Building Type-Safe APIs with Next.js

Why Type Safety Matters

Type safety is fundamentally important for building reliable APIs. When your types are wrong, your runtime errors are just waiting to happen.

We use Zod schemas to validate request bodies at runtime, ensuring our z.infer<typeof schema> types always match reality.

The Next.js documentation covers Route Handlers in detail, including how to type request and response objects.

Setting Up Route Handlers

import { z } from "zod";
import { NextResponse } from "next/server";

const CreatePostSchema = z.object({
  title: z.string().min(1),
  slug: z.string().regex(/^[a-z0-9-]+$/),
  body: z.string(),
  publishedAt: z.string().datetime().optional(),
});

type CreatePostInput = z.infer<typeof CreatePostSchema>;

export async function POST(request: Request) {
  const json = await request.json();
  const result = CreatePostSchema.safeParse(json);

  if (!result.success) {
    return NextResponse.json(
      { error: result.error.flatten() },
      { status: 400 },
    );
  }

  const data: CreatePostInput = result.data;
  // ... persist to database
  return NextResponse.json({ success: true, data });
}

• Catch malformed requests before they reach your business logic
• Get autocomplete and type checking throughout your handler code
• Generate API documentation directly from your schema definitions

curl -X POST http://localhost:3000/api/posts \
  -H "Content-Type: application/json" \
  -d '{"title": "Hello World", "slug": "hello-world", "body": "First post!"}'

Type safety isn’t about preventing all bugs — it’s about catching the boring ones automatically so you can focus on the interesting problems.

Lessons Learned

Building type-safe APIs is an investment that pays off quickly. Start with Zod schemas for your most critical endpoints and expand from there. The compile-time safety net lets you refactor with confidence and ship faster.