it on two different routes: /blog and /blog/[id].

We import it in pages/blog/[id].js:

import moment from 'moment' ... const Post = props => { return ( 

{props.post.title}

Published on {moment().format('dddd D MMMM YYYY')}

{props.post.content}

) }

I'm just adding today's date, as an example.

This will include Moment.js in the blog post page bundle, as you can see by running npm run analyze:

See that we now have a red entry in /blog/[id], the route that we added Moment.js to!

It went from ~1kB to 350kB, quite a big deal. And this is because the Moment.js library itself is 349kB.

The client bundles visualization now shows us that the bigger bundle is the page one, which before was very little. And 99% of its code is Moment.js.

Every time we load a blog post we are going to have all this code transferred to the client. Which is not ideal.

One fix would be to look for a library with a smaller size, as Moment.js is not known for being lightweight (especially out of the box with all the locales included), but let's assume for the sake of the example that we must use it.

What we can do instead is separating all the Moment code in a separate bundle.

How? Instead of importing Moment at the component level, we perform an async import inside getInitialProps, and we calculate the value to send to the component.

Remember that we can't return complex objects inside the getInitialProps() returned object, so we calculate the date inside it:

import posts from '../../posts.json' const Post = props => { return ( 

{props.post.title}

Published on {props.date}

{props.post.content}

) } Post.getInitialProps = async ({ query }) => { const moment = (await import('moment')).default() return { date: moment.format('dddd D MMMM YYYY'), post: posts[query.id] } } export default Post

See that special call to .default() after await import? It's needed to reference the default export in a dynamic import (see //v8.dev/features/dynamic-import)

Now if we run npm run analyze again, we can see this:

Our /blog/[id] bundle is again very small, as Moment has been moved to its own bundle file, loaded separately by the browser.

Where to go from here

There is a lot more to know about Next.js. I didn't talk about managing user sessions with login, serverless, managing databases, and so on.

The goal of this Handbook is not to teach you everything, but instead it aims to introduce you, gradually, to all the power of Next.js.

The next step I recommend is to take a good read at the Next.js official documentation to find out more about all the features and functionality I didn't talk about, and take a look at all the additional functionalities introduced by Next.js plugins, some of which are pretty amazing.

You can reach me on Twitter @flaviocopes.

Also check out my website, flaviocopes.com.

Note: you can download a PDF / ePub / Mobi version of this tutorial so you can read it offline!