TypeScript Mistakes That Are Silently Killing Your Next.js App in 2026

What you'll learn:

• The TypeScript mistakes that look harmless until they hit production

• Why as and any are more dangerous in Next.js 15 than you think

• Next.js App Router specific TypeScript pitfalls most guides miss

• How to catch these errors before they reach production

• The one config change that exposes hidden type errors immediately

I shipped a Next.js app to production last year with zero TypeScript errors in the build output. Three days later, a deeply nested undefined property access crashed a checkout flow for real users. The error was invisible at compile time because I had used as to force a type three layers up in the data flow. TypeScript never complained. The app did at the worst possible moment. These are the mistakes that do exactly that: they pass the type checker, pass the build, and fail in production.

Mistake 1: Using ignoreBuildErrors: true and Forgetting About It

This one is in a category by itself because it disables the entire safety net. In next.config.ts, setting typescript: { ignoreBuildErrors: true } tells Next.js to ship to production even when TypeScript errors are present. It does not suppress errors it bypasses the check entirely.

I have seen this in three production codebases. In every case, the original developer added it to "temporarily" unblock a deployment. It stayed there for months. Hundreds of type errors accumulated underneath, invisible to everyone, because the build never complained. The Next.js docs themselves label this option with a double warning "!! WARN !!" for exactly this reason.

// next.config.ts — DO NOT do this in production
import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  typescript: {
    ignoreBuildErrors: true, // <-- This is a time bomb
  },
}

export default nextConfig

Fix: Remove it. Run npx tsc --noEmit to see the full list of errors that have accumulated. Fix them one by one or if there are hundreds, add // @ts-expect-error with a comment explaining why on specific lines as a tracked workaround, never a blanket bypass.

Mistake 2: Type Assertions (as) Instead of Type Guards

Type assertions tell TypeScript "trust me, I know what this is." TypeScript does trust you completely and without verification. That's the problem. When you write const user = data as User, TypeScript stops checking. If data is actually null, or if the API changed and User.email is now User.emailAddress, you will not find out until runtime.

// ❌ Wrong — TypeScript trusts you blindly
const user = apiResponse.data as User
console.log(user.email) // crashes if data is null or shape changed

// ✅ Correct — use a type guard
function isUser(data: unknown): data is User {
  return (
    typeof data === 'object' &&
    data !== null &&
    'email' in data &&
    typeof (data as User).email === 'string'
  )
}

if (isUser(apiResponse.data)) {
  console.log(apiResponse.data.email) // safe — TypeScript AND runtime verified
}

For API responses specifically, pair type guards with a validation library. Zod is the standard choice in Next.js projects you define the schema once and get both runtime validation and automatic TypeScript inference. Our guide on type-safe API validation with Zod in Next.js covers exactly this pattern end to end.

Mistake 3: Spreading any Through Your Codebase

any is contagious. One any in a function parameter means every value that touches that parameter loses its type. Pass it to another function and that function's return type becomes any. Return it from a component and the prop types downstream become any. Sound familiar?

The most common source in Next.js apps is untyped API responses. You fetch data from an endpoint, TypeScript infers the return as any because you haven't typed the response, and that any spreads to every component that consumes it. By the time something breaks, tracing it back to the source takes longer than fixing it would have originally.

// ❌ Wrong — any spreads silently
async function getOrder(id: string) {
  const res = await fetch(`/api/orders/${id}`)
  return res.json() // return type: Promise<any>
}

// ✅ Correct — type the response explicitly
interface OrderResponse {
  id: string
  total: number
  status: 'pending' | 'shipped' | 'delivered'
}

async function getOrder(id: string): Promise<OrderResponse> {
  const res = await fetch(`/api/orders/${id}`)
  if (!res.ok) throw new Error(`Failed to fetch order ${id}`)
  return res.json() as Promise<OrderResponse>
}

Enable "noImplicitAny": true in your tsconfig.json it's included in "strict": true. This forces you to explicitly type anything TypeScript can't infer. Every place you would have written implicit any, you now have to make a conscious decision. If you generate your TypeScript interfaces from JSON responses, the JSON to TypeScript Converter handles this in seconds paste the API response and get clean typed interfaces ready to import.

