Writing comprehensive unit tests for your Zod schemas is essential to ensure their correctness and2 prevent regressions as your application evolves. Here are key strategies for testing Zod schemas:

Test Valid Inputs

Create test cases with valid input data that should successfully parse against your schema. Assert that schema.parse() or schema.safeParse() returns the expected validated data without errors.

import { z } from 'zod';
import { describe, it, expect } from 'vitest'; // Example using Vitest

const userSchema = z.object({
	name: z.string().min(2),
	age: z.number().positive(),
});

describe('User Schema', () => {
	it('should parse valid user data', () => {
		const validUserData = { name: 'Alice', age: 30 };
		const parsedUser = userSchema.parse(validUserData);
		expect(parsedUser).toEqual(validUserData); // Assert parsed data is as expected
	});
});

Test Invalid Inputs

Create test cases with invalid input data that should fail validation. Assert that schema.parse() throws a ZodError or that schema.safeParse() returns success: false. Examine the error.errors array to verify specific error messages and error paths are as expected.

import { z } from 'zod';
import { describe, it, expect } from 'vitest';

const userSchema = z.object({
	name: z.string().min(2),
	age: z.number().positive(),
});

describe('User Schema', () => {
	it('should throw ZodError for invalid age', () => {
		const invalidUserData = { name: 'Bob', age: -5 };
		expect(() => userSchema.parse(invalidUserData)).toThrowError(z.ZodError); // Assert ZodError is thrown
		try {
			userSchema.parse(invalidUserData);
		} catch (error) {
			if (error instanceof z.ZodError) {
				expect(error.errors[0].message).toBe('Number must be greater than 0'); // Assert specific error message
				expect(error.errors[0].path).toEqual(['age']); // Assert error path
			}
		}
	});

	it('should return success: false and ZodError for safeParse with invalid name', () => {
		const invalidUserData = { name: 'C', age: 25 };
		const result = userSchema.safeParse(invalidUserData);
		expect(result.success).toBe(false); // Assert safeParse returns failure
		if (!result.success) {
			expect(result.error).toBeInstanceOf(z.ZodError); // Assert error is ZodError
			expect(result.error.errors[0].message).toBe('String must be at least 2 characters'); // Assert error message
		}
	});
});

Test Edge Cases and Boundary Conditions

Consider edge cases and boundary conditions for your schemas (e.g., empty strings, null values where nullable is not specified, maximum allowed lengths, minimum/maximum values). Create test cases to ensure your schemas handle these scenarios correctly.

Test Custom Refinements and Transformations

Thoroughly test your .refine(), .superRefine(), and .transform() methods to ensure your custom validation and transformation logic works as expected.