List types in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query with list types?

Given the following GraphQL schema snippet:

type Query {
  numbers: [Int!]!
}

And the resolver returns: [1, 2, 3]

What will be the result of the query { numbers }?

GraphQL

{ numbers }

A {"data":{"numbers":[1,2,3]}}

B {"data":{"numbers":null}}

C {"errors":[{"message":"Cannot return null for non-nullable field Query.numbers"}]}

D {"data":{"numbers":[1,null,3]}}

💡 Hint

Remember that [Int!]! means the list itself cannot be null and no element inside can be null.

The schema specifies numbers as a non-null list of non-null integers. The resolver returns a valid list of integers without nulls, so the query returns the list inside data.

📝 Syntax

intermediate

Which GraphQL list type syntax is valid for a list of nullable strings?

Choose the correct GraphQL type syntax for a field that returns a list which can be null, and whose elements can also be null strings.

A [String]!

B [String!]

C [String]

D [String!]!

💡 Hint

Think about which parts are nullable: the list itself and the elements inside.

[String] means the list can be null and elements can be null. Other options make either the list or elements non-null.

❓ optimization

advanced

How to optimize a GraphQL query fetching a large list with pagination?

You have a GraphQL field items: [Item!]! returning thousands of items. Which approach optimizes performance best?

A Use a single query without pagination and cache the result

B Use arguments like <code>first</code> and <code>after</code> to paginate the list

C Change the schema to <code>items: [Item]</code> to allow nulls

D Fetch all items at once and filter on the client side

💡 Hint

Think about how to reduce data transfer and server load.

Pagination with first and after arguments limits the number of items returned, improving performance and user experience.

🔧 Debug

advanced

Why does this GraphQL query return an error with list types?

Given the schema:

type Query {
  users: [User!]!
}

type User {
  id: ID!
  name: String!
}

The resolver returns [{id: "1", name: "Alice"}, null]. What error occurs and why?

A Error: Field users must return a nullable list

B No error, returns list with null element

C Error: Missing required field id in User

D Error: Cannot return null for non-nullable field User in list

💡 Hint

Check the non-null constraints on list elements.

The list is non-null and elements are non-null. Returning a null element violates User! and causes an error.

🧠 Conceptual

expert

What is the difference between [String!] and [String] in GraphQL?

Explain the difference between the GraphQL list types [String!] and [String] in terms of nullability of the list and its elements.

A <p><code>[String!]</code> means the list can be null but elements cannot be null; <code>[String]</code> means both list and elements can be null.</p>

B <p><code>[String!]</code> means the list cannot be null but elements can be null; <code>[String]</code> means list can be null but elements cannot be null.</p>

C <p><code>[String!]</code> means the list can be null and elements can be null; <code>[String]</code> means list cannot be null and elements cannot be null.</p>

D <p><code>[String!]</code> means the list cannot be null and elements cannot be null; <code>[String]</code> means list cannot be null but elements can be null.</p>

💡 Hint

Focus on which parts the exclamation mark applies to: the list or the elements.

[String!] means a list that may be null but all elements inside must be non-null strings. [String] means the list may be null and elements may also be null.