Default resolvers in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query using default resolvers?

Given this GraphQL schema and data, what will be the result of the query below?

type Query {
  book: Book
}

type Book {
  title: String
  author: Author
}

type Author {
  name: String
}

const data = {
  book: {
    title: "1984",
    author: {
      name: "George Orwell"
    }
  }
};

Query:
{
  book {
    title
    author {
      name
    }
  }
}

GraphQL

const data = {
  book: {
    title: "1984",
    author: {
      name: "George Orwell"
    }
  }
};

A { "data": { "book": null } }

B { "data": { "book": { "title": null, "author": { "name": null } } } }

C { "data": { "book": { "title": "1984", "author": { "name": "George Orwell" } } } }

D { "errors": [ { "message": "Field 'title' not found" } ] }

💡 Hint

Default resolvers return the value from the object if the field name matches.

The default resolver returns the value of the field from the parent object. Since the data has matching fields, the query returns the full nested data.

🧠 Conceptual

intermediate

How do default resolvers behave when a field is missing in the data?

Consider a GraphQL schema with a field author of type Author. The data object for book does not have the author property. What will the default resolver return for author?

A It throws a runtime error because the field is missing.

B It returns <code>null</code> for the <code>author</code> field.

C It returns an empty object <code>{}</code> for <code>author</code>.

D It returns the string "undefined".

💡 Hint

Default resolvers return null if the field is not present in the object.

If the field is missing in the data object, the default resolver returns null instead of throwing an error or returning other values.

📝 Syntax

advanced

Which option correctly defines a default resolver function in GraphQL?

Choose the correct JavaScript function that acts as a default resolver for a field.

A function defaultResolver(parent, args, context, info) { return parent[info.fieldName]; }

B function defaultResolver(parent, args) { return args[info.fieldName]; }

C function defaultResolver(info) { return parent[info.fieldName]; }

D function defaultResolver(parent, args, context) { return parent.fieldName; }

💡 Hint

The default resolver uses the info.fieldName to get the field name dynamically.

The default resolver function receives parent, args, context, and info. It returns the property of parent matching info.fieldName.

❓ optimization

advanced

How can default resolvers impact performance in deeply nested queries?

When using default resolvers on a deeply nested GraphQL query, what is a common performance concern?

A Default resolvers always cache results automatically, improving performance.

B Default resolvers prevent any data fetching beyond the first level.

C Default resolvers batch all nested field requests into a single database call by default.

D Default resolvers may cause multiple redundant data fetches if not optimized.

💡 Hint

Think about how default resolvers access nested fields and data fetching.

Default resolvers simply access properties on objects and do not optimize data fetching. This can lead to multiple redundant calls if the data source is not optimized or batched.

🔧 Debug

expert

Why does this GraphQL query return null for a nested field despite data existing?

Given this schema and data, the query returns null for author.name. Identify the cause.

type Query {
  book: Book
}

type Book {
  title: String
  author: Author
}

type Author {
  name: String
}

const data = {
  book: {
    title: "Dune",
    authorName: "Frank Herbert"
  }
};

Query:
{
  book {
    title
    author {
      name
    }
  }
}

A The data object uses 'authorName' instead of nested 'author' object, so default resolver returns null.

B The schema is missing a resolver for 'author', causing a runtime error.

C The query syntax is invalid and causes the server to return null.

D The 'name' field is not defined in the 'Author' type, so it returns null.

💡 Hint

Check how the data structure matches the schema fields.

The data has 'authorName' as a flat property instead of an 'author' object with 'name'. Default resolvers look for 'author' object but find none, so return null.