Resolver unit tests in GraphQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a basic resolver function that returns a fixed string.

GraphQL

const resolvers = { Query: { hello: () => [1] } };

Drag options to blanks, or click blank then click option'

A"Hello, world!"

BHello, world!

Creturn "Hello, world!"

Dconsole.log("Hello, world!")

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string

Using console.log instead of returning a value

The resolver must return a string value. Using quotes returns the string correctly.

2fill in blank

medium

Complete the resolver to accept arguments and return the name argument.

GraphQL

const resolvers = { Query: { greet: (_, [1]) => name } };

Drag options to blanks, or click blank then click option'

Acontext

Bargs

C{ name }

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong parameter name

Not destructuring the argument object

The resolver receives arguments as the second parameter. Destructuring { name } extracts the name argument.

3fill in blank

hard

Fix the error in the resolver that tries to return a user by id from a data source.

GraphQL

const resolvers = { Query: { user: (_, { id }, [1]) => [1].dataSources.userAPI.getUserById(id) } };

Drag options to blanks, or click blank then click option'

Ainfo

Bcontext

Cargs

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter name for context

Trying to access dataSources from args

The third parameter is the context, which contains dataSources. Using 'context' allows access to userAPI.

4fill in blank

hard

Fill both blanks to write a test that mocks the data source and checks the resolver output.

GraphQL

test('user resolver returns user data', async () => {
  const mockUser = { id: '1', name: 'Alice' };
  const context = { dataSources: { userAPI: { getUserById: jest.fn().mockResolvedValue([1]) } } } };
  const result = await resolvers.Query.user(null, { id: '1' }, [2]);
  expect(result).toEqual(mockUser);
});

Drag options to blanks, or click blank then click option'

AmockUser

Bcontext

CmockUser.id

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Not passing context to the resolver

Mocking the wrong return value

The mockResolvedValue should return the mockUser object. The context parameter must be passed to the resolver.

5fill in blank

hard

Fill all three blanks to write a test that verifies the resolver throws an error when user is not found.

GraphQL

test('user resolver throws error if user not found', async () => {
  const context = { dataSources: { userAPI: { getUserById: jest.fn().mockResolvedValue([1]) } } } };
  await expect(resolvers.Query.user(null, { id: '2' }, [2])).rejects.toThrow([3]);
});

Drag options to blanks, or click blank then click option'

Anull

Bcontext

C"User not found"

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Not mocking null return value

Not passing context to resolver

Expecting wrong error message

The mock returns null to simulate no user found. The context is passed to the resolver. The test expects an error with message 'User not found'.

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