Understanding next.js routing

This article is a quick introduction to next.js routing where I explain the steps I took to understand it.

When you first create a NEXT.js app with

npx create-next-app

The file structure looks like this:

the pages folder contains the page which will be rendered.

Let's add a simple page contact.js

contact.js

export default function Contact() {
    return (
        <div>
            <h1>Contact page</h1>
        </div>
    )
}

open http://localhost:3000/contact
and you should see your page title

Creating a large number of pages manually becomes quickly cumbersome. Fortunately NEXT.js provides dynamic routes.

Dynamic Routes

A dynamic route is basically a javascript file under pages/ folder with special characters in its name.

basis examples

• [id].js
• [slug].js
• [user_id].js
• [name].js

catch all examples

• [...id].js
• [...slug].js
• [...user_id].js
• [...name].js

Let's create a dynamic route.

[slug].js

export default function DynamicPage() {
  return (
    <div>
      <p>dynamic page</p>
    </div>
  );
}

export async function getStaticPaths() {
  const posts = ["post/a", "post/b"];
  const paths = posts.map((post) => ({
    params: { slug: post },
  }));

  // { fallback: false } means other routes should 404.
  return { paths, fallback: false };
}

When you navigate to this page you get a 404 error page.

Seems legit since the folder structure pages/[slug].js tells next.js that the path to these pages is at the root of the site. Something like /my-page should work. right ?

Let's navigate to /a. And then you get this error.

Error: getStaticPaths was added without a getStaticProps in /[slug]. Without getStaticProps, getStaticPaths does nothing

reproduction repo

Let's add a getStaticProps

[slug].js

export default function DynamicPage(props) {
  return (
    <div>
      <p>dynamic page</p>
      <p>{props.message}</p>
    </div>
  );
}

export async function getStaticProps(context) {
  return {
    props: { message: "dynamic page part 2" }, // will be passed to the page component as props
  };
}

export async function getStaticPaths() {
  const posts = ["post/a", "post/b"];
  const paths = posts.map((post) => ({
    params: { slug: post },
  }));

  return { paths, fallback: false };
}

The error is gone but the page is still not found.

reproduction repo

Fix: [slug].js

You need to add the page a in getStaticPaths

[slug].js

export async function getStaticPaths() {
  const posts = ["post/a", "post/b", "a"];
  const paths = posts.map((post) => ({
    params: { slug: post },
  }));

  return { paths, fallback: false };
}

And it works

But /post/a is still not found.

Fix: /post/a

Remember that next.js routing is based on the folder and files structure. For /post/a to work we need to create one of these files:

• pages/post/a.js
• pages/post/[slug].js
• pages/post/[id].js

The examples use slug and id but you can actually use any name since next.js use file structure to map the url path.

This file structure tells next.js to render pages if an http client requests one of the following path:

• /a
• /post/a
• /post/b
• /post/c
• /post/d
• ...

Any path starting with /post/ becomes a valid path.

• /post/a path will be rendered using pages/post/a.js
• /post/<any> path will be rendered using pages/post/[slug].js

At this point, in pages/[slug].js paths post/a, post/b become irrelevant. We can delete them.

reproduction repo

Let's assume you have only two posts a and b

You don't want to render a page for non existing posts. To achieve this, you need to add getStaticPaths AND getStaticProps to file pages/post/[slug].js

pages/post/[slug].js

export default function DynamicPage(props) {
  return (
    <div>
      <p>dynamic page </p>
    </div>
  );
}
export async function getStaticProps(context) {
  return {
    props: { message: "some post" }, 
  };
}

export async function getStaticPaths() {
  const posts = ["a", "b"]; // <= the magic happens here
  const paths = posts.map((post) => ({
    params: { slug: post },
  }));

  return { paths, fallback: false };
}

This limits the valid url paths to:

• /post/a
• /post/b

reproduction repo

IMPORTANT

When you use getStaticPaths the paths params must match the filename.

This will throw an error.

Error

Error: A required parameter (id) was not provided as a string in getStaticPaths for /post/[id]

documentation

official doc