Problem: Enforce a Required Build Sequence in a Signup Form

hard

40 min

Create a builder that enforces a specific method call sequence—username and password must be set before optional fields or .build().

Problem statement

You’re designing a secure sign-up flow for a backend service. Each sign-up form must include a username and password before proceeding to optional steps, such as email, profile image, or phone number.

You’ve been asked to implement a SignupFormBuilder that enforces this order of construction. Developers must call .withUsername() and .withPassword() first—in any order—but no other method (including .build()) should be allowed until both are set.

Goal

Implement a SignupFormBuilder with the following methods:

.withUsername(username: string): Sets the username (Required).
.withPassword(password: string): Sets the password (Required).
.withEmail(email: string): Adds an optional email.
.withPhone(phone: string): Adds an optional phone number.
.withAvatar(url: string): Adds an optional avatar URL.
.build(): Returns an object with shape: { username: string, password: string, email?: string, phone?: string, avatar?: string }.

Calling any optional method or .build() before both required fields are set must throw an error.

Constraints

Enforce the required sequence via internal state tracking—not static typing.
Do not allow .build() or optional setters to proceed until both required fields are set.
Avoid returning partial objects or silently accepting incomplete builds.

Sample output

The examples below illustrate what the output should look like:

// Valid
const form1 = new SignupFormBuilder()
  .withUsername('alice')
  .withPassword('secure123')
  .withEmail('alice@example.com')
  .withAvatar('https://cdn/avatar.png')
  .build();
console.log(form1);
/* Expected output:
{ username: 'alice',
  password: 'secure123',
  email: 'alice@example.com',
  avatar: 'https://cdn/avatar.png' } */
// Invalid: build too early
try {
  new SignupFormBuilder()
    .withUsername('bob')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call build before setting both username and password */
// Invalid: optional field before required
try {
  new SignupFormBuilder()
    .withEmail('x@x.com')
    .withUsername('x')
    .withPassword('y')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call withEmail before setting both username and password */

Problem: Enforce a Required Build Sequence in a Signