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Grokking Node.js Design Patterns for Interviews

Problem: Construct a Form Schema Builder with Validation Rules

Hard
40 min
Build a fluent, nested form schema builder that supports field-level validation rules and serializes to JSON.

Problem statement

Your team is building a dynamic form engine. Forms are described using JSON schemas, where each field has a name, type, and optional validation rules, such as required, minLength, maxLength, etc.

Currently, developers construct these schemas manually using raw objects. You’ve been asked to build a fluent FormSchemaBuilder that cleanly assembles field definitions, including embedded validation logic.

Goal

Implement a FormSchemaBuilder with:

  • .addField(name: string): Creates a new field, returns a FieldBuilder.

  • FieldBuilder.withType(type: string): Sets the field type (e.g., 'string', 'number').

  • FieldBuilder.required(): Marks the field as required.

  • FieldBuilder.minLength(n: number): String only.

  • FieldBuilder.maxLength(n: number): String only.

  • FieldBuilder.done(): Finalizes the field and returns to FormSchemaBuilder.

  • .build(): Returns a JSON object with the following structure.

{
  fields: {
    username: {
      type: 'string',
      required: true,
      minLength: 3,
      maxLength: 20
    },
    age: {
      type: 'number'
    }
  }
}

Constraints

  • Do not allow .build() until all added fields have called .done().

  • Do not allow duplicate field names.

  • Only include validation keys that were explicitly set.

Sample output

The examples below illustrate what the output should look like:

// Valid schema with rules
const schema1 = new FormSchemaBuilder()
  .addField('username')
    .withType('string')
    .required()
    .minLength(3)
    .maxLength(20)
    .done()
  .addField('age')
    .withType('number')
    .done()
  .build();
console.log(schema1);
/* Expected output:
 { fields:
  { username:
      { type: 'string', required: true, minLength: 3, maxLength: 20 },
    age: { type: 'number' } } } */
// Invalid: build called before .done()
try {
  const builder = new FormSchemaBuilder();
  builder.addField('email').withType('string');
  builder.build(); // should throw
} catch (err) {
  console.log('Failed to build schema2:', err.message);
}
/* Expected output:
Failed to build schema2: Cannot build schema before calling .done() on the last field */
Invalid: duplicate field
try {
  new FormSchemaBuilder()
    .addField('email')
      .withType('string')
      .done()
    .addField('email')
      .withType('string')
      .done();
} catch (err) {
  console.log('Failed to build schema3:', err.message);
}
/* Expected output:
Failed to build schema3: Field "email" already defined */

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

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Home
Courses
Grokking Node.js Design Patterns for Interviews

Problem: Construct a Form Schema Builder with Validation Rules

Hard
40 min
Build a fluent, nested form schema builder that supports field-level validation rules and serializes to JSON.

Problem statement

Your team is building a dynamic form engine. Forms are described using JSON schemas, where each field has a name, type, and optional validation rules, such as required, minLength, maxLength, etc.

Currently, developers construct these schemas manually using raw objects. You’ve been asked to build a fluent FormSchemaBuilder that cleanly assembles field definitions, including embedded validation logic.

Goal

Implement a FormSchemaBuilder with:

  • .addField(name: string): Creates a new field, returns a FieldBuilder.

  • FieldBuilder.withType(type: string): Sets the field type (e.g., 'string', 'number').

  • FieldBuilder.required(): Marks the field as required.

  • FieldBuilder.minLength(n: number): String only.

  • FieldBuilder.maxLength(n: number): String only.

  • FieldBuilder.done(): Finalizes the field and returns to FormSchemaBuilder.

  • .build(): Returns a JSON object with the following structure.

{
  fields: {
    username: {
      type: 'string',
      required: true,
      minLength: 3,
      maxLength: 20
    },
    age: {
      type: 'number'
    }
  }
}

Constraints

  • Do not allow .build() until all added fields have called .done().

  • Do not allow duplicate field names.

  • Only include validation keys that were explicitly set.

Sample output

The examples below illustrate what the output should look like:

// Valid schema with rules
const schema1 = new FormSchemaBuilder()
  .addField('username')
    .withType('string')
    .required()
    .minLength(3)
    .maxLength(20)
    .done()
  .addField('age')
    .withType('number')
    .done()
  .build();
console.log(schema1);
/* Expected output:
 { fields:
  { username:
      { type: 'string', required: true, minLength: 3, maxLength: 20 },
    age: { type: 'number' } } } */
// Invalid: build called before .done()
try {
  const builder = new FormSchemaBuilder();
  builder.addField('email').withType('string');
  builder.build(); // should throw
} catch (err) {
  console.log('Failed to build schema2:', err.message);
}
/* Expected output:
Failed to build schema2: Cannot build schema before calling .done() on the last field */
Invalid: duplicate field
try {
  new FormSchemaBuilder()
    .addField('email')
      .withType('string')
      .done()
    .addField('email')
      .withType('string')
      .done();
} catch (err) {
  console.log('Failed to build schema3:', err.message);
}
/* Expected output:
Failed to build schema3: Field "email" already defined */

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

Similar Problems
Design a URL Builder with Conditional Parameters
Implement a Chainable Logger Config Builder
Enforce a Required Build Sequence in a Sign-Up Form
Model a GraphQL Query Using a Nested Builder
Build a Minimal Email Template Builder
Create a Fluent Pizza Order Builder
Build a Simple Validation Rule Builder
Build a Flexible Product Query Builder
Node.js
class FormSchemaBuilder {
  // implement builder logic here
}
class FieldBuilder {
  // implement field-level chaining logic here
}
// Example usage:
// Valid schema with rules
const schema1 = new FormSchemaBuilder()
  .addField('username')
    .withType('string')
    .required()
    .minLength(3)
    .maxLength(20)
    .done()
  .addField('age')
    .withType('number')
    .done()
  .build();
console.log(schema1);
/* Expected output:
 { fields:
  { username:
      { type: 'string', required: true, minLength: 3, maxLength: 20 },
    age: { type: 'number' } } } */
// Invalid: build called before .done()
try {
  const builder = new FormSchemaBuilder();
  builder.addField('email').withType('string');
  builder.build(); // should throw
} catch (err) {
  console.log('Failed to build schema2:', err.message);
}
/* Expected output:
Failed to build schema2: Cannot build schema before calling .done() on the last field */
Invalid: duplicate field
try {
  new FormSchemaBuilder()
    .addField('email')
      .withType('string')
      .done()
    .addField('email')
      .withType('string')
      .done();
} catch (err) {
  console.log('Failed to build schema3:', err.message);
}
/* Expected output:
Failed to build schema3: Field "email" already defined */