Context setup in GraphQL

Introduction

Context setup helps share information like user details or database connections across all parts of a GraphQL server.

When you want to know which user is making a request to check permissions.

When you need to reuse a database connection for all queries in one request.

When you want to pass common data like language or timezone to all resolvers.

When you want to log or track requests with shared info.

When you want to keep your code clean by avoiding passing data manually to every resolver.

Syntax

GraphQL

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

The context function runs for every request and returns an object shared by all resolvers.

You can add anything to context like user info, database clients, or helpers.

Examples

Simple context with a database client shared to all resolvers.

GraphQL

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

Context that extracts user info from request headers for authentication.

GraphQL

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

Async context example to fetch user data before resolvers run.

GraphQL

context: async ({ req }) => {
  const user = await getUserFromToken(req.headers.token);
  return { user };
}

Sample Program

This example sets up a GraphQL server with context that reads user info from request headers. The query returns the current user's name or 'Guest' if no user is found.

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 === 'secret-token') {
    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: 'secret-token' } };

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

Important Notes

Context runs once per request, so avoid putting heavy operations inside it.

Always return a plain object from context for easy access in resolvers.

You can pass extra info like loaders or helpers in context to keep resolvers clean.

Summary

Context shares data like user info or database clients across all resolvers.

It runs once per request and returns an object accessible in every resolver.

Use context to keep your GraphQL server organized and secure.

Practice

1. What is the main purpose of the context in a GraphQL server?

easy

A. To share data like user info or database clients across all resolvers

B. To define the GraphQL schema types

C. To write the query resolvers

D. To format the GraphQL query response

Solution

1. Step 1: Understand what context provides

   Context is an object that runs once per request and is accessible in every resolver, allowing shared data like user info or database clients.

2. Step 2: Compare with other options

   Defining schema types, writing resolvers, and formatting responses are separate tasks not handled by context.

3. Final Answer:

   To share data like user info or database clients across all resolvers -> Option A

4. Quick Check:

   Context shares data across resolvers [OK]

Hint: Context shares common data across resolvers per request [OK]

Common Mistakes:

• Confusing context with schema definition
• Thinking context formats responses
• Assuming context writes resolver logic

2. Which of the following is the correct way to define a context function in Apollo Server?

easy

A. context = { user: getUser() }

B. context: () => ({ user: getUser() })

C. context: { user: getUser() }

D. context() { return user: getUser() }

Solution

1. Step 1: Recall Apollo Server context syntax

   Context is defined as a function returning an object, e.g., context: () => ({ user: getUser() }).

2. Step 2: Identify syntax errors in other options

   context = { user: getUser() } uses assignment instead of a function, C uses an object directly without a function, and D has invalid syntax missing parentheses and braces.

3. Final Answer:

   context: () => ({ user: getUser() }) -> Option B

4. Quick Check:

   Context is a function returning an object [OK]

Hint: Context must be a function returning an object [OK]

Common Mistakes:

• Defining context as a plain object
• Missing parentheses in function syntax
• Using assignment instead of property

3. Given this context setup in Apollo Server:

context: () => ({ user: { id: 5, name: 'Alice' } })

What will be the value of context.user.name inside a resolver?

medium

A. undefined

B. 5

C. null

D. 'Alice'

Solution

1. Step 1: Analyze the context object

   The context function returns an object with a user property containing { id: 5, name: 'Alice' }.

2. Step 2: Access user.name in resolver

   Accessing context.user.name will give the string 'Alice' as defined.

3. Final Answer:

   'Alice' -> Option D

4. Quick Check:

   context.user.name = 'Alice' [OK]

Hint: Context returns user object; access properties directly [OK]

Common Mistakes:

• Confusing user id with name
• Assuming context is undefined
• Expecting null instead of string

4. You wrote this context function:

context: () => { user: getUser() }

Why does context.user return undefined in resolvers?

medium

A. Because user is not a valid property

B. Because getUser() is undefined

C. Because the function does not return the object explicitly

D. Because context must be an object, not a function

Solution

1. Step 1: Check function return behavior

   The function uses curly braces but lacks a return statement, so it returns undefined by default.

2. Step 2: Understand impact on context

   Since nothing is returned, context.user is undefined in resolvers.

3. Final Answer:

   Because the function does not return the object explicitly -> Option C

4. Quick Check:

   Missing return in function = undefined context [OK]

Hint: Use return or parentheses to return object in arrow function [OK]

Common Mistakes:

• Omitting return in arrow function with braces
• Assuming getUser() is undefined without checking
• Thinking context must be an object, not function

5. You want to add a database client and user info to context for each request. Which is the best way to do this in Apollo Server?

hard

A. context: async () => { const db = await connectDB(); const user = await getUser(); return { db, user }; }

B. context: { db: connectDB(), user: getUser() }

C. context: () => { db: connectDB(); user: getUser(); }

D. context: () => ({ db: connectDB(), user: getUser() })

Solution

1. Step 1: Understand async context needs

   Connecting to a database and fetching user info are asynchronous operations, so context must be async and await these calls.

2. Step 2: Evaluate options for async correctness

   context: async () => { const db = await connectDB(); const user = await getUser(); return { db, user }; } correctly uses async function and awaits results before returning an object. context: { db: connectDB(), user: getUser() } is a plain object, not a function. context: () => { db: connectDB(); user: getUser(); } lacks return and awaits. context: () => ({ db: connectDB(), user: getUser() }) returns an object but does not await async calls.

3. Final Answer:

   context: async () => { const db = await connectDB(); const user = await getUser(); return { db, user }; } -> Option A

4. Quick Check:

   Async context with awaited data [OK]

Hint: Use async function and await for async data in context [OK]

Common Mistakes:

• Not awaiting async calls in context
• Using plain object instead of function
• Missing return statement in function