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Grokking Node.js Design Patterns for Interviews

Problem: Model a GraphQL Query Using a Nested Builder

Hard
40 min
Build a fluent API that constructs a nested GraphQL query string with optional fields and subfields.

Problem statement

You’re building a backend system that generates dynamic GraphQL queries for server-to-server communication. Developers need a way to programmatically construct deeply nested query strings using a clear, fluent API.

Your job is to implement a GraphQLQueryBuilder that allows developers to:

  • Start a root query.

  • Add fields to that query.

  • For any field, optionally nest a sub-selection using another builder.

  • Output a final query string that’s valid GraphQL syntax.

Goal

Build a fluent API using GraphQLQueryBuilder that supports:

  • .field(name: string): Adds a top-level field.

  • .select(field: string, subBuilderFn: (b: GraphQLQueryBuilder) => void): Adds a field with a nested selection set, defined using a new GraphQLQueryBuilder instance.

  • .build(): Returns the full query string as a formatted GraphQL query, with proper indentation and nested braces.

{
  field1
  field2
  parent {
    child1
    child2
  }
}

Constraints

  • Do not hardcode field names—support any valid field string passed by the user.

  • .select() must create a new builder instance for nested fields, preserving parent-child structure.

  • .build() must return a single formatted string—not an object, array, or intermediary structure.

Sample output

The examples below illustrate what the output should look like:

const query1 = new GraphQLQueryBuilder()
  .field('id')
  .field('title')
  .select('author', (b) => {
    b.field('name').field('email');
  })
  .build();
console.log(query1);
/* Expected output:
{
  id
  title
  author {
    name
    email
  }
}
*/
const query2 = new GraphQLQueryBuilder()
  .select('user', (b) => {
    b.field('id')
     .select('profile', (p) => {
       p.field('bio').field('avatar');
     });
  })
  .build();
console.log(query2);
/* Expected output:
{
  user {
    id
    profile {
      bio
      avatar
    }
  }
}
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

Similar Problems
Build a Flexible Product Query Builder
Design a URL Builder with Conditional Parameters
Implement a Chainable Logger Config Builder
Enforce a Required Build Sequence in a Sign-Up Form
Construct a Form Schema Builder with Validation Rules
Build a Minimal Email Template Builder
Create a Fluent Pizza Order Builder
Build a Simple Validation Rule Builder
Home
Courses
Grokking Node.js Design Patterns for Interviews

Problem: Model a GraphQL Query Using a Nested Builder

Hard
40 min
Build a fluent API that constructs a nested GraphQL query string with optional fields and subfields.

Problem statement

You’re building a backend system that generates dynamic GraphQL queries for server-to-server communication. Developers need a way to programmatically construct deeply nested query strings using a clear, fluent API.

Your job is to implement a GraphQLQueryBuilder that allows developers to:

  • Start a root query.

  • Add fields to that query.

  • For any field, optionally nest a sub-selection using another builder.

  • Output a final query string that’s valid GraphQL syntax.

Goal

Build a fluent API using GraphQLQueryBuilder that supports:

  • .field(name: string): Adds a top-level field.

  • .select(field: string, subBuilderFn: (b: GraphQLQueryBuilder) => void): Adds a field with a nested selection set, defined using a new GraphQLQueryBuilder instance.

  • .build(): Returns the full query string as a formatted GraphQL query, with proper indentation and nested braces.

{
  field1
  field2
  parent {
    child1
    child2
  }
}

Constraints

  • Do not hardcode field names—support any valid field string passed by the user.

  • .select() must create a new builder instance for nested fields, preserving parent-child structure.

  • .build() must return a single formatted string—not an object, array, or intermediary structure.

Sample output

The examples below illustrate what the output should look like:

const query1 = new GraphQLQueryBuilder()
  .field('id')
  .field('title')
  .select('author', (b) => {
    b.field('name').field('email');
  })
  .build();
console.log(query1);
/* Expected output:
{
  id
  title
  author {
    name
    email
  }
}
*/
const query2 = new GraphQLQueryBuilder()
  .select('user', (b) => {
    b.field('id')
     .select('profile', (p) => {
       p.field('bio').field('avatar');
     });
  })
  .build();
console.log(query2);
/* Expected output:
{
  user {
    id
    profile {
      bio
      avatar
    }
  }
}
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

Similar Problems
Build a Flexible Product Query Builder
Design a URL Builder with Conditional Parameters
Implement a Chainable Logger Config Builder
Enforce a Required Build Sequence in a Sign-Up Form
Construct a Form Schema Builder with Validation Rules
Build a Minimal Email Template Builder
Create a Fluent Pizza Order Builder
Build a Simple Validation Rule Builder
Node.js
class GraphQLQueryBuilder {
  // implement methods here
}
// Example usage:
const query1 = new GraphQLQueryBuilder()
  .field('id')
  .field('title')
  .select('author', (b) => {
    b.field('name').field('email');
  })
  .build();
console.log(query1);
/* Expected output:
{
  id
  title
  author {
    name
    email
  }
}
*/
const query2 = new GraphQLQueryBuilder()
  .select('user', (b) => {
    b.field('id')
     .select('profile', (p) => {
       p.field('bio').field('avatar');
     });
  })
  .build();
console.log(query2);
/* Expected output:
{
  user {
    id
    profile {
      bio
      avatar
    }
  }
}
*/