Mocking resolvers in GraphQL - Step-by-Step Execution

Concept Flow - Mocking resolvers

Define schema with types

↓

Create mock resolvers for fields

↓

Query execution triggers resolver

↓

Mock resolver returns fake data

↓

Query returns mocked data to client

This flow shows how GraphQL queries use mock resolvers to return fake data based on the schema.

Execution Sample

GraphQL

type Query {
  book: Book
}

type Book {
  title: String
  author: String
}

// Mock resolver
const mocks = {
  Query: { book: () => ({ title: "Mock Title", author: "Mock Author" }) }
};

Defines a simple schema and a mock resolver that returns fake book data when queried.

Execution Table

Step	Action	Resolver Called	Returned Value	Query Result
1	Start query for 'book'	No	N/A	N/A
2	Resolver for Query.book called	Query.book	{ title: "Mock Title", author: "Mock Author" }	N/A
3	Resolver for Book.title called	Book.title	"Mock Title"	"title": "Mock Title"
4	Resolver for Book.author called	Book.author	"Mock Author"	"author": "Mock Author"
5	Assemble final response	No	N/A	{ book: { title: "Mock Title", author: "Mock Author" } }

💡 Query completes after all fields resolved with mocked data.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
book	undefined	{ title: "Mock Title", author: "Mock Author" }	{ title: "Mock Title", author: "Mock Author" }	{ title: "Mock Title", author: "Mock Author" }	{ title: "Mock Title", author: "Mock Author" }
title	undefined	undefined	"Mock Title"	"Mock Title"	"Mock Title"
author	undefined	undefined	undefined	"Mock Author"	"Mock Author"

Key Moments - 2 Insights

Why does the Query.book resolver return an object instead of a simple string?

Because the 'book' field returns a Book type object, the resolver must return an object with fields 'title' and 'author' as shown in execution_table step 2.

How does the mock resolver know what fields to provide for the Book type?

The mock resolver returns an object matching the Book type shape. The GraphQL engine then calls resolvers for each field like 'title' and 'author' as in steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what value does the Query.book resolver return at step 2?

A "Mock Title"

B { title: "Mock Title", author: "Mock Author" }

C "Mock Author"

D undefined

💡 Hint

Check the 'Returned Value' column at step 2 in the execution_table.

At which step does the resolver for the 'author' field return its value?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Resolver Called' column to find when 'Book.author' is resolved.

If the mock resolver for Query.book returned null, what would the final query result be?

A { book: { title: "Mock Title" } }

B An error would occur

C { book: null }

D { book: {} }

💡 Hint

Consider what happens when a resolver returns null for an object field in GraphQL.

Concept Snapshot

Mocking resolvers in GraphQL:
- Define schema types
- Create mock resolvers returning fake data
- Query triggers resolvers
- Resolvers return mocked values
- Query returns mocked data to client

Full Transcript

This visual execution shows how mocking resolvers work in GraphQL. First, a schema defines types like Query and Book. Then, mock resolvers are created to return fake data for these types. When a query requests the 'book' field, the Query.book resolver is called and returns a mock object with title and author. Next, the GraphQL engine calls resolvers for each field of Book, returning mocked strings. Finally, the query response assembles all mocked data and returns it to the client. This process helps developers test and develop without real data sources.