Default resolvers in GraphQL - Mini Project: Build & Apply

GraphQL Default Resolvers Practice

📖 Scenario: You are building a simple GraphQL API for a bookstore. The data is stored in a JavaScript object. You want to use GraphQL's default resolvers to fetch book information without writing custom resolver functions.

🎯 Goal: Create a GraphQL schema with types and queries that use default resolvers to return book data from a JavaScript object.

📋 What You'll Learn

Create a Book type with fields id, title, and author.

Create a Query type with a field books that returns a list of Book.

Create a JavaScript object bookData with exactly three books, each having id, title, and author.

Set up the GraphQL server schema using the default resolvers without writing any custom resolver functions.

Add a query to fetch all books using the default resolver.

💡 Why This Matters

🌍 Real World

GraphQL default resolvers let you quickly build APIs when your data structure matches your schema, saving time and effort.

💼 Career

Understanding default resolvers is essential for backend developers working with GraphQL to efficiently expose data without unnecessary code.

1

Create the book data object

Create a JavaScript constant called bookData that is an array of three objects. Each object must have the exact keys and values: { 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 const bookData = [ ... ] with three objects inside the array.

2

Define the GraphQL schema types

Create a GraphQL schema string called typeDefs that defines a Book type with fields id (ID!), title (String!), and author (String!). Also define a Query type with a field books that returns a list of Book (use square brackets).

GraphQL

const bookData = [
  { 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 backticks to create a multi-line string for typeDefs. Define the Book and Query types exactly as described.

3

Set up the resolver map using default resolvers

Create a JavaScript object called resolvers with a Query field. Inside Query, create a field books that returns the bookData array. Do not write any custom resolver functions for the Book fields.

GraphQL

const bookData = [
  { 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 typeDefs = `
  type Book {
    id: ID!
    title: String!
    author: String!
  }

  type Query {
    books: [Book]
  }
`;

# Your code here

Hint

Define resolvers as an object with a Query property. Inside Query, add a books field that returns bookData.

4

Complete the GraphQL server setup

Create an Apollo Server instance called server using typeDefs and resolvers. Export the server as the default export.

GraphQL

const bookData = [
  { 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 typeDefs = `
  type Book {
    id: ID!
    title: String!
    author: String!
  }

  type Query {
    books: [Book]
  }
`;

const resolvers = {
  Query: {
    books: () => bookData
  }
};

# Your code here

Hint

Import ApolloServer, create a new server with typeDefs and resolvers, then export it as default.

Practice

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

easy

A. To write custom logic for every field in the schema

B. To automatically return data by matching schema fields to data keys

C. To validate user input before querying the database

D. To handle errors during query execution

Solution

1. Step 1: Understand default resolver behavior

   Default resolvers automatically map schema fields to matching data keys without extra code.

2. Step 2: Compare with other options

   Options B, C, and D describe custom logic, validation, and error handling, which are not default resolver roles.

3. Final Answer:

   To automatically return data by matching schema fields to data keys -> Option B

4. Quick Check:

   Default resolver = automatic field-to-key mapping [OK]

Hint: Default resolvers match schema fields to data keys automatically [OK]

Common Mistakes:

• Thinking default resolvers handle validation
• Confusing default resolvers with custom resolvers
• Assuming default resolvers manage errors

2. Which of the following is the correct way to define a GraphQL schema field that uses a default resolver?

easy

A. type Query { user: User() }

B. type Query { user: User @customResolver }

C. type Query { user: User }

D. type Query { user: User -> resolver }

Solution

1. Step 1: Identify correct schema syntax

   GraphQL schema fields are defined as fieldName: Type without extra syntax for default resolvers.

2. Step 2: Analyze options

   type Query { user: User } uses correct syntax. Options B, C, and D include invalid or non-standard syntax.

3. Final Answer:

   type Query { user: User } -> Option C

4. Quick Check:

   Default resolver needs simple field: Type syntax [OK]

Hint: Default resolvers need simple field: Type syntax in schema [OK]

Common Mistakes:

• Adding parentheses after type name
• Using custom directives without definition
• Using arrow or other symbols in schema

3. Given the schema type Query { book: Book } and data { book: { title: "1984", author: "Orwell" } }, what will the default resolver return for the field book?

medium

A. { title: "1984", author: "Orwell" }

B. "1984"

C. null

D. Error: Resolver not found

Solution

1. Step 1: Understand default resolver behavior

   The default resolver returns the data object matching the field name if it exists.

2. Step 2: Match field to data

   The field book matches the data key book which contains the object with title and author.

3. Final Answer:

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

4. Quick Check:

   Field book returns matching data object [OK]

Hint: Default resolver returns matching data object for field name [OK]

Common Mistakes:

• Returning only one property instead of whole object
• Expecting null if no custom resolver is defined
• Assuming error if resolver is not explicitly written

4. You wrote a GraphQL schema with type Query { user: User } but your query returns null for user. What is a likely cause related to default resolvers?

medium

A. The data source does not have a matching key named 'user'

B. You forgot to write a custom resolver for 'user'

C. The schema syntax is invalid

D. GraphQL does not support default resolvers

Solution

1. Step 1: Recall default resolver behavior

   Default resolvers return data matching the field name from the data source.

2. Step 2: Identify cause of null result

   If the data source lacks a key named 'user', the default resolver returns null.

3. Final Answer:

   The data source does not have a matching key named 'user' -> Option A

4. Quick Check:

   Missing data key causes null from default resolver [OK]

Hint: Check data keys match schema fields for default resolver to work [OK]

Common Mistakes:

• Assuming custom resolver is always required
• Blaming schema syntax without checking data
• Thinking default resolvers are unsupported

5. You have a schema type Query { product: Product } and data { product: { id: 1, name: "Pen", price: 2.5 } }. You want to return the price with a 10% discount but keep default resolvers for other fields. How should you implement this?

hard

A. Use default resolvers only; custom resolvers cannot be mixed

B. Write a custom resolver for the entire 'product' field in Query

C. Modify the data source to store discounted price

D. Write a custom resolver only for the 'price' field inside Product type

Solution

1. Step 1: Understand resolver layering

   Default resolvers work per field; you can override specific fields with custom resolvers.

2. Step 2: Apply custom resolver to 'price' only

   Write a custom resolver for 'price' to apply discount, leaving other fields to default resolvers.

3. Step 3: Avoid overriding entire 'product' resolver

   Overriding entire 'product' would require manually returning all fields, losing default resolver benefits.

4. Final Answer:

   Write a custom resolver only for the 'price' field inside Product type -> Option D

5. Quick Check:

   Custom resolver per field + default resolvers for others [OK]

Hint: Override only needed fields with custom resolvers, keep defaults for rest [OK]

Common Mistakes:

• Overriding entire parent field unnecessarily
• Trying to mix default and custom resolvers at root level only
• Modifying data source instead of resolver logic