Resolver unit tests in GraphQL - Mini Project: Build & Apply

Resolver Unit Tests for GraphQL

📖 Scenario: You are building a simple GraphQL API for a bookstore. You want to make sure your resolver functions work correctly by writing unit tests for them.

🎯 Goal: Write unit tests for GraphQL resolvers that fetch book data from a mock database.

📋 What You'll Learn

Create a mock data array called books with 3 book objects

Create a configuration variable called bookIdToTest with the value 2

Write a resolver function called getBookById that returns the book matching bookIdToTest

Write a unit test function called testGetBookById that asserts the resolver returns the correct book

💡 Why This Matters

🌍 Real World

Unit testing GraphQL resolvers helps catch bugs early and ensures your API returns correct data.

💼 Career

Writing and testing resolvers is a key skill for backend developers working with GraphQL APIs.

1

Create mock book data

Create a list called books with exactly these three book objects: { id: 1, title: "1984", author: "George Orwell" }, { id: 2, title: "Brave New World", author: "Aldous Huxley" }, and { id: 3, title: "Fahrenheit 451", author: "Ray Bradbury" }.

GraphQL

// Your code here

Hint

Use an array of objects with the exact keys and values given.

2

Add test book ID configuration

Create a constant called bookIdToTest and set it to the number 2.

GraphQL

const books = [
  { id: 1, title: "1984", author: "George Orwell" },
  { id: 2, title: "Brave New World", author: "Aldous Huxley" },
  { id: 3, title: "Fahrenheit 451", author: "Ray Bradbury" }
];

// Your code here

Hint

Use const bookIdToTest = 2; exactly.

3

Write the resolver function

Write a function called getBookById that takes no parameters and returns the book object from books where id equals bookIdToTest. Use the find method.

GraphQL

const books = [
  { id: 1, title: "1984", author: "George Orwell" },
  { id: 2, title: "Brave New World", author: "Aldous Huxley" },
  { id: 3, title: "Fahrenheit 451", author: "Ray Bradbury" }
];

const bookIdToTest = 2;

// Your code here

Hint

Use books.find(book => book.id === bookIdToTest) inside the function.

4

Write the unit test function

Write a function called testGetBookById that calls getBookById() and checks if the returned book's title is exactly "Brave New World". Use an if statement to compare and throw an error with the message "Test failed" if it does not match.

GraphQL

const books = [
  { id: 1, title: "1984", author: "George Orwell" },
  { id: 2, title: "Brave New World", author: "Aldous Huxley" },
  { id: 3, title: "Fahrenheit 451", author: "Ray Bradbury" }
];

const bookIdToTest = 2;

function getBookById() {
  return books.find(book => book.id === bookIdToTest);
}

// Your code here

Hint

Call getBookById(), check the title, and throw an error if it is wrong.

Practice

1. What is the main purpose of a resolver unit test in GraphQL?

easy

A. To verify the database connection settings

B. To test the entire GraphQL schema at once

C. To check if a resolver returns the correct data

D. To style the GraphQL playground interface

Solution

1. Step 1: Understand resolver role

   Resolvers are functions that fetch and return data for GraphQL queries.

2. Step 2: Purpose of unit tests

   Unit tests check small parts of code, here specifically if resolvers return correct data.

3. Final Answer:

   To check if a resolver returns the correct data -> Option C

4. Quick Check:

   Resolver unit tests = check resolver output [OK]

Hint: Resolvers return data; tests check if data is correct [OK]

Common Mistakes:

• Confusing unit tests with integration tests
• Thinking tests check UI or styling
• Assuming tests check database setup

2. Which syntax correctly defines a simple resolver unit test using Jest?

easy

A. describe('Test', () => { it('checks resolver', () => { expect(resolver()).toBe(data); }); });

B. test('Test', () => { describe('checks resolver', () => { expect(resolver()).toBe(data); }); });

C. it('Test', () => { expect(resolver()).toBe(data); }); describe('checks resolver', () => {});

D. expect('Test', () => { it('checks resolver', () => { resolver(); }); });

Solution

1. Step 1: Identify Jest test structure

   Jest uses describe to group tests and it or test for individual tests.

2. Step 2: Check correct nesting and syntax

   describe('Test', () => { it('checks resolver', () => { expect(resolver()).toBe(data); }); }); correctly nests it inside describe and uses expect properly.

3. Final Answer:

   describe('Test', () => { it('checks resolver', () => { expect(resolver()).toBe(data); }); }); -> Option A

4. Quick Check:

   describe + it + expect = correct test syntax [OK]

Hint: Use describe for groups, it for tests, expect for checks [OK]

Common Mistakes:

• Swapping describe and it blocks
• Missing expect or incorrect nesting
• Using expect without a matcher

3. Given this resolver and test code, what will the test output be?

const resolver = () => ({ id: 1, name: 'Alice' });

describe('User resolver', () => {
  it('returns correct user', () => {
    expect(resolver()).toEqual({ id: 1, name: 'Alice' });
  });
});

medium

A. Test throws runtime error

B. Test fails due to wrong object

C. Syntax error in test code

D. Test passes successfully

Solution

1. Step 1: Analyze resolver output

   The resolver returns an object { id: 1, name: 'Alice' } exactly.

2. Step 2: Check test expectation

   The test expects the same object using toEqual, which compares object values deeply.

3. Final Answer:

   Test passes successfully -> Option D

4. Quick Check:

   Exact object match = test passes [OK]

Hint: toEqual checks deep equality; objects must match exactly [OK]

Common Mistakes:

• Confusing toBe with toEqual for objects
• Expecting test to fail with correct data
• Misreading object properties

4. Identify the error in this resolver unit test code:

describe('Test resolver', () => {
  it('returns data', () => {
    expect(resolver).toBe(data);
  });
});

medium

A. No error; code is correct

B. Missing parentheses to call resolver function

C. Wrong matcher; should use toEqual instead of toBe

D. Incorrect use of describe block

Solution

1. Step 1: Check resolver usage

   The test uses resolver without parentheses, so it tests the function itself, not its return value.

2. Step 2: Correct function call

   To test the returned data, the resolver must be called as resolver().

3. Final Answer:

   Missing parentheses to call resolver function -> Option B

4. Quick Check:

   Call resolver() to get data, not resolver [OK]

Hint: Call functions with () to test their output [OK]

Common Mistakes:

• Forgetting to call the resolver function
• Confusing toBe and toEqual for objects
• Misplacing describe and it blocks

5. You want to test a resolver that fetches a user by ID from a mock database. Which approach best ensures your unit test is isolated and reliable?

hard

A. Mock the database call inside the resolver test to return fixed data

B. Connect to the real database and fetch actual user data

C. Skip testing the resolver and test only the database separately

D. Write tests that depend on network availability

Solution

1. Step 1: Understand unit test isolation

   Unit tests should test one part only, without relying on external systems like databases.

2. Step 2: Use mocking for database calls

   Mocking replaces real database calls with fixed data, making tests fast and reliable.

3. Final Answer:

   Mock the database call inside the resolver test to return fixed data -> Option A

4. Quick Check:

   Mock external calls for isolated unit tests [OK]

Hint: Mock external dependencies to isolate resolver tests [OK]

Common Mistakes:

• Using real database in unit tests
• Skipping resolver tests entirely
• Writing flaky tests depending on network