Add a Table of Contents with Smooth scroll using Gatsby and MDX

If you’re having issues with the clicks not scrolling to link IDs see Not scrolling to ID

The main purpose for me documenting this is to demonstrate implementing a table of contents with smooth scroll to the anchors in a Gatsby project using MDX.

In the process I’m also setting up the Gatsby starter with MDX.

TL;DR, go here: Make a TOC component

I like using styled-components for my styling and would like to use them in this example, so I’m going to clone the Gatsby starter I made in a previous post.

Clone the Gatsby Default Starter with styled-components

Spin up a new project using the template I made:

1npx gatsby new \
2  gatsby-toc-example \
3  https://github.com/spences10/gatsby-starter-styled-components

Once that has finished installing I’m going to cd into the project (cd gatsby-toc-example) and install dependencies for using MDX in Gatsby.

1# you can use npm if you like
2yarn add gatsby-plugin-mdx \
3  @mdx-js/mdx \
4  @mdx-js/react

Add some content

Create a posts directory with a toc-example directory in it which contains the index.mdx file I’ll be adding the content to.

1mkdir -p posts/toc-example
2touch posts/toc-example/index.mdx

I’ll paste in some content, I’ll take from the markdown from this post!

Configure the project to use MDX

To enable MDX in the project I’ll add the gatsby-plugin-mdx configuration to the gatsby-config.js file.

1{
2  resolve: `gatsby-plugin-mdx`,
3  options: {
4    extensions: [`.mdx`, `.md`],
5    gatsbyRemarkPlugins: [],
6  },
7},

I’ll also need to add the posts directory to the gatsby-source-filesystem config as well.

1{
2  resolve: `gatsby-source-filesystem`,
3  options: {
4    name: `posts`,
5    path: `${__dirname}/posts`,
6  },
7},

Stop the dev server (Ctrl+c in the terminal) and start with the new configuration.

Once the dev server has started back up, I’ll validate the Gatsby MDX config by seeing if allMdx is available in the GraphiQL explorer (localhost:8000/___graphql).

1{
2  allMdx {
3    nodes {
4      excerpt
5    }
6  }
7}

Configure Gatsby node to create the fields and pages

Here I’ll make all the paths for the files in the posts directory, currently it’s only gatsby-toc-example. I’ll do that with createFilePath when creating the node fields with createNodeField.

1const { createFilePath } = require(`gatsby-source-filesystem`)
2
3exports.onCreateNode = ({ node, actions, getNode }) => {
4  const { createNodeField } = actions
5  if (node.internal.type === `Mdx`) {
6    const value = createFilePath({ node, getNode })
7    createNodeField({
8      name: `slug`,
9      node,
10      value,
11    })
12  }
13}

Stop and start the gatsby dev server again as I changed gatsby-node.js.

In the Gatsby GraphQL explorer (GraphiQL) validate that the fields are being created.

1{
2  allMdx {
3    nodes {
4      fields {
5        slug
6      }
7    }
8  }
9}

Create a post template

To make the pages for the content in the posts directory, I’ll need a template to use with the Gatsby createPages API.

To do that, I’ll create a templates directory in src then make a post-template.js file.

1mkdir src/templates
2touch src/templates/post-template.js

For now, I’m going to return a h1 with Hello template so I can validate the page was created by Gatsby node.

1import React from 'react'
2
3export default () => {
4  return (
5    <>
6      <h1>Hello template</h1>
7    </>
8  )
9}

Save the template, now to create the pages in gatsby-node.js I’m adding the following.