Mistake 4: Treating TypeScript Types as Runtime Validation

This is the most conceptually important mistake on this list. TypeScript types are a compile-time feature only. They are completely erased in the compiled JavaScript output. At runtime, your app has no knowledge of any TypeScript interface, type alias, or union type you defined.

What this means practically: if an external API sends you a response where a field that should be string is actually null, TypeScript cannot protect you. Your interface says it's a string. The interface is not there at runtime to enforce that. The null value goes through, your .toUpperCase() call throws, and users see a 500 error.

import { z } from 'zod'

// Define once — get both runtime validation + TypeScript types
const ProductSchema = z.object({
  id: z.string(),
  name: z.string(),
  price: z.number().positive(),
  inStock: z.boolean(),
})

// TypeScript type derived automatically — no duplication
type Product = z.infer<typeof ProductSchema>

// Runtime validation — catches API surprises before they crash the app
const result = ProductSchema.safeParse(apiData)
if (!result.success) {
  console.error('API response shape changed:', result.error)
  return null
}
const product = result.data // fully typed AND runtime-verified

Zod with safeParse is the pattern that eliminates the gap between what TypeScript checks at compile time and what actually arrives at runtime. The combination of strict TypeScript at compile time and Zod validation at runtime covers both failure modes. For a reference on common Next.js patterns around regex, validation, and structured data handling, our Regex Tester helps you build and verify the patterns used in Zod string validations before adding them to your schema.

Mistake 5: Wrong Types for Next.js 15 App Router Params

This one is specific to Next.js 15 and catches a lot of developers who migrated from the Pages Router or upgraded from Next.js 14. In Next.js 15, dynamic route params the params object passed to page components and route handlers are now Promises, not plain objects.

// ❌ Wrong — Next.js 14 pattern that breaks in Next.js 15
type Props = {
  params: { slug: string } // <-- This type is wrong in Next.js 15
}

export default function BlogPost({ params }: Props) {
  return <h1>{params.slug}</h1> // params.slug is undefined — params is a Promise
}

// ✅ Correct — Next.js 15 App Router pattern
type Props = {
  params: Promise<{ slug: string }>
}

export default async function BlogPost({ params }: Props) {
  const { slug } = await params // must await params first
  return <h1>{slug}</h1>
}

The same applies to searchParams in page components. Both are now async in Next.js 15. TypeScript will not always catch this immediately because the old types can still satisfy the compiler in some configurations but at runtime the component receives a Promise object, not the values you expect.

Mistake 6: Not Enabling Strict Mode

A Next.js project without "strict": true in tsconfig.json is using TypeScript at about 40% of its capability. Strict mode enables a bundle of checks that are individually optional but collectively essential: strictNullChecks, noImplicitAny, strictFunctionTypes, and several others.

strictNullChecks alone prevents the single most common class of runtime error in JavaScript accessing a property on a value that might be null or undefined. Without it, TypeScript happily lets you write user.name.toUpperCase() even when user could be null. With it, that line is a compile error until you handle the null case.

// tsconfig.json — minimum for a serious Next.js project
{
  "compilerOptions": {
    "strict": true,        // enables strictNullChecks, noImplicitAny + more
    "noUncheckedIndexedAccess": true,  // array[0] returns T | undefined, not T
    "exactOptionalPropertyTypes": true // optional props can't be set to undefined
  }
}

If your existing project doesn't have strict mode and you enable it now, you'll likely see hundreds of errors. That's not a problem those errors existed before, TypeScript just wasn't telling you. Fix them incrementally, starting with the most critical paths: authentication, data fetching, and payment flows.

Mistake 7: Untyped Environment Variables

process.env.DATABASE_URL has type string | undefined in TypeScript. Most developers access it directly without checking and get a false sense of security when the build passes. In production, a missing environment variable produces undefined which then crashes at the first database call, often with a confusing error message that points nowhere near the actual problem.

