Mocking resolvers in GraphQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a mock resolver for a GraphQL query named hello that returns a string.

GraphQL

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

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

Atrue

B12345

C"Hello, world!"

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a number or boolean instead of a string.

Not using quotes around the string value.

The mock resolver for a string field should return a string value, such as "Hello, world!".

2fill in blank

medium

Complete the code to mock a resolver for a user query that returns an object with a name field.

GraphQL

const mocks = {
  Query: {
    user: () => ({ name: [1] })
  }
};

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

Anull

B123

Cfalse

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a number or boolean instead of a string.

Returning null which may cause errors if not handled.

The name field expects a string, so the mock should return a string like "Alice".

3fill in blank

hard

Fix the error in the mock resolver that should return a list of strings for the tags field.

GraphQL

const mocks = {
  Query: {
    tags: () => [1]
  }
};

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

A["graphql", "mock", "resolver"]

B"graphql, mock, resolver"

Cnull

D123

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a single string instead of an array.

Returning null or a number which does not match the expected type.

The tags field expects a list of strings, so the mock should return an array of strings.

4fill in blank

hard

Fill both blanks to mock a resolver for a post query returning an object with title and likes fields.

GraphQL

const mocks = {
  Query: {
    post: () => ({ title: [1], likes: [2] })
  }
};

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

A"GraphQL Basics"

B42

C"100"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string for the likes field.

Using a boolean for either field.

The title field expects a string, so use a string value. The likes field expects a number, so use a number like 42.

5fill in blank

hard

Fill all three blanks to mock a resolver for a comment query returning an object with author, content, and likes fields.

GraphQL

const mocks = {
  Query: {
    comment: () => ({ author: [1], content: [2], likes: [3] })
  }
};

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

A"Bob"

B"Nice post!"

C10

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a boolean for any field.

Using numbers for string fields.

The author and content fields expect strings, so use string values. The likes field expects a number, so use a number like 10.

Practice

1. What is the main purpose of mocking resolvers in GraphQL?

easy

A. To simulate API responses without needing real data

B. To optimize database queries for faster performance

C. To secure the API by hiding sensitive data

D. To automatically generate GraphQL schemas

Solution

1. Step 1: Understand mocking resolvers

   Mocking resolvers are used to create fake data responses for GraphQL fields without connecting to a real database.

2. Step 2: Identify the main purpose

   This helps frontend developers test and build UI without waiting for backend data.

3. Final Answer:

   To simulate API responses without needing real data -> Option A

4. Quick Check:

   Mocking = Simulate data [OK]

Hint: Mocks simulate data, not optimize or secure APIs [OK]

Common Mistakes:

• Confusing mocking with database optimization
• Thinking mocks secure the API
• Assuming mocks generate schemas automatically

2. Which of the following is the correct way to define a mock resolver for a GraphQL field user that returns a fixed name?

easy

A. const mocks = { Query: { user: () => 'Alice' } };

B. const mocks = { user: { Query: () => 'Alice' } };

C. const mocks = { Query: { user: 'Alice' } };

D. const mocks = { Query: { user: () => ({ name: 'Alice' }) } };

Solution

1. Step 1: Understand mock resolver structure

   Mock resolvers are objects where the type (e.g., Query) maps to functions returning objects matching the schema.

2. Step 2: Check the correct syntax

   The user field should be a function returning an object with a name property, so const mocks = { Query: { user: () => ({ name: 'Alice' }) } }; is correct.

3. Final Answer:

   const mocks = { Query: { user: () => ({ name: 'Alice' }) } }; -> Option D

4. Quick Check:

   Mock resolver returns object with fields [OK]

Hint: Mock functions return objects matching schema fields [OK]

Common Mistakes:

• Returning a string instead of an object
• Swapping Query and user keys
• Not using a function for the resolver

3. Given the mock resolver below, what will be the output of the GraphQL query { book { title author } }?

const mocks = {
  Query: {
    book: () => ({ title: '1984', author: 'George Orwell' })
  }
};

medium

A. { "data": { "book": { "title": "1984", "author": "George Orwell" } } }

B. { "data": { "book": "1984" } }

C. { "data": { "book": { "title": "1984" } } }

D. Error: Resolver must return a string

Solution

1. Step 1: Analyze the mock resolver return value

   The book resolver returns an object with title and author fields as strings.

2. Step 2: Match query fields with returned object

   The query requests title and author, both present in the returned object, so the output includes both.

3. Final Answer:

   { "data": { "book": { "title": "1984", "author": "George Orwell" } } } -> Option A

4. Quick Check:

   Returned object matches query fields [OK]

Hint: Mock returns object matching query fields exactly [OK]

Common Mistakes:

• Expecting only one field returned
• Thinking resolver must return string
• Ignoring requested fields in query

4. Identify the error in the following mock resolver code snippet:

const mocks = {
  Query: {
    user: () => {
      name: 'Bob'
    }
  }
};

medium

A. Extra comma after 'name' property

B. Missing return statement inside the user resolver function

C. Resolver should return a string, not an object

D. Incorrect key name; should be 'User' instead of 'user'

Solution

1. Step 1: Check function body syntax

   The user resolver uses curly braces but does not return an object explicitly.

2. Step 2: Understand JavaScript function return rules

   Without a return statement, the function returns undefined, causing the mock to fail.

3. Final Answer:

   Missing return statement inside the user resolver function -> Option B

4. Quick Check:

   Functions with braces need explicit return [OK]

Hint: Use return or parentheses for object in arrow functions [OK]

Common Mistakes:

• Assuming implicit return with braces
• Confusing type names case sensitivity
• Expecting string return instead of object

5. You want to mock a GraphQL resolver for a product field that returns a list of products with id and price. Which mock resolver correctly returns two products with ids 1 and 2 and prices 10.5 and 20.0 respectively?

hard

A. const mocks = { Query: { product: [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } };

B. const mocks = { Query: { product: () => ({ id: [1, 2], price: [10.5, 20.0] }) } };

C. const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } };

D. const mocks = { Query: { product: () => { id: 1; price: 10.5; } } };

Solution

1. Step 1: Understand the expected return type

   The product field should return a list (array) of objects, each with id and price fields.

2. Step 2: Check each option's return value

   const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } }; returns an array of two objects with correct fields and values. Others either return wrong types or syntax errors.

3. Final Answer:

   const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } }; -> Option C

4. Quick Check:

   Return array of objects for list fields [OK]

Hint: Return array of objects for list fields in mocks [OK]

Common Mistakes:

• Returning object with arrays instead of array of objects
• Assigning array directly without function
• Missing return or using wrong syntax in function