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  "path": "/gary_killen37/common-nextjs-errors-and-how-i-solved-them-193o",
  "publishedAt": "2026-06-30T19:21:32.000Z",
  "site": "https://dev.to",
  "tags": [
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  "textContent": "##  Common Next.js Errors (and How I Solved Them)\n\nIf you've spent any amount of time building with Next.js, you've probably discovered that learning React is only half the battle. The other half is staring at an error message that makes absolutely no sense—until you finally figure it out hours later.\n\nI've been building projects with Next.js over the past several months, and while the framework is incredible, I've hit my fair share of confusing errors. The good news? Every one of them taught me something valuable.\n\nHere are some of the most common Next.js errors I ran into and how I eventually solved them.\n\n##  1. `ReferenceError: document is not defined`\n\nThis was one of the first errors that completely confused me.\n\n\n\n    ReferenceError: document is not defined\n\n\nAt first, I thought something was wrong with my code. The problem wasn't my code—it was _where_ it was running.\n\nNext.js renders pages on the server before sending them to the browser. Since the server doesn't have access to browser APIs like `document` or `window`, trying to use them immediately causes this error.\n\n###  What caused it?\n\n\n    const width = window.innerWidth;\n\n\nThis code executes during server-side rendering, where `window` doesn't exist.\n\n###  How I fixed it\n\nI moved the browser-specific code inside a `useEffect()` hook.\n\n\n\n    useEffect(() => {\n      console.log(window.innerWidth);\n    }, []);\n\n\nOr, if necessary, I checked whether the code was running in the browser before accessing `window`.\n\n###  💡 Lesson Learned\n\nIf you're using browser APIs, make sure your code only runs on the client.\n\n##  2. Environment Variables Returning `undefined`\n\nFew things are more frustrating than seeing this:\n\n\n\n    process.env.MONGODB_URI\n\n\n…and getting:\n\n\n\n    undefined\n\n\nIn my case, I had simply forgotten to create or correctly configure my `.env.local` file.\n\nOther times, I changed an environment variable but forgot to restart the development server.\n\n###  My Debugging Checklist\n\n  * Is the variable inside `.env.local`?\n  * Did I restart `npm run dev`?\n  * Does the variable need the `NEXT_PUBLIC_` prefix?\n  * Did I accidentally misspell the variable name?\n\n\n\n###  💡 Lesson Learned\n\nEnvironment variables are loaded when the development server starts—not while it's already running.\n\n##  3. Hydration Errors\n\nHydration errors can look intimidating.\n\n\n\n    Hydration failed because the initial UI does not match what was rendered on the server.\n\n\nThis usually means the HTML generated on the server doesn't match what React renders in the browser.\n\nCommon causes include:\n\n  * Using `Date.now()`\n  * Using `Math.random()`\n  * Rendering different content on the client\n  * Reading from `localStorage` during rendering\n\n\n\n###  How I fixed it\n\nInstead of rendering dynamic values immediately, I waited until the component mounted.\n\n\n\n    const [mounted, setMounted] = useState(false);\n\n    useEffect(() => {\n      setMounted(true);\n    }, []);\n\n    if (!mounted) return null;\n\n\n###  💡 Lesson Learned\n\nThe first render should produce the same output on both the server and the client.\n\n##  4. MongoDB Connection Problems\n\nDatabase errors always seemed more complicated than they actually were.\n\nSometimes the error looked like this:\n\n\n\n    MongoServerError: authentication failed\n\n\nOther times:\n\n\n\n    querySrv ECONNREFUSED\n\n\nOr:\n\n\n\n    MONGODB_URI is undefined\n\n\nThe fixes were usually simple:\n\n  * Double-check the connection string.\n  * Verify the username and password.\n  * Confirm your IP address is allowed in MongoDB Atlas.\n  * Make sure your internet connection isn't blocking DNS lookups.\n  * Verify your environment variables.\n\n\n\n###  💡 Lesson Learned\n\nDatabase errors are often configuration problems rather than coding problems.\n\n##  5. `Module not found`\n\nEveryone has seen this one.\n\n\n\n    Module not found\n\n\nSometimes I had:\n\n  * Misspelled an import\n  * Used the wrong file extension\n  * Imported from the wrong folder\n  * Forgotten to install a package\n\n\n\n###  My Debugging Checklist\n\n  * Does the file actually exist?\n  * Is the import path correct?\n  * Is the package installed?\n  * Did I restart the development server?\n\n\n\n###  💡 Lesson Learned\n\nMost \"module not found\" errors are caused by simple oversights. Start with the basics before assuming something is seriously broken.\n\n##  6. React Hook Errors\n\nOne of my favorites:\n\n\n\n    Invalid hook call\n\n\nUsually this happened because I accidentally placed a hook:\n\n  * Inside an `if` statement\n  * Inside a loop\n  * Inside another function\n\n\n\nReact hooks must always be called in the same order.\n\n###  ❌ Incorrect\n\n\n    if (loggedIn) {\n      useEffect(() => {});\n    }\n\n\n###  ✅ Correct\n\n\n    useEffect(() => {\n      if (loggedIn) {\n        // do something\n      }\n    }, [loggedIn]);\n\n\n###  💡 Lesson Learned\n\nAlways call hooks at the top level of your component.\n\n##  7. Importing a Server Component into a Client Component\n\nThis one took me a while to understand.\n\nWith the App Router, Next.js separates components into:\n\n  * Server Components\n  * Client Components\n\n\n\nIf I needed state, event handlers, or browser APIs, I had to mark the component with:\n\n\n\n    'use client';\n\n\nOtherwise, Next.js assumed it was a Server Component.\n\n###  💡 Lesson Learned\n\nNot every component should be a Client Component. Add `'use client'` only when you actually need client-side features.\n\n##  8. Images Not Loading\n\nI eventually discovered that Next.js doesn't automatically allow images from every domain.\n\nThe fix was adding the domain to `next.config.js`.\n\n\n\n    images: {\n      remotePatterns: [\n        {\n          protocol: 'https',\n          hostname: 'example.com',\n        },\n      ],\n    }\n\n\n###  💡 Lesson Learned\n\nIf you're using the `next/image` component with external images, make sure the image domain is configured.\n\n##  Final Thoughts\n\nWhen I first started with Next.js, every error felt like I had broken something beyond repair.\n\nOver time, I realized that these errors weren't telling me I was a bad developer—they were teaching me how the framework actually works.\n\nUnderstanding concepts like server-side rendering, client-side rendering, hydration, and environment variables made debugging much less intimidating.\n\nThe biggest improvement in my development skills didn't come from memorizing APIs.\n\nIt came from solving bugs.\n\nEvery confusing error forced me to understand another piece of how modern web applications work.\n\nAnd honestly, that's where the real learning happens.\n\n##  What About You?\n\nWhat's the most frustrating Next.js error you've encountered?\n\nShare it in the comments—I’d love to hear your debugging stories and how you solved them.",
  "title": "Common Next.js Errors (and How I Solved Them)"
}