// ❌ Wrong — silently breaks if env var is missing in production
const db = new MongoClient(process.env.MONGODB_URI) // type error: string | undefined

// ✅ Correct — validate at startup, fail loudly and early
function getEnvVar(key: string): string {
  const value = process.env[key]
  if (!value) {
    throw new Error(`Missing required environment variable: ${key}`)
  }
  return value
}

const MONGODB_URI = getEnvVar('MONGODB_URI') // string — guaranteed
const db = new MongoClient(MONGODB_URI)

For larger projects, use @t3-oss/env-nextjs it validates all environment variables against a Zod schema at build time and gives you fully typed access throughout the codebase. Missing env vars fail the build, not the runtime.

Mistake 8: Missing the next-env.d.ts in Version Control

This one causes mysterious build failures on CI/CD that work perfectly locally. Next.js automatically generates a next-env.d.ts file in your project root. It contains type declarations that Next.js needs including how to handle CSS module imports. If this file is in your .gitignore and therefore missing on Vercel or your CI server, you get Cannot find module errors that are impossible to reproduce locally.

Fix: Make sure next-env.d.ts is committed to your repository and not listed in .gitignore. Also verify your tsconfig.json includes it explicitly: "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx"].

How to Catch These Before They Reach Production

The most effective single change you can make: add npx tsc --noEmit as a step in your CI pipeline before deployment. This runs the full TypeScript compiler without producing output just error checking. Combined with "strict": true in your tsconfig, it catches every mistake on this list at the pull request stage rather than in production.

Use the Next.js TypeScript plugin in VS Code it provides advanced type checking specific to Next.js patterns, including the App Router param types and Server Component constraints. Enable it by opening the VS Code command palette, searching for "TypeScript: Select TypeScript Version", and choosing "Use Workspace Version". This activates Next.js-aware type checking in your editor, not just during builds.

For runtime validation at API boundaries, Zod with safeParse is the pattern that eliminates the gap between what TypeScript checks at compile time and what actually arrives at runtime. The combination of strict TypeScript at compile time and Zod validation at runtime covers both failure modes. For a reference on common Next.js patterns around regex, validation, and structured data handling, our Regex Tester helps you build and verify the patterns used in Zod string validations before adding them to your schema.

Frequently Asked Questions

Should I use interface or type in Next.js projects?

For plain object shapes like API responses and component props, interface is generally preferred it produces clearer error messages, supports declaration merging for extending library types, and is the conventional choice in most Next.js codebases. Use type for union types, mapped types, and utility type operations where interfaces fall short.

Why does TypeScript not catch errors that crash at runtime?

TypeScript is a compile-time tool all type information is erased in the compiled JavaScript. It cannot verify what an external API actually sends, what environment variables are actually set, or what actually exists in a database. For any data that crosses a trust boundary, TypeScript needs to be paired with runtime validation (Zod, Valibot, or similar). Think of TypeScript as catching developer mistakes and Zod as catching external data surprises you need both.

How is @ts-expect-error different from @ts-ignore?

@ts-ignore suppresses any TypeScript error on the next line, even if no error exists. @ts-expect-error suppresses an error but itself becomes an error if the next line is actually type-safe. This means it self-documents: when you fix the underlying issue, TypeScript tells you the suppression is no longer needed. Always prefer @ts-expect-error with a comment explaining why over a silent @ts-ignore.

What changed about params in Next.js 15?

In Next.js 15, the params and searchParams props in App Router page components became asynchronous they are now Promise objects rather than plain objects. Pages must be async functions and must await params before accessing values. The old type { params: { slug: string } } is wrong for Next.js 15 it must be { params: Promise<{ slug: string }> }.

What is the fastest way to add TypeScript types to an existing JavaScript Next.js project?

Rename files from .js to .ts or .tsx one directory at a time. Start with "strict": false to get the project compiling, then enable strict incrementally per file using // @ts-check. For API response types, paste the JSON into the JSON to TypeScript Converter to generate interfaces instantly rather than writing them manually.

Is TypeScript strict mode worth enabling on an existing large project?

Yes , always. The errors strict mode exposes already exist in your code. TypeScript was just not reporting them. Enable it incrementally using // @ts-nocheck on files you haven't migrated yet and tackle the strictest paths first: authentication, data fetching, and payment flows. Every strict-mode error you fix is a runtime crash you've prevented before it happened.