March 02, 2019 • 9 min read

tl;dr Developing a gatsby-plugin-react-helmet-canonical-urls because gatsby-plugin-canonical-urls and gatsby-plugin-react-helmet do not play well together when using the canonical link.

Recently Dan Abramov announced that he would stop writing in medium, so, trying to set up my own gatsby blog was a great week-end project. I really didn’t want something fancy, just a blog site with a clean look that didn’t take me so many time to set up.

My first step was opening gatsby tutorial. Although, I must admit that the people from gatsby did a great work writing the docs, it seems to be targeted to new comers, i just didn’t want to read how to install node and all that stuff. I wanted to throw a one liner in the terminal and figure out thins on the way.

I finally found the generator cli and borrowing some ideas from Dan’s Overreacted, I picked gatsby’s starter blog which, to be fair, met all my needs.

> npx gatsby new nckweb.com.ar https://github.com/gatsbyjs/gatsby-starter-blog

Mission accomplish 🚀.

I need to fix two SEO related issues and I was going to be ready to go:

I planned to import my medium stories, I’ve to figure out how to set the canonical tag in order not to be penalized by Google because of duplicate content.

I publish my site with www and without it, so I’ve to automatically set the canonical tag for all the page (e.g: www.nckweb.com.ar/foo page should have a canonical tag with the value: nckweb.com.ar/foo ).

Custom Canonical data

The boilerplate file structure seemed a bit straight forward:

. ├── content │ ├── assets │ │ ├── icon.png │ │ └── profile-pic.jpg │ └── blog │ └── first-post │ └── index.md ├── gatsby-browser.js ├── gatsby-config.js ├── gatsby-node.js ├── package.json ├── README.md ├── src │ ├── components │ │ ├── bio.js │ │ ├── layout.js │ │ └── seo.js │ ├── pages │ │ ├── 404.js │ │ └── index.js │ ├── templates │ │ └── blog-post.js │ └── utils │ ├── global.css │ └── typography.js ├── static │ ├── favicon.ico │ └── robots.txt └── yarn.lock

Checking the markdown of an example post, you figure out that properties are written with a front matter syntax (just like jekyll’s). I just added a new one called canonical_url which will have the canonical value. (As dev.to uses that name to set the canonical, I was in the hope 🤞 that it will automagically work, but i wasn’t my lucky day).

I wanted to do some seo stuff, my first bet was checking the component with that name. Luckily, that component was using React Helmet. So, I just needed to figure out how to pass the front-matter canonical_url value to this seo component.

The next lead was the blog-post component:

class BlogPostTemplate extends React . Component { render ( ) { const post = this . props . data . markdownRemark const siteTitle = this . props . data . site . siteMetadata . title const { previous , next } = this . props . pageContext return ( < Layout location = { this . props . location } title = { siteTitle } > < SEO title = { post . frontmatter . title } description = { post . excerpt } /> ... </ Layout > ) } } export const pageQuery = graphql ` query BlogPostBySlug($slug: String!) { site { siteMetadata { title author } } markdownRemark(fields: { slug: { eq: $slug } }) { id excerpt(pruneLength: 160) html frontmatter { title date(formatString: "MMMM DD, YYYY") } } } ` ;

Yay! 🎉. It was using the seo component and judging from the graphql query, getting the canonical_url was just adding the property to the frontmatter query.

So far, so good, I got the canonical for the imported posts.

Plugins, plugins, plugins

I didn’t want to reinvent the wheel, so my first attempt was looking for a plugin and I came across gatsby-plugin-canonical-urls which did exactly what I needed.

Setting it up was as easy as reading its docs. But, things got a little bit ugly:

Two canonical links, why?

Getting hands dirty

I was decided to fix this issue, so I jumped directly into code.

As far as i understand (checking gatsby-plugin-canonical-urls and gatsby-plugin-react-helmet ) a plugin consist of the following structure:

. ├── index.js ├── package.json ├── .babelrc ├── README.md ├── src │ ├── gatsby-browser.js │ └── gatsby-ssr.js └── yarn.lock

The relevant files are gatsby-browser.js and gatsby-ssr.js , index.js is just a stub. Those two first files allows the use of the browser api and the ssr api respectively.

