Type error during next build Next.js Origin: On your device Last updated 5 Nov 2026

Next.js: Failed to compile. ./src/app/page.tsx:15:9 Type error: Type 'string | undefined' is not assignable to type 'string'

TypeScript type checking failed during next build, which blocks the deployment.

Exact message you may see Failed to compile. Type error: Type 'string | undefined' is not assignable to type 'string'Linting and checking validity of types ... Type errorType error: Property 'x' does not exist on typeType error: Argument of type is not assignable to parameter of typeFailed to compile. Type error
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Seen on: Next.js TypeScript

Meaning

Fix the type errors reported (file:line). Dev server doesn't block on types, so errors first surface in build/CI. Run npx tsc --noEmit locally to see them all. Avoid ignoreBuildErrors: true except as an emergency measure.

Common causes

  • Possibly undefined values (strict null checks)
  • Wrong prop types
  • Missing types for libraries (@types/*)
  • Next 15 async params type changes
  • Version upgrades changing types

⚡ Quick fix

  1. npx tsc --noEmit locally and fix errors
  2. Install missing @types/...
  3. Narrow values (if (!x) return) before use

Detailed fix by platform

Next.js

  1. npx tsc --noEmit --pretty

How to diagnose

  1. Error — File and line?
  2. Strict — Null checks?
  3. Types — @types installed?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: On your device. Errors that are often confused with or caused by this one (click to open):

[Vue warn]: Component is missing template or render function — Vue: [Vue warn]: Component is missing template or render function. (Vue)[Vue warn]: Component…Cannot find module './App.vue' or its corresponding type declarations — TypeScript: error TS2307: Cannot find module './App.vue' or its corresponding type declarations. (Vue)Cannot find module '.…Could not find a production build in the '.next' directory — Next.js: Error: Could not find a production build in the '.next' directory. Try building your app with 'next build' before starting the production server (Next.js)Could not find a prod…ERR_UNKNOWN_FILE_EXTENSION — TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" (Node.js)ERR_UNKNOWN_FILE_EXTE…Failed to compile: ESLint errors during next build — Next.js: Failed to compile. ./src/app/page.tsx 12:7 Error: 'x' is assigned a value but never used. @typescript-eslint/no-unused-vars (Next.js)Failed to compile: ES…Feature flags __VUE_OPTIONS_API__ not explicitly defined (esm-bundler) — Vue: You are running the esm-bundler build of Vue — configure your bundler to explicitly replace feature flag globals (__VUE_OPTIONS_API__) (Vue)Feature flags __VUE_O…next build: fetch failed ECONNREFUSED (build-time API) — Next.js: Error occurred prerendering page "/blog" TypeError: fetch failed { cause: Error: connect ECONNREFUSED 127.0.0.1:3000 } (Next.js)next build: fetch fai…next build: JavaScript heap out of memory — Next.js: FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory (during next build) (Next.js)next build: JavaScrip…Type erro…

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