The gatsby-ssr.js API file

The file gatsby-ssr.js lets you alter the content of static HTML files as they are being Server-Side Rendered (SSR) by Gatsby and Node.js. To use the Gatsby SSR APIs, create a file called gatsby-ssr.js in the root of your site. Export any of the APIs you wish to use in this file.

The APIs wrapPageElement and wrapRootElement exist in both the SSR and browser APIs. If you use one of them, consider if you should implement it in both gatsby-ssr.js and gatsby-browser.js so that pages generated through SSR with Node.js are the same after being hydrated with browser JavaScript.