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.

Practice

1. What does a list type in GraphQL represent?
[Int] means a list of what?

easy

A. A string value

B. A single integer

C. A collection of integers

D. A boolean value

Solution

1. Step 1: Understand list notation in GraphQL

   Square brackets [] indicate a list type that holds multiple values of the same type.

2. Step 2: Interpret [Int]

   This means a list containing zero or more integers.

3. Final Answer:

   A collection of integers -> Option C

4. Quick Check:

   List type = multiple values [OK]

Hint: Square brackets mean list of that type [OK]

Common Mistakes:

• Thinking it holds a single value
• Confusing list with string or boolean
• Ignoring the brackets meaning

2. Which of the following is the correct syntax to define a list of strings in GraphQL?

easy

A. string[]

B. [String]

C. (String[])

D. {String}

Solution

1. Step 1: Recall GraphQL list syntax

   GraphQL uses square brackets [] around a type to define a list.

2. Step 2: Identify correct option

   [String] is the correct way to define a list of strings in GraphQL.

3. Final Answer:

   [String] -> Option B

4. Quick Check:

   List syntax = brackets around type [OK]

Hint: Use square brackets around type for lists [OK]

Common Mistakes:

• Using language-specific syntax like string[]
• Using parentheses or curly braces instead of brackets
• Missing brackets entirely

3. Given this GraphQL schema snippet:

type Query {
  favoriteNumbers: [Int]
}

What is a valid response for favoriteNumbers?

medium

A. {1, 2, 3}

B. 42

C. "[1, 2, 3]"

D. [1, 2, 3]

Solution

1. Step 1: Understand the field type

   favoriteNumbers is a list of integers, so the response must be a list (array) of numbers.

2. Step 2: Check each option

   [1, 2, 3] is a list of integers. 42 is a single integer, not a list. "[1, 2, 3]" is a string, not a list. {1, 2, 3} uses curly braces which is invalid syntax.

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   List field returns list value [OK]

Hint: List fields return arrays, not single values [OK]

Common Mistakes:

• Returning a single value instead of a list
• Returning a string instead of a list
• Using wrong brackets for lists

4. Identify the error in this GraphQL list type declaration:

type User {
  tags: [String
}

medium

A. Missing closing bracket "]" for the list type

B. List types cannot hold strings

C. Square brackets should be replaced with parentheses

D. Field name cannot be "tags"

Solution

1. Step 1: Check list type syntax

   List types require both opening and closing square brackets around the type.

2. Step 2: Locate syntax error

   The closing bracket "]" is missing after String, causing a syntax error.

3. Final Answer:

   Missing closing bracket "]" for the list type -> Option A

4. Quick Check:

   List syntax needs matching brackets [OK]

Hint: Always close brackets in list types [OK]

Common Mistakes:

• Forgetting the closing bracket
• Using wrong brackets like parentheses
• Thinking list can't hold strings

5. You want to define a GraphQL field that can return null or a list of non-null integers, but the list itself can be empty. Which type declaration is correct?

hard

A. [Int!]

B. [Int]!

C. [Int!]!

D. Int!

Solution

1. Step 1: Understand non-null and list syntax

   Int! means non-null integer, [Int!] means list of non-null integers, list itself can be null or empty.

2. Step 2: Analyze options

   [Int!]: List of non-null integers, list nullable (can be empty).[Int]!: Non-null list of nullable integers.[Int!]!: Non-null list of non-null integers (list cannot be null).Int!: Single non-null integer, not a list.

3. Final Answer:

   [Int!] -> Option A

4. Quick Check:

   Non-null items inside nullable list = [Int!] [OK]

Hint: Non-null items inside list use [Type!] [OK]

Common Mistakes:

• Making list itself non-null when it should be nullable
• Confusing non-null item vs non-null list
• Using single type instead of list