Defining API Schemas with Zod and TypeScript

Why Use Zod with TypeScript

Zod is a TypeScript-first validation library that lets you:

• Validate data at runtime (like req.body)
• Automatically generate TypeScript types from schemas
• Reduce duplication between validation and type declarations

Perfect for full-stack TypeScript projects.

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Installing Zod

npm install zod

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Creating and Using Schemas

import { z } from "zod";

const UserSchema = z.object({
  name: z.string(),
  age: z.number().min(18)
});

Now you can validate and parse input:

const result = UserSchema.safeParse(req.body);

if (!result.success) {
  return res.status(400).json(result.error);
}

const user = result.data;

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Inferring Types from Zod Schemas

Use z.infer<typeof schema> to generate matching types:

type User = z.infer<typeof userschema>;

const createUser = (user: User) =&gt; {
  // fully typed, matches schema exactly
};

This ensures type and validation are always in sync.

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Validating API Request Data

You can validate:

• req.body
• req.query
• req.params

Example:

const QuerySchema = z.object({
  page: z.string().regex(/^d+$/)
});

const queryResult = QuerySchema.safeParse(req.query);

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Reusable Middleware for Express

Create a custom middleware:

function validate(schema: z.ZodTypeAny) {
  return (req: Request, res: Response, next: NextFunction) => {
    const result = schema.safeParse(req.body);
    if (!result.success) {
      return res.status(400).json(result.error.flatten());
    }
    req.body = result.data;
    next();
  };
}

// Usage:
app.post("/users", validate(UserSchema), (req, res) => {
  const user = req.body; // now fully typed + validated
  res.status(201).json(user);
});

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Summary

• Zod is the ideal partner for TypeScript APIs
• Define your schema once — and infer types automatically
• Use .safeParse() for clean validation with no exceptions
• Build reusable middleware to keep your Express routes clean

Next up: Lesson 19 – Integrating TypeScript with Zustand or Redux