Reading the code:

import React from "react" import url from "url" exports . onRenderBody = ( { setHeadComponents , pathname = `/` } , pluginOptions ) => { if ( pluginOptions && pluginOptions . siteUrl ) { const parsedUrl = url . parse ( pluginOptions . siteUrl ) const myUrl = ` ${ pluginOptions . siteUrl } ${ pathname } ` setHeadComponents ( [ < link rel = " canonical " key = { myUrl } href = { myUrl } data-baseprotocol = { parsedUrl . protocol } data-basehost = { parsedUrl . host } /> , ] ) } } import { Helmet } from "react-helmet" exports . onRenderBody = ( { setHeadComponents , setHtmlAttributes , setBodyAttributes , } ) => { const helmet = Helmet . renderStatic ( ) if ( setHtmlAttributes ) { setHtmlAttributes ( helmet . htmlAttributes . toComponent ( ) ) } if ( setBodyAttributes ) { setBodyAttributes ( helmet . bodyAttributes . toComponent ( ) ) } setHeadComponents ( [ helmet . title . toComponent ( ) , helmet . link . toComponent ( ) , helmet . meta . toComponent ( ) , helmet . noscript . toComponent ( ) , helmet . script . toComponent ( ) , helmet . style . toComponent ( ) , ] ) }

Both plugins implement the onRenderBodyapi :

Called after every page Gatsby server renders while building HTML so you can set head and body components to be rendered in your html.js . Gatsby does a two-pass render for HTML. It loops through your pages first rendering only the body and then takes the result body HTML string and passes it as the body prop to your html.js to complete the render. It’s often handy to be able to send custom components to your html.js. For example, it’s a very common pattern for React.js libraries that support server rendering to pull out data generated during the render to add to your HTML.

At this point, I got a clear picture of what was happening:

Both plugins added elements on the head calling setHeadComponents

Gatsby didn’t prevent from link rel="canonical" repetition

repetition Both plugins didn’t take care if other plugin added the canonical tag.

Possible solutions

As I didn’t want to reinvent the wheel, my first attempt was to “fix” any of the mentioned issues.

Making Gatsby’s setHeadComponents aware that link rel="canonical" have not to be repeated.

Although this was one of my firsts thoughts, I wasn’t really convinced by it. First of all depended on the plugins execution order (ie.: the first call setHeadComponents will be the one which win). And in order to develop this, I needed a better understanding of how Gatsby worked.

Implement the onPreRenderHTML api on any of the plugins (or create a new plugin that implement this api) in order to remove the duplicated link rel="canonical" .

In order to avoid the repetition I needed to use getHeadComponent to get all head components, check if there is a duplicated canonical and then use replaceHeadComponents to set the head components deduplicated. Doing this will also depend on the plugin’s execution order which was something I wanted to avoid. In addition, the documentation has a uppercased warning (“WARNING if multiple plugins implement this API it’s the last plugin that ‘wins’”) which discouraged me from this approach.

React Helmet deals with this kind of duplication, if the canonical’s plugin instead of using setHeadComponents set the link with react-helmet’s api, the problem will be solved.

As I was using react-helmet in order to set meta tags, this solution actually made sense. I had just to figure out how to add the Helmet component.

Canonicals with Helmet

With this approach I’ll have to add the Helmet element. As it has to be done before the onRenderBody is called, i shouldn’t use that api. In addition, in order to give the lower priority to the default canonical tag, this component has to be placed first (ie, on a higher level in the react’s component tree) than the Helmet component that the page uses.

A simple way to achieve those things is wrapping current page and inserting a Helmet component in that wrap:

const Wrapper = ( { children , ... props } ) => ( < > < Helmet > link= { [ rel : 'canonical' ... ] } /> { children } </ > );

Gatsby has the wrapPageElement which allows to exactly do that:

const React = require ( 'react' ) ; const { Helmet } = require ( 'react-helmet' ) ; exports . wrapPageElement = ( { element , props } , pluginOptions ) => { if ( pluginOptions && pluginOptions . siteUrl ) { const myUrl = ` ${ pluginOptions . siteUrl } ${ props . location . pathname || '/' } ${ props . location . search } ${ props . location . hash } ` ; return ( < > < Helmet link = { [ { rel : 'canonical' , key : myUrl , href : myUrl , } ] } /> { element } </ > ) ; } return element ; } ;

What about client side rendering?

In order to be consistent, the plugin should also add the Helmet component on the client side. Although, if I don’t add it, I won’t trigger any React hydratation issue (the DOM tree that react hydratates is the same with or without this default canonical Helmet component), the canonical link is being removed when the client side react runs.

In addition, if you check gatsby-plugin-canonical-urls , you’ll notice that it also implement gatsby-browser :

exports . onRouteUpdate = ( { location } ) => { const domElem = document . querySelector ( `link[rel='canonical']` ) var existingValue = domElem . getAttribute ( `href` ) var baseProtocol = domElem . getAttribute ( `data-baseProtocol` ) var baseHost = domElem . getAttribute ( `data-baseHost` ) if ( existingValue && baseProtocol && baseHost ) { domElem . setAttribute ( `href` , ` ${ baseProtocol } // ${ baseHost } ${ location . pathname } ${ location . search } ${ location . hash } ` ) } }

Luckily, gatsby’s browser api implements the wrapPageElement method. So fixing this client side issues is just exposing the same method under the browser’s api.

So far the experience implementing a Gatbsy blog has been great!. The API is very simple, so if the long list of plugins doesn’t suit you, you can easily implement yours.

Community around the project is very active, I even got a gift swag for fixing a little doc issue while doing this post!