1const { createFilePath } = require(`gatsby-source-filesystem`)
2const path = require(`path`)
3
4exports.createPages = ({ actions, graphql }) => {
5  const { createPage } = actions
6  const postTemplate = path.resolve('src/templates/post-template.js')
7
8  return graphql(`
9    {
10      allMdx(sort: { fields: [frontmatter___date], order: DESC }) {
11        nodes {
12          fields {
13            slug
14          }
15        }
16      }
17    }
18  `).then(result => {
19    if (result.errors) {
20      throw result.errors
21    }
22
23    const posts = result.data.allMdx.nodes
24
25    posts.forEach((post, index) => {
26      createPage({
27        path: post.fields.slug,
28        component: postTemplate,
29        context: {
30          slug: post.fields.slug,
31        },
32      })
33    })
34  })
35}
36
37exports.onCreateNode = ({ node, actions, getNode }) => {
38  const { createNodeField } = actions
39  if (node.internal.type === `Mdx`) {
40    const value = createFilePath({ node, getNode })
41    createNodeField({
42      name: `slug`,
43      node,
44      value,
45    })
46  }
47}

I know there’s a lot in there to unpack, so, if you need more detail check out the sections in the ”Build a coding blog from scratch with Gatsby and MDX”, listed here:

Confirm the pages were created with Gatsby’s built in 404 page

Stop and start the dev server as there’s been changes to Gatsby node.

Check the page has been created, to do that add /404.js to the dev server url which will show all the available pages in the project.

From here I can select the path created to /toc-example/ and confirm the page was created.

Build out the post template to use the MDXRenderer

Now I can add the data to the post-template.js page from a GraphQL query. I’ll do that with the Gatsby graphql tag and query some frontmatter, body and the table of contents.

This query is taking the String! parameter of slug passed to it from createPage in gatsby-node.js.

1query PostBySlug($slug: String!) {
2  mdx(fields: { slug: { eq: $slug } }) {
3    frontmatter {
4      title
5      date(formatString: "YYYY MMMM Do")
6    }
7    body
8    excerpt
9    tableOfContents
10    timeToRead
11    fields {
12      slug
13    }
14  }
15}

Destructure the body and frontmatter data from data.mdx, data is the results of the PostBySlug query. Wrap the body data in the <MDXRenderer> component.

The frontmatter.title and frontmatter.date can be used in h1 and p tags for now.

1import { graphql } from 'gatsby'
2import { MDXRenderer } from 'gatsby-plugin-mdx'
3import React from 'react'
4
5export default ({ data }) => {
6  const { body, frontmatter } = data.mdx
7  return (
8    <>
9      <h1>{frontmatter.title}</h1>
10      <p>{frontmatter.date}</p>
11      <MDXRenderer>{body}</MDXRenderer>
12    </>
13  )
14}
15
16export const query = graphql`
17  query PostBySlug($slug: String!) {
18    mdx(fields: { slug: { eq: $slug } }) {
19      frontmatter {
20        title
21        date(formatString: "YYYY MMMM Do")
22      }
23      body
24      excerpt
25      tableOfContents
26      timeToRead
27      fields {
28        slug
29      }
30    }
31  }
32`

I’m going to be using tableOfContents later when I make a table of contents component.

Add page elements for the MDXProvider

The content (headings, paragraphs, etc.) were reset with styled-reset in the template being used so will need to be added in.

I’m going to be amending the already existing H1 and <P> styled-components to be React components so that I can spread in the props I need for the heading ID.

1import React from 'react'
2import styled from 'styled-components'
3
4export const StyledH1 = styled.h1`
5  font-size: ${({ theme }) => theme.fontSize['4xl']};
6  font-family: ${({ theme }) => theme.font.serif};
7  margin-top: ${({ theme }) => theme.spacing[8]};
8  line-height: ${({ theme }) => theme.lineHeight.none};
9`
10
11export const H1 = props => {
12  return <StyledH1 {...props}>{props.children}</StyledH1>
13}

Create a <H2> component based off of the <H1>, adjust the spacing and font size.

1import React from 'react'
2import styled from 'styled-components'
3
4export const StyledH2 = styled.h2`
5  font-size: ${({ theme }) => theme.fontSize['3xl']};
6  font-family: ${({ theme }) => theme.font.serif};
7  margin-top: ${({ theme }) => theme.spacing[6]};
8  line-height: ${({ theme }) => theme.lineHeight.none};
9`
10
11export const H2 = props => {
12  return <StyledH2 {...props}>{props.children}</StyledH2>
13}

I’ll need to add the newly created H2 to the index file for page-elements:

1export * from './h1'
2export * from './h2'
3export * from './p'

Same with the <P> as I did with the H1, I’ll switch it to use React.

