Gatsby SSR Error Fix

In this tutorial, you'll learn about Gatsby SSR Error Fix. We cover key concepts, practical examples, and best practices.

The Problem

error Building static HTML failed for path "/my-page/"
See above for specific error messages. The following HTML is not valid:

Gatsby's static HTML build fails because a component throws during SSR.

Wrong

function Page() {
  const data = fetch('/api/data')  // HTTP request at top level
  return <div>{data}</div>
}

Output: Error: fetch is not defined during SSR.

fetch is not available in Node.js without a polyfill, and HTTP requests should not be made at the module level.

Right

Use Gatsby's data layer (GraphQL) for build-time data:

export const query = graphql`
  query {
    site {
      siteMetadata {
        title
      }
    }
  }
`

function Page({ data }) {
  return <div>{data.site.siteMetadata.title}</div>
}

For client-only data, use useEffect:

function Page() {
  const [data, setData] = useState(null)

  useEffect(() => {
    fetch('/api/data')
      .then(res => res.json())
      .then(setData)
  }, [])

  return <div>{data ? data.name : 'Loading...'}</div>
}

Output: static HTML builds successfully, client data loads after hydration.

Prevention

• Do not make HTTP requests at the top level of components
• Use Gatsby's GraphQL layer for build-time data
• Wrap client-only code in useEffect or event handlers

Common Mistakes with ssr error

1. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
2. Using head and tail instead of pattern matching, causing runtime errors on empty lists
3. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks

These mistakes appear frequently in real-world GATSBY code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