Context argument in GraphQL

Introduction

The context argument helps share common data or tools across all parts of a GraphQL request. It makes it easy to access things like user info or database connections without repeating code.

When you want to check who is making a request to control access.

When you need to share a database connection with all resolvers.

When you want to pass user settings or preferences to many parts of your API.

When you want to log or track requests in one place.

When you want to share helper functions or services across resolvers.

Syntax

GraphQL

const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: ({ req }) => {
    // create and return context object
    return { user: getUserFromReq(req), db };
  }
});

The context function runs once per request and returns an object.

Resolvers receive this context as their third argument.

Examples

Simple context returning a database connection.

GraphQL

context: () => ({ db: myDatabase })

Context that extracts user info from request headers.

GraphQL

context: ({ req }) => {
  const user = authenticate(req.headers.authorization);
  return { user };
}

Context with user info and helper functions.

GraphQL

context: ({ req }) => {
  return {
    user: getUser(req),
    helpers: { formatDate, calculateAge }
  };
}

Sample Program

This example shows how the context argument passes user info to resolvers. The resolver returns the user's name if logged in, or 'Guest' otherwise.

GraphQL

const { ApolloServer, gql } = require('apollo-server');

const typeDefs = gql`
  type Query {
    currentUserName: String
  }
`;

const resolvers = {
  Query: {
    currentUserName: (parent, args, context) => {
      return context.user ? context.user.name : 'Guest';
    }
  }
};

function getUserFromReq(req) {
  // Simulate user extraction
  if (req.headers.authorization === 'token123') {
    return { name: 'Alice' };
  }
  return null;
}

const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: ({ req }) => {
    const user = getUserFromReq(req);
    return { user };
  }
});

// Simulate a request with authorization header
const fakeReq = { headers: { authorization: 'token123' } };

server.executeOperation({ query: '{ currentUserName }' }, { req: fakeReq }).then(response => {
  console.log(response.data);
});

Important Notes

Context is created fresh for each request to avoid sharing data between users.

Use context to keep your resolvers clean and focused on data fetching.

Summary

The context argument shares common data for all resolvers during a request.

It is useful for authentication, database access, and shared helpers.

Resolvers receive context as their third argument to use shared info easily.

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