Input validation patterns in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this GraphQL query with input validation?

Given the following GraphQL schema snippet that validates an input argument age to be a positive integer, what will be the result of this query?

type Query {
  user(age: Int!): String
}

# Resolver logic:
# if age <= 0, return error "Invalid age"
# else return "User age is <age>"

Query:

{ user(age: -5) }

A Error: age must be an integer

B User age is -5

C null

D Error: Invalid age

💡 Hint

Think about how the resolver handles invalid input values.

The resolver checks if the age is less than or equal to zero and returns an error message. Since -5 is invalid, it returns "Error: Invalid age".

📝 Syntax

intermediate

Which input validation directive syntax is correct in GraphQL?

Choose the correct syntax to validate that a string input email matches a simple email pattern using a custom directive @pattern.

A input UserInput { email: String! @pattern(regex: "^[^@\s]+@[^@\s]+\.[^@\s]+$") }

B input UserInput { email: String! @pattern(value: "^[^@\s]+@[^@\s]+\.[^@\s]+$") }

C input UserInput { email: String! @validate(regex: "^[^@\s]+@[^@\s]+\.[^@\s]+$") }

D input UserInput { email: String! @pattern("^[^@\s]+@[^@\s]+\.[^@\s]+$") }

💡 Hint

Look for the correct argument name and syntax for directives.

The directive uses the argument name regex with colon syntax. Option A matches this pattern correctly.

🧠 Conceptual

advanced

Why is input validation important in GraphQL APIs?

Which of the following is the best reason to implement input validation in GraphQL APIs?

A To prevent invalid or malicious data from causing errors or security issues

B To make queries run faster by skipping validation

C To allow clients to send any data without restrictions

D To reduce the size of the GraphQL schema

💡 Hint

Think about security and data integrity.

Input validation helps ensure only valid data is processed, preventing errors and security vulnerabilities.

🔧 Debug

advanced

Identify the error in this GraphQL input validation resolver

Consider this resolver function for a mutation that validates a username length:

async function createUser(parent, args) {
  if (args.username.length < 3) {
    throw new Error("Username too short");
  }
  // create user logic
  return { id: 1, username: args.username };
}

What error will occur if args.username is null?

A No error, user created successfully

B Error: Username too short

C TypeError: Cannot read property 'length' of null

D SyntaxError: Unexpected token

💡 Hint

What happens when you try to access .length of null?

Accessing length on null causes a TypeError because null has no properties.

❓ optimization

expert

How to optimize input validation for nested GraphQL inputs?

You have a mutation that accepts a nested input object with multiple fields requiring validation. Which approach optimizes validation performance and maintainability?

A Validate all fields manually inside the resolver function with nested if statements

B Use reusable custom scalar types and directives for validation on each field

C Skip validation and trust client inputs to reduce server load

D Validate only the top-level input object and ignore nested fields

💡 Hint

Think about reusability and separation of concerns.

Using custom scalars and directives allows declarative, reusable validation close to the schema, improving performance and maintainability.

Practice

1. What is the main purpose of input validation in GraphQL schemas?

easy

A. To ensure data is safe and correct before processing

B. To speed up query execution

C. To automatically generate UI forms

D. To store data in multiple databases

Solution

1. Step 1: Understand input validation role

   Input validation checks data before it is saved or used to avoid errors or security issues.

2. Step 2: Identify main goal in GraphQL

   GraphQL uses input validation to keep data safe and correct before processing.

3. Final Answer:

   To ensure data is safe and correct before processing -> Option A

4. Quick Check:

   Input validation = data safety and correctness [OK]

Hint: Input validation means checking data before use [OK]

Common Mistakes:

• Confusing validation with performance optimization
• Thinking validation auto-generates UI
• Assuming validation stores data

2. Which of the following is a valid way to enforce input validation in a GraphQL schema?

easy

A. Using HTML tags in schema definitions

B. Writing SQL queries inside the schema

C. Adding CSS styles to input fields

D. Using custom scalar types for specific formats

Solution

1. Step 1: Review GraphQL schema capabilities

   GraphQL schemas can define custom scalar types to validate formats like email or date.

2. Step 2: Eliminate invalid options

   SQL queries, CSS, and HTML are unrelated to schema validation.

3. Final Answer:

   Using custom scalar types for specific formats -> Option D

4. Quick Check:

   Custom scalars = validation in schema [OK]

Hint: Custom scalars validate input formats in GraphQL [OK]

Common Mistakes:

• Confusing schema with UI styling
• Trying to embed SQL in schema
• Using HTML tags in schema definitions

3. Given this GraphQL input type and resolver snippet, what happens if the input 'username' is an empty string?

input UserInput {
  username: String!
}

resolver createUser(input: UserInput) {
  if (input.username.length === 0) {
    throw new Error('Username cannot be empty');
  }
  return saveUser(input);
}

medium

A. The resolver ignores the username field

B. The resolver throws an error and user is not saved

C. The user is saved with an empty username

D. The schema rejects the query before resolver runs

Solution

1. Step 1: Analyze resolver input check

   The resolver checks if username length is zero and throws an error if true.

2. Step 2: Understand effect of empty string input

   Empty string triggers the error, so user is not saved.

3. Final Answer:

   The resolver throws an error and user is not saved -> Option B

4. Quick Check:

   Empty username triggers error in resolver [OK]

Hint: Empty string triggers error in resolver check [OK]

Common Mistakes:

• Assuming schema rejects empty string automatically
• Thinking empty username is saved
• Ignoring resolver error handling

4. Identify the error in this GraphQL input validation snippet:

input ProductInput {
  price: Float!
}

resolver addProduct(input: ProductInput) {
  if (input.price < 0) {
    return 'Price must be positive';
  }
  saveProduct(input);
  return 'Product added';
}

medium

A. Returning a string error instead of throwing an error

B. Missing input type declaration

C. Using Float instead of Int for price

D. No validation for price being zero

Solution

1. Step 1: Check error handling in resolver

   The resolver returns a string on error instead of throwing an error, which may not stop execution properly.

2. Step 2: Understand proper error signaling

   Throwing an error is standard to halt processing and signal failure in GraphQL.

3. Final Answer:

   Returning a string error instead of throwing an error -> Option A

4. Quick Check:

   Errors should be thrown, not returned as strings [OK]

Hint: Throw errors, don't return error strings in resolvers [OK]

Common Mistakes:

• Returning error messages instead of throwing
• Confusing Float and Int types
• Ignoring zero price validation

5. You want to enforce that a user's email input is always lowercase and matches a valid email format in GraphQL. Which combination is the best approach?

hard

A. Use a directive to reject any uppercase letters without transformation

B. Only rely on GraphQL's String type without extra checks

C. Use a custom scalar for email format and transform input to lowercase in resolver

D. Validate email format in the database after saving

Solution

1. Step 1: Understand validation needs

   Email must be valid format and lowercase before saving or processing.

2. Step 2: Choose best GraphQL validation method

   Custom scalar can enforce format; resolver can transform input to lowercase.

3. Step 3: Evaluate other options

   Relying only on String misses validation; directives alone can't transform; database validation is late.

4. Final Answer:

   Use a custom scalar for email format and transform input to lowercase in resolver -> Option C

5. Quick Check:

   Custom scalar + resolver transform = best validation [OK]

Hint: Combine custom scalar and resolver for format and case [OK]

Common Mistakes:

• Skipping format validation
• Expecting directives to transform input
• Validating only after saving data