Complete Guide with Claude Code

Design and Implement Your Test Strategy with Claude Code

We all know tests should be written, but deciding what to test and how far to go is often the hard part. With Claude Code, you can efficiently design and implement a strategic test plan based on the testing pyramid.

The Testing Pyramid in Practice

> Propose a test strategy for the project.
> Based on the testing pyramid, organize the ratio
> and scope of unit tests, integration tests, and E2E tests.

Test Type	Ratio	Scope	Tool
Unit Tests	70%	Business logic, utilities	Vitest
Integration Tests	20%	API, DB integration, components	Vitest + Testing Library
E2E Tests	10%	Key user flows	Playwright

Auto-Generating Unit Tests

> Create unit tests for all functions under src/services/.
> Cover success cases, error cases, and boundary values.
> Target 80%+ coverage.

// src/services/pricing.ts
export function calculatePrice(
  basePrice: number,
  quantity: number,
  discountRate: number
): number {
  if (quantity < 0) throw new Error("Quantity must be 0 or greater");
  if (discountRate < 0 || discountRate > 1) {
    throw new Error("Discount rate must be between 0 and 1");
  }
  const subtotal = basePrice * quantity;
  return Math.round(subtotal * (1 - discountRate));
}

// __tests__/services/pricing.test.ts
import { describe, it, expect } from "vitest";
import { calculatePrice } from "../../services/pricing";

describe("calculatePrice", () => {
  // Success cases
  it("returns base price multiplied by quantity", () => {
    expect(calculatePrice(100, 5, 0)).toBe(500);
  });

  it("applies the discount", () => {
    expect(calculatePrice(1000, 1, 0.1)).toBe(900);
  });

  it("rounds decimal places correctly", () => {
    expect(calculatePrice(333, 3, 0.1)).toBe(899);
  });

  // Boundary values
  it("returns 0 for quantity of 0", () => {
    expect(calculatePrice(100, 0, 0)).toBe(0);
  });

  it("returns 0 for 100% discount rate", () => {
    expect(calculatePrice(100, 5, 1)).toBe(0);
  });

  // Error cases
  it("throws error for negative quantity", () => {
    expect(() => calculatePrice(100, -1, 0)).toThrow("Quantity must be 0 or greater");
  });

  it("throws error for out-of-range discount rate", () => {
    expect(() => calculatePrice(100, 1, 1.5)).toThrow("Discount rate must be between 0 and 1");
    expect(() => calculatePrice(100, 1, -0.1)).toThrow("Discount rate must be between 0 and 1");
  });
});

Generating Integration Tests

Generate integration tests for API endpoints.

> Create integration tests for the /api/orders endpoint.
> Test actual CRUD operations using a test database.
> Ensure test data doesn't interfere between tests.

import { describe, it, expect, beforeEach, afterAll } from "vitest";
import request from "supertest";
import { app } from "../../app";
import { prisma } from "../../lib/prisma";

describe("Orders API Integration", () => {
  beforeEach(async () => {
    await prisma.order.deleteMany();
    await prisma.user.upsert({
      where: { id: "test-user" },
      update: {},
      create: { id: "test-user", email: "[email protected]", name: "Test" },
    });
  });

  afterAll(async () => {
    await prisma.$disconnect();
  });

  it("creates an order via POST /api/orders", async () => {
    const res = await request(app)
      .post("/api/orders")
      .set("Authorization", "Bearer test-token")
      .send({
        items: [{ productId: "P1", quantity: 2, price: 1000 }],
      });

    expect(res.status).toBe(201);
    expect(res.body.data.id).toBeDefined();
    expect(res.body.data.totalAmount).toBe("2000");
  });

  it("retrieves order list via GET /api/orders", async () => {
    // Create test data
    await prisma.order.create({
      data: {
        userId: "test-user",
        totalAmount: 5000,
        status: "confirmed",
      },
    });

    const res = await request(app)
      .get("/api/orders")
      .set("Authorization", "Bearer test-token");

    expect(res.status).toBe(200);
    expect(res.body.data).toHaveLength(1);
  });
});

Generating E2E Tests

Test key user flows with Playwright.

> Create Playwright E2E tests for the user registration
> through login flow.

import { test, expect } from "@playwright/test";

test.describe("User Registration and Login Flow", () => {
  test("from new user registration to dashboard display", async ({ page }) => {
    // Navigate to registration page
    await page.goto("/register");
    await expect(page.getByRole("heading", { name: "Sign Up" })).toBeVisible();

    // Fill in the form
    await page.getByLabel("Email").fill("[email protected]");
    await page.getByLabel("Password").fill("SecurePass123");
    await page.getByLabel("Confirm Password").fill("SecurePass123");

    // Submit registration
    await page.getByRole("button", { name: "Sign Up" }).click();

    // Redirect to dashboard
    await expect(page).toHaveURL("/dashboard");
    await expect(page.getByText("Welcome")).toBeVisible();
  });

  test("shows error for invalid email address", async ({ page }) => {
    await page.goto("/register");
    await page.getByLabel("Email").fill("invalid-email");
    await page.getByRole("button", { name: "Sign Up" }).click();

    await expect(page.getByText("Please enter a valid email address")).toBeVisible();
  });
});

Improving Test Coverage

> Generate a test coverage report and add tests
> to files with low coverage.
> Target 80%+.

# Commands Claude Code would run
npx vitest --coverage --run

For test-first development with TDD, see TDD and Claude Code. For automated test execution in CI/CD, see the CI/CD Pipeline Guide. For using tests in debugging, also check out the Complete Debugging Guide.

Summary

Building a test strategy hinges on deciding “what to test and at what level.” When you guide Claude Code with the testing pyramid in mind, it can efficiently build an effective test suite. Start with unit tests and gradually expand your coverage.

For test framework details, refer to the official Vitest documentation. For Claude Code, see the official Anthropic documentation.