Context argument in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the context argument in GraphQL?

The context argument is an object passed to every resolver in a GraphQL server. It holds shared data like authentication info, database connections, or user details that resolvers can access.

beginner

Why do we use the context argument instead of passing data directly to resolvers?

Using context avoids repeating the same data in every resolver call. It provides a single place to store shared info, making code cleaner and easier to maintain.

intermediate

How is the context argument typically created in a GraphQL server?

The context is usually created once per request. It can include user authentication info, database clients, or other services needed by resolvers during that request.

beginner

Give an example of data you might store in the context argument.

You might store the current user's ID, their roles or permissions, a database connection object, or a logger instance in the context.

intermediate

Can the context argument change during a single GraphQL request?

No, the context is created once per request and remains the same for all resolvers during that request to ensure consistent data access.

What is the main purpose of the context argument in GraphQL?

A To store query results

B To define the GraphQL schema

C To format the response data

D To share data like authentication info across all resolvers

The context argument shares common data such as authentication info with all resolvers during a request.

When is the context argument created in a GraphQL server?

A Once per GraphQL request

B Once per server startup

C Once per resolver call

D Once per schema definition

The context is created once for each request to provide consistent data to all resolvers handling that request.

Which of the following is NOT typically stored in the context argument?

A User authentication info

B Database connection

C GraphQL query string

D Logger instance

The GraphQL query string is usually passed separately, not stored in context.

How do resolvers access the context argument?

A As the third argument

B As the first argument

C As the second argument

D As the fourth argument

Resolvers receive (parent, args, context, info) where context is the third argument.

Why is it better to use context instead of global variables for shared data?

A Global variables are recommended for security

B Context is request-specific and avoids conflicts between users

C Context is harder to use

D Global variables are faster

Context is created per request, so it keeps data isolated per user and avoids conflicts.

Explain what the context argument is in GraphQL and why it is useful.

Think about how multiple resolvers can access the same info without repeating it.

Describe how you would use the context argument to handle user authentication in a GraphQL server.

Consider how to get user data from a request and make it available to all resolvers.

Practice

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

easy

A. To format the response data before sending it to the client

B. To define the GraphQL schema types

C. To specify the query operation type (query, mutation, subscription)

D. To share common data like authentication or database access across all resolvers during a request

Solution

1. Step 1: Understand the role of context in GraphQL

   The context argument is designed to hold data shared by all resolvers during a single request, such as authentication info or database connections.

2. Step 2: Compare with other options

   Defining schema types, specifying operation types, or formatting responses are handled elsewhere, not by context.

3. Final Answer:

   To share common data like authentication or database access across all resolvers during a request -> Option D

4. Quick Check:

   Context = Shared data for resolvers [OK]

Hint: Context holds shared info for all resolvers in a request [OK]

Common Mistakes:

• Confusing context with schema definition
• Thinking context formats responses
• Assuming context defines operation types

2. Which of the following is the correct way to access the context argument in a GraphQL resolver function?

easy

A. resolver(parent, args, context) { ... }

B. resolver(context, args, parent) { ... }

C. resolver(args, parent, context) { ... }

D. resolver(args, context) { ... }

Solution

1. Step 1: Recall resolver function signature

   GraphQL resolvers receive arguments in the order: parent, args, context.

2. Step 2: Match the correct order

   Only resolver(parent, args, context) { ... } matches this order correctly.

3. Final Answer:

   resolver(parent, args, context) { ... } -> Option A

4. Quick Check:

   Resolver args order = parent, args, context [OK]

Hint: Remember resolver args order: parent, args, context [OK]

Common Mistakes:

• Swapping context and args positions
• Omitting the parent argument
• Using only two arguments instead of three

3. Given this resolver code snippet:

const resolver = (parent, args, context) => {
  return context.db.getUserById(args.id);
};

What will resolver(null, {id: 5}, context) return if context.db.getUserById(5) returns { id: 5, name: 'Alice' }?

medium

A. null

B. { id: 5, name: 'Alice' }

C. undefined

D. Error: getUserById is not a function

Solution

1. Step 1: Understand the resolver call

   The resolver calls context.db.getUserById with args.id which is 5.

2. Step 2: Check the return value from context

   Given context.db.getUserById(5) returns { id: 5, name: 'Alice' }, the resolver returns this object.

3. Final Answer:

   { id: 5, name: 'Alice' } -> Option B

4. Quick Check:

   Resolver returns context.db.getUserById(args.id) result [OK]

Hint: Resolver returns what context function returns for given args [OK]

Common Mistakes:

• Assuming resolver returns null or undefined
• Confusing parent argument with context
• Expecting an error without checking context structure

4. Consider this resolver code:

const resolver = (parent, args, context) => {
  return context.getUser(args.id);
};

If running this code causes TypeError: context.getUser is not a function, what is the most likely cause?

medium

A. The context object does not have a getUser method

B. The args.id is undefined

C. The parent argument is missing

D. The resolver function is missing the args parameter

Solution

1. Step 1: Analyze the error message

   The error says getUser is not a function on context, meaning context.getUser is undefined or not callable.

2. Step 2: Check other options

   Undefined args.id or missing parent won't cause this specific error. Missing args parameter would cause a different error.

3. Final Answer:

   The context object does not have a getUser method -> Option A

4. Quick Check:

   TypeError means missing method on context [OK]

Hint: TypeError means method missing on context object [OK]

Common Mistakes:

• Blaming args.id for method not found error
• Assuming parent argument causes this error
• Ignoring that context must have the method

5. You want to add user authentication info to all resolvers using the context argument. Which approach correctly sets up the context for this purpose?

hard

A. Pass user info as an argument in every query instead of context

B. Define user info inside each resolver separately without using context

C. Create context once per request including user info, then pass it to all resolvers

D. Use global variables to store user info accessible by resolvers

Solution

1. Step 1: Understand context usage for shared data

   Context is designed to be created once per request and shared with all resolvers, perfect for authentication info.

2. Step 2: Evaluate other options

   Defining user info inside each resolver duplicates code; passing user info as query args is insecure and inconvenient; global variables are unsafe for concurrent requests.

3. Final Answer:

   Create context once per request including user info, then pass it to all resolvers -> Option C

4. Quick Check:

   Context = single shared object per request [OK]

Hint: Set context once per request with user info for all resolvers [OK]

Common Mistakes:

• Duplicating user info in every resolver
• Passing sensitive info as query arguments
• Using global variables causing data leaks