Fix environment variables that are undefined at runtime

Environment variables showing as undefined is a common issue caused by missing .env files, wrong variable naming conventions, or accessing server-side variables in client-side code without the proper prefix.

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Error messages you might see

TypeError: Cannot read properties of undefined (reading 'VARIABLE_NAME')
Error: Missing required environment variable: DATABASE_URL

Why this happens in AI-generated code

1

Missing NEXT_PUBLIC_ prefix for client-side variables

AI-generated code accesses process.env.API_KEY in client components, but Next.js only exposes variables prefixed with NEXT_PUBLIC_ to the browser bundle.

2

.env file not in the correct location or format

AI tools reference environment variables assuming a .env file exists at the project root, but it might be missing, misnamed (.env.local vs .env), or have syntax errors like spaces around the = sign.

3

Variables not configured in deployment environment

AI-generated code works locally with .env.local but the deployment platform (Vercel, Railway, etc.) doesn't have those variables configured in its settings.

How to fix it

1

Use the correct variable naming convention

For Next.js, prefix any variable needed in client-side code with NEXT_PUBLIC_. Server-only variables (database URLs, API secrets) should NOT have this prefix.

2

Verify .env file location and restart the dev server

Ensure your .env.local file is in the project root (same level as package.json). After adding or changing variables, restart the dev server — changes aren't picked up automatically.

3

Get professional help

Still stuck? Our engineers can audit your environment configuration across all environments. Visit /products to get started.

Related technologies

Next.jsNode.js

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Frequently asked questions

What is the difference between .env, .env.local, and .env.production?

In Next.js, .env loads in all environments, .env.local overrides for local development (and is gitignored), and .env.production loads only during production builds. .env.local takes highest priority locally.

Should I commit my .env file to git?

Never commit .env.local or any file containing secrets. You can commit .env with non-sensitive defaults or placeholder values. Always add .env.local and .env*.local to your .gitignore.

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