1import React from 'react'
2import styled from 'styled-components'
3
4export const StyledP = styled.p`
5  margin-top: ${({ theme }) => theme.spacing[3]};
6  strong {
7    font-weight: bold;
8  }
9  em {
10    font-style: italic;
11  }
12`
13
14export const P = props => {
15  const { children, ...rest } = props
16  return <StyledP {...rest}>{children}</StyledP>
17}

Importing the modified components into the root-wrapper.js I can now pass them into the <MDXProvider> which is used to map to the HTML elements created in markdown.

There’s a complete listing of all the HTML elements that can be customised on the MDX table of components.

In this example I’m mapping the H1, H2 and P components to the corresponding HTML elements and passing them into the <MDXProvider>.

1import { MDXProvider } from '@mdx-js/react'
2import React from 'react'
3import { ThemeProvider } from 'styled-components'
4import Layout from './src/components/layout'
5import { H1, H2, P } from './src/components/page-elements'
6import { GlobalStyle, theme } from './src/theme/global-style'
7
8const components = {
9  h1: props => <H1 {...props} />,
10  h2: props => <H2 {...props} />,
11  p: props => <P {...props} />,
12}
13
14export const wrapPageElement = ({ element }) => (
15  <ThemeProvider theme={theme}>
16    <GlobalStyle />
17    <MDXProvider components={components}>
18      <Layout>{element}</Layout>
19    </MDXProvider>
20  </ThemeProvider>
21)

Add gatsby-remark-autolink-headers for adding id’s to headers

Now I have a page, with some content and headers I should now be able to navigate to the individual headings, right?

Well, not quite, although the headers are there, there’s no IDs in them to scroll to yet.

I can use gatsby-remark-autolink-headers to create the heading IDs.

1yarn add gatsby-remark-autolink-headers

Add gatsby-remark-autolink-headers in the Gatsby MDX config.

1{
2  resolve: `gatsby-plugin-mdx`,
3  options: {
4    extensions: [`.mdx`, `.md`],
5    gatsbyRemarkPlugins: [`gatsby-remark-autolink-headers`],
6  },
7},

As I’ve changed the gatsby-config.js file I’ll need to stop and start the dev server.

Fix the weird positioning on the SVGs for the links added by gatsby-remark-autolink-headers.

Do that by making some reusable CSS with a tagged template literal, I’ll put it in it’s own file heading-link.js.

1touch src/components/page-elements/heading-link.js

Then add the CSS in as a template literal:

1export const AutoLink = `
2  a {
3    float: left;
4    padding-right: 4px;
5    margin-left: -20px;
6  }
7  svg {
8    visibility: hidden;
9  }
10  &:hover {
11    a {
12      svg {
13        visibility: visible;
14      }
15    }
16  }
17`

Then I’m going to use that (AutoLink) in the H2 and anywhere else that could have a link applied to it (any heading element).

1import React from 'react'
2import styled from 'styled-components'
3import { AutoLink } from './linked-headers'
4
5export const StyledH2 = styled.h2`
6  font-size: ${({ theme }) => theme.fontSize['3xl']};
7  font-family: ${({ theme }) => theme.font.serif};
8  margin-top: ${({ theme }) => theme.spacing[6]};
9  line-height: ${({ theme }) => theme.lineHeight.none};
10  ${AutoLink}
11`
12
13export const H2 = props => {
14  return <StyledH2 {...props}>{props.children}</StyledH2>
15}

Clicking around on the links now should scroll to each one smoothly and have the SVG for the link only visible on hover.

Make a TOC component

From here onwards is what the whole post boils down to! I did want to go through the process of how you would do something similar yourself though, so I’m hoping this has helped in some way.

For the TOC with smooth scroll you need several things:

scroll-behavior: smooth; added to your html, this is part of the starter I made in a previous post.
IDs in the headings to scroll to, this is done with gatsby-remark-autolink-headers.
A table of contents which is provided by Gatsby MDX with tableOfContents.

The first two parts have been covered so now to create a TOC component, with styled-components.

In the post-template.js I’ll create a Toc component for some positioning and create a scrollable div to use inside of that.

1const Toc = styled.ul`
2  position: fixed;
3  left: calc(50% + 400px);
4  top: 110px;
5  max-height: 70vh;
6  width: 310px;
7  display: flex;
8  li {
9    line-height: ${({ theme }) => theme.lineHeight.tight};
10    margin-top: ${({ theme }) => theme.spacing[3]};
11  }
12`
13
14const InnerScroll = styled.div`
15  overflow: hidden;
16  overflow-y: scroll;
17`

The main content is overlapping with the TOC here so I’m going to add a maxWidth inline on the layout.js component.

1<main style={{ maxWidth: '640px' }}>{children}</main>

Conditionally render the TOC

Time to map over the tableOfContents object:

1{
2  typeof tableOfContents.items === 'undefined' ? null : (
3    <Toc>
4      <InnerScroll>
5        <H2>Table of contents</H2>
6        {tableOfContents.items.map(i => (
7          <li key={i.url}>
8            <a href={i.url} key={i.url}>
9              {i.title}
10            </a>
11          </li>
12        ))}
13      </InnerScroll>
14    </Toc>
15  )
16}

Here’s the full post-template.js file, I’ve reused the page-elements components for the h1, h2 on the TOC and p:

1import { graphql } from 'gatsby'
2import { MDXRenderer } from 'gatsby-plugin-mdx'
3import React from 'react'
4import styled from 'styled-components'
5import { H1, H2, P } from '../components/page-elements'
6
7const Toc = styled.ul`
8  position: fixed;
9  left: calc(50% + 400px);
10  top: 110px;
11  max-height: 70vh;
12  width: 310px;
13  display: flex;
14  li {
15    line-height: ${({ theme }) => theme.lineHeight.tight};
16    margin-top: ${({ theme }) => theme.spacing[3]};
17  }
18`
19
20const InnerScroll = styled.div`
21  overflow: hidden;
22  overflow-y: scroll;
23`
24
25export default ({ data }) => {
26  const { body, frontmatter, tableOfContents } = data.mdx
27  return (
28    <>
29      <H1>{frontmatter.title}</H1>
30      <P>{frontmatter.date}</P>
31      {typeof tableOfContents.items === 'undefined' ? null : (
32        <Toc>
33          <InnerScroll>
34            <H2>Table of contents</H2>
35            {tableOfContents.items.map(i => (
36              <li key={i.url}>
37                <a href={i.url} key={i.url}>
38                  {i.title}
39                </a>
40              </li>
41            ))}
42          </InnerScroll>
43        </Toc>
44      )}
45      <MDXRenderer>{body}</MDXRenderer>
46    </>
47  )
48}
49
50export const query = graphql`
51  query PostBySlug($slug: String!) {
52    mdx(fields: { slug: { eq: $slug } }) {
53      frontmatter {
54        title
55        date(formatString: "YYYY MMMM Do")
56      }
57      body
58      excerpt
59      tableOfContents
60      timeToRead
61      fields {
62        slug
63      }
64    }
65  }
66`

That’s it, I can play around navigating between headings now from the TOC.

📺 Here’s a video detailing the process.

Demo and Sauce

Source code: https://github.com/spences10/gatsby-smooth-scroll

Demo: https://gatsby-smooth-scroll.vercel.app/toc-example/

Not scrolling to ID

There’s been a recent issue with gatsby-react-router-scroll causing inconsistencies with clicking and scrolling to the IDs, this is being addressed by the Gatsby team.

If it’s not resolved when you’re reading this (dated: 2020-07-08) add the following to your package.json:

1"resolutions": {
2  "gatsby-react-router-scroll": "3.0.3"
3}

Resources that helped me

Thanks for reading 🙏

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