Mocking resolvers in GraphQL - Time & Space Complexity

Time Complexity: Mocking resolvers

O(n)

Understanding Time Complexity

When we mock resolvers in GraphQL, we simulate data fetching without real database calls.

We want to know how the time to get data changes as we ask for more items.

Scenario Under Consideration

Analyze the time complexity of the following GraphQL mocking resolver snippet.

const mocks = {
  Query: () => ({
    books: () => new Array(booksCount).fill(null).map(() => ({
      title: 'Sample Book',
      author: 'Author Name'
    }))
  })
};
    

This code creates a list of books with fixed data, simulating a database response.

Identify Repeating Operations

Look for repeated actions in the code.

• Primary operation: Creating each book object in the array.
• How many times: Once for each book requested (booksCount times).

How Execution Grows With Input

As the number of books requested grows, the time to create them grows too.

Input Size (booksCount)	Approx. Operations
10	10 book objects created
100	100 book objects created
1000	1000 book objects created

Pattern observation: The work grows directly with the number of books requested.

Final Time Complexity

Time Complexity: O(n)

This means the time to mock data grows in a straight line as you ask for more items.

Common Mistake

[X] Wrong: "Mocking data is instant and does not depend on how many items we create."

[OK] Correct: Even though data is fake, creating each item takes time, so more items mean more work.

Interview Connect

Understanding how mocking scales helps you explain performance in testing and development setups.

Self-Check

What if we changed the mock to return a fixed small list regardless of the requested size? How would the time complexity change?

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