How to Fix Undefined Variable in Gatsby

Fixing Undefined Variable in Gatsby

Gatsby's build process runs in Node.js while components run in the browser. Undefined variable errors typically occur during SSR when browser APIs aren't available.

Common Causes

• window, document, or localStorage used during SSR
• GraphQL query results not checked for null
• Missing page context variables

Solution

Check for browser environment and handle null data:

import React from 'react';
import { graphql } from 'gatsby';

const isBrowser = typeof window !== 'undefined';

export default function BlogPost({ data }) {
  const post = data?.markdownRemark;

  if (!post) {
    return <div>Post not found</div>;
  }

  // Only access browser APIs after checking
  const savedTheme = isBrowser
    ? localStorage.getItem('theme')
    : 'light';

  return (
    <article>
      <h1>{post.frontmatter?.title ?? 'Untitled'}</h1>
      <div dangerouslySetInnerHTML={{ __html: post.html }} />
    </article>
  );
}

export const query = graphql`
  query($slug: String!) {
    markdownRemark(fields: { slug: { eq: $slug } }) {
      html
      frontmatter { title }
    }
  }
`;

Use optional chaining on all GraphQL data access and gate browser APIs behind environment checks.

Bugsly for Gatsby

Bugsly separates build-time SSR errors from client-side runtime errors in Gatsby, so you can see if the undefined variable issue only occurs during gatsby build or also affects users in the browser.