TESTING & DEBUGGING / LESSON 08

Testing & Debugging with Claude.

Learn how to use Claude to write tests, debug errors, generate test data, and explain stack traces

Written by Andrew Lee JenkinsPublished Oct 5, 202614 min readFree lesson

You've learned testing techniques, patterns, and debugging tools. Now let's bring in some backup... Claude. Using AI as a pair programming buddy for testing and debugging can speed things up a CRAZY amount.

For a sense of scale, in one demo Claude Code spun up 13 subaccounts, crawled the sites, and rebuilt the forms in just under 12 minutes. Yeah. Twelve minutes.

Think of Claude as a coworker who never gets tired, has read basically every testing tutorial, has seen thousands of error messages, and can whip up test data in seconds. Your job is asking the right questions, and that's a skill you can get good at.

Writing Tests with Claude

One of the best ways to use Claude is having it write full test suites for your code. Instead of typing every test by hand, just describe what you want tested.

Example

// Your prompt to Claude:
"Write unit tests for this function using Vitest.
Include happy path, edge cases, and error cases."

function validateEmail(email: string): boolean {
  const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
  return regex.test(email);
}

And Claude will come back with tests like these.

Example

import { describe, it, expect } from 'vitest';
import { validateEmail } from './validate';

describe('validateEmail', () => {
  // Happy path
  it('should accept valid email addresses', () => {
    expect(validateEmail('user@example.com')).toBe(true);
    expect(validateEmail('first.last@domain.co.uk')).toBe(true);
  });

  // Edge cases
  it('should reject emails without @', () => {
    expect(validateEmail('userexample.com')).toBe(false);
  });

  it('should reject emails with spaces', () => {
    expect(validateEmail('user @example.com')).toBe(false);
  });

  // Error cases
  it('should reject empty string', () => {
    expect(validateEmail('')).toBe(false);
  });
});

Debugging Errors with Claude

When you hit an error that makes zero sense, paste the full error message and the related code into Claude.

Example

// Your prompt:
"I'm getting this error. What's wrong and how do I fix it?"

Error: Hydration failed because the server rendered text didn't
match the client.

// Component code:
function Greeting() {
  return <p>Hello, it is {new Date().toLocaleTimeString()}</p>;
}

Claude will walk you through it.

The time is different on the server vs the client (the server renders first, then the client takes over)
That mismatch between the server HTML and the client HTML is what causes the hydration error
The fix is to use useEffect so the time only gets set on the client side

Generating Test Data

Need realistic data for your tests? Claude can make it in seconds.

Example

// Your prompt:
"Generate 5 realistic test user objects with name, email,
age, and role. Include edge cases like very long names
and special characters."

Here's what comes back.

Example

export const testUsers = [
  { name: 'Alice Johnson', email: 'alice@example.com', age: 28, role: 'user' },
  { name: 'Bob O\'Brien-Smith', email: 'bob.obrien@company.co.uk', age: 65, role: 'admin' },
  { name: 'A', email: 'a@b.co', age: 18, role: 'user' },  // Minimum lengths
  { name: 'Maria del Carmen Rodriguez Fernandez', email: 'maria@test.com', age: 42, role: 'moderator' },
  { name: '张三', email: 'zhangsan@example.cn', age: 30, role: 'user' }  // Unicode name
];

Explaining Stack Traces

Stack traces can be scary looking, ESPECIALLY when they're full of framework guts. Claude can translate them into plain English.

Example

// Your prompt:
"Explain this stack trace in simple terms. Where is my bug?"

Unhandled Runtime Error
TypeError: Cannot destructure property 'data' of 'undefined'
    at UserDashboard (src/pages/dashboard.tsx:23:11)
    at renderWithHooks (react-dom-client.development.js:14985:18)
    at updateFunctionComponent (react-dom-client.development.js:17811:13)
    at beginWork (react-dom-client.development.js:19049:16)

Claude will break it down.

On line 23 of dashboard.tsx, you're destructuring data from something that's undefined
That probably means an API call or hook hasn't returned its data yet
The fix is a loading check before you destructure

Using Claude for Test-Driven Development

You can even use Claude for TDD (Test-Driven Development), where the tests get written first.

Example

// Your prompt:
"I want to build a shopping cart. Write the tests first,
then I'll implement the code to make them pass.
Features: add items, remove items, calculate total, apply discount codes."

Now you've got a clear target to code against, and every feature is tested from day one.

Effective Prompting for Testing

Here are some templates for the testing and debugging requests you'll make most.

Step 1. For Writing Tests

"Write [unit/integration] tests for this [function/component/API endpoint] using [Vitest/Jest]. Cover happy path, edge cases, and error handling."

Step 2. For Debugging

"I expected [X] to happen but got [Y] instead. Here's my code and the error message. What's wrong?"

Step 3. For Code Review

"Review this test file. Are there any missing test cases? Are the assertions correct? What edge cases am I missing?"

Step 4. For Understanding

"Explain what this test is checking and why it might be failing. I'm new to testing."

Common Patterns to Ask Claude About

"Why is my mock not being called?"
"How do I test a component that uses React context?"
"What is the difference between shallow and deep rendering?"
"How do I test a custom hook?"
"My test passes locally but fails in CI. What could cause this?"
"How do I test code that depends on the current date?"

TL;DR

Claude can write full test suites from your code
Paste the full error message and the code for the best debugging help
Use Claude to make realistic test data, edge cases included
Claude can explain gnarly stack traces in plain English
Good prompts include context, code and what you expected to happen
ALWAYS review generated code before it goes into your project

What's Next?

And that's a wrap on Testing and Debugging! You know why testing matters, how to write unit and integration tests, the common testing patterns, how to debug like a detective, which tools to reach for, and how to put Claude to work as your testing partner.

Here's my challenge for ya. Start your next project with tests on day one... future you is gonna be REAL grateful.

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