Mocking resolvers in GraphQL

Introduction

Mocking resolvers helps you test and develop your GraphQL API without needing a real database or backend.

When you want to try out your GraphQL queries before the real data is ready.

When you need to test your frontend app with sample data quickly.

When you want to check how your API behaves with different data shapes.

When you want to develop parts of your app independently from the backend.

When you want to share a working API example without exposing real data.

Syntax

GraphQL

const mocks = {
  Query: {
    fieldName: () => mockedValue,
  },
  TypeName: {
    fieldName: () => mockedValue,
  },
};

Resolvers are functions that return data for each field in your GraphQL schema.

Mocking resolvers means writing simple functions that return fake data instead of real data.

Examples

This mocks the hello field in the Query type to always return 'Hello, world!'.

GraphQL

const mocks = {
  Query: {
    hello: () => 'Hello, world!'
  }
};

This mocks the User type fields name and age with fixed values.

GraphQL

const mocks = {
  User: {
    name: () => 'Alice',
    age: () => 30
  }
};

This mocks a randomNumber field to return a random number between 0 and 99 each time.

GraphQL

const mocks = {
  Query: {
    randomNumber: () => Math.floor(Math.random() * 100)
  }
};

Sample Program

This example creates a simple GraphQL server with a greeting field mocked to always return 'Hi there!'.

GraphQL

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

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

const mocks = {
  Query: {
    greeting: () => 'Hi there!'
  }
};

const server = new ApolloServer({ typeDefs, mocks });

server.listen().then(({ url }) => {
  console.log(`Server ready at ${url}`);
});

Important Notes

Mocking is great for early development but replace mocks with real resolvers when your backend is ready.

You can use libraries like graphql-tools to help create mocks easily.

Mocks can return static values or dynamic values like random numbers or dates.

Summary

Mocking resolvers lets you simulate API responses without real data.

Use mocks to speed up frontend development and testing.

Mocks are simple functions returning fake data for GraphQL fields.

Practice

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

easy

A. To simulate API responses without needing real data

B. To optimize database queries for faster performance

C. To secure the API by hiding sensitive data

D. To automatically generate GraphQL schemas

Solution

1. Step 1: Understand mocking resolvers

   Mocking resolvers are used to create fake data responses for GraphQL fields without connecting to a real database.

2. Step 2: Identify the main purpose

   This helps frontend developers test and build UI without waiting for backend data.

3. Final Answer:

   To simulate API responses without needing real data -> Option A

4. Quick Check:

   Mocking = Simulate data [OK]

Hint: Mocks simulate data, not optimize or secure APIs [OK]

Common Mistakes:

• Confusing mocking with database optimization
• Thinking mocks secure the API
• Assuming mocks generate schemas automatically

2. Which of the following is the correct way to define a mock resolver for a GraphQL field user that returns a fixed name?

easy

A. const mocks = { Query: { user: () => 'Alice' } };

B. const mocks = { user: { Query: () => 'Alice' } };

C. const mocks = { Query: { user: 'Alice' } };

D. const mocks = { Query: { user: () => ({ name: 'Alice' }) } };

Solution

1. Step 1: Understand mock resolver structure

   Mock resolvers are objects where the type (e.g., Query) maps to functions returning objects matching the schema.

2. Step 2: Check the correct syntax

   The user field should be a function returning an object with a name property, so const mocks = { Query: { user: () => ({ name: 'Alice' }) } }; is correct.

3. Final Answer:

   const mocks = { Query: { user: () => ({ name: 'Alice' }) } }; -> Option D

4. Quick Check:

   Mock resolver returns object with fields [OK]

Hint: Mock functions return objects matching schema fields [OK]

Common Mistakes:

• Returning a string instead of an object
• Swapping Query and user keys
• Not using a function for the resolver

3. Given the mock resolver below, what will be the output of the GraphQL query { book { title author } }?

const mocks = {
  Query: {
    book: () => ({ title: '1984', author: 'George Orwell' })
  }
};

medium

A. { "data": { "book": { "title": "1984", "author": "George Orwell" } } }

B. { "data": { "book": "1984" } }

C. { "data": { "book": { "title": "1984" } } }

D. Error: Resolver must return a string

Solution

1. Step 1: Analyze the mock resolver return value

   The book resolver returns an object with title and author fields as strings.

2. Step 2: Match query fields with returned object

   The query requests title and author, both present in the returned object, so the output includes both.

3. Final Answer:

   { "data": { "book": { "title": "1984", "author": "George Orwell" } } } -> Option A

4. Quick Check:

   Returned object matches query fields [OK]

Hint: Mock returns object matching query fields exactly [OK]

Common Mistakes:

• Expecting only one field returned
• Thinking resolver must return string
• Ignoring requested fields in query

4. Identify the error in the following mock resolver code snippet:

const mocks = {
  Query: {
    user: () => {
      name: 'Bob'
    }
  }
};

medium

A. Extra comma after 'name' property

B. Missing return statement inside the user resolver function

C. Resolver should return a string, not an object

D. Incorrect key name; should be 'User' instead of 'user'

Solution

1. Step 1: Check function body syntax

   The user resolver uses curly braces but does not return an object explicitly.

2. Step 2: Understand JavaScript function return rules

   Without a return statement, the function returns undefined, causing the mock to fail.

3. Final Answer:

   Missing return statement inside the user resolver function -> Option B

4. Quick Check:

   Functions with braces need explicit return [OK]

Hint: Use return or parentheses for object in arrow functions [OK]

Common Mistakes:

• Assuming implicit return with braces
• Confusing type names case sensitivity
• Expecting string return instead of object

5. You want to mock a GraphQL resolver for a product field that returns a list of products with id and price. Which mock resolver correctly returns two products with ids 1 and 2 and prices 10.5 and 20.0 respectively?

hard

A. const mocks = { Query: { product: [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } };

B. const mocks = { Query: { product: () => ({ id: [1, 2], price: [10.5, 20.0] }) } };

C. const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } };

D. const mocks = { Query: { product: () => { id: 1; price: 10.5; } } };

Solution

1. Step 1: Understand the expected return type

   The product field should return a list (array) of objects, each with id and price fields.

2. Step 2: Check each option's return value

   const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } }; returns an array of two objects with correct fields and values. Others either return wrong types or syntax errors.

3. Final Answer:

   const mocks = { Query: { product: () => [{ id: 1, price: 10.5 }, { id: 2, price: 20.0 }] } }; -> Option C

4. Quick Check:

   Return array of objects for list fields [OK]

Hint: Return array of objects for list fields in mocks [OK]

Common Mistakes:

• Returning object with arrays instead of array of objects
• Assigning array directly without function
• Missing return or using wrong syntax in function