Error Handling and Custom Messages in Zod

Learn how to implement robust error handling and custom error messages using Zod in Node.js applications.

Master data validation with practical examples and best practices.

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If you’re building Node.js applications, data validation is crucial for maintaining application reliability. Zod has emerged as a powerful schema validation library, but its true strength lies in its error handling capabilities. Let’s dive into how we can leverage Zod to create robust error handling with meaningful error messages.

Understanding Zod’s Error Structure

When Zod encounters validation errors, it provides detailed information about what went wrong. By default, these errors might be too technical for end-users. Here’s how we can make them more user-friendly:

import { z } from 'zod';

const userSchema = z.object({
  username: z.string()
    .min(3, 'Username must be at least 3 characters')
    .max(20, 'Username cannot exceed 20 characters'),
  email: z.string()
    .email('Please provide a valid email address'),
  age: z.number()
    .min(18, 'You must be at least 18 years old')
});

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Implementing Custom Error Messages

Custom error messages make your application more user-friendly. Let’s explore different ways to customize error messages:

const orderSchema = z.object({
  items: z.array(z.object({
    productId: z.string(),
    quantity: z.number().positive()
  })).nonempty({
    message: "Your cart cannot be empty"
  }),
  shippingAddress: z.string().min(10, {
    message: "Please provide a complete shipping address"
  })
}).refine(
  (data) => data.items.length <= 10,
  { message: "Maximum 10 items allowed per order" }
);

Handling Errors in Express.js

Here’s how we can integrate Zod validation with Express.js middleware:

const validateRequest = (schema: z.ZodSchema) => {
  return async (req: Request, res: Response, next: NextFunction) => {
    try {
      const validatedData = await schema.parseAsync(req.body);
      req.body = validatedData;
      next();
    } catch (error) {
      if (error instanceof z.ZodError) {
        return res.status(400).json({
          status: 'error',
          errors: error.errors.map(err => ({
            field: err.path.join('.'),
            message: err.message
          }))
        });
      }
      next(error);
    }
  };
};

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Advanced Error Handling Patterns

Sometimes you need more complex validation rules. Zod’s refinements and transformations can help:

const passwordSchema = z.string()
  .min(8, "Password must be at least 8 characters")
  .refine(
    (password) => {
      const hasUpperCase = /[A-Z]/.test(password);
      const hasLowerCase = /[a-z]/.test(password);
      const hasNumber = /[0-9]/.test(password);
      return hasUpperCase && hasLowerCase && hasNumber;
    },
    {
      message: "Password must contain uppercase, lowercase, and number"
    }
  );

Error handling doesn’t have to be complicated. With Zod, you can create clear, consistent, and user-friendly error messages that make your application more professional and maintainable.

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