7 Debugging Guides

Next.js Debugging

Step-by-step debugging guides for Next.js App Router issues, deployment failures, API route errors, hydration mismatches, and environment variable problems.

High Priority

Why is process.env Undefined in Next.js?

Your API keys work perfectly on localhost, but crash the production build. Stop guessing and fix your Next.js environment variables instantly.

Next.jsDeploymentEnvironment VariablesVercel
6 min readUpdated 6/4/2026
High Priority

Next.js API Routes Failing in Production? Let's Fix It.

404s, 504 timeouts, and Edge runtime crashes. Here is the exact debugging playbook to rescue your Next.js App Router backend.

Next.jsAPI RoutesVercelServerless
8 min readUpdated 5/28/2026
High: Running 'npm run build' fails due to TypeScript type check errors in the console. Priority

Fix: Next.js TypeScript Build Errors & Route Props

Solve compilation errors, resolve missing module declarations, and define type-safe Page props.

Next.jsTypeScriptBuild FailureDebugging
5 min readUpdated 5/20/2026
High: Your fetch requests fail with 'Access to fetch at ... from origin ... has been blocked by CORS policy.' Priority

Fix: Next.js API Route Blocked by CORS Policy

Stop cross-origin resource sharing errors from breaking your frontend integrations.

Next.jsCORSAPI RoutesBackend
6 min readUpdated 5/20/2026
Critical Priority

Rescuing Failed Next.js Deployments on Vercel

Your app runs flawlessly on localhost, but Vercel rejects the build. Decode the logs and fix the pipeline right now.

Next.jsVercelDeploymentDebugging
10 min readUpdated 5/14/2026
Your Next.js build is failing on Vercel because of a ReferenceError. Priority

ReferenceError: window is not defined in Next.js

The exact debugging workflow to resolve Server-Side Rendering (SSR) crashes caused by browser-only objects.

Next.jsSSRBuild ErrorDebugging
5 min readUpdated 5/7/2026
High Priority

Next.js Production Debugging: The Ultimate Rescue Guide

Master Next.js troubleshooting. Fix hydration errors, window is not defined, Vercel build failures, and API route timeouts.

Next.jsReactDebuggingVercel
12 min readUpdated 5/6/2026

Can't Find Your Next.js Error?

Send me your exact error message and I'll help you debug it directly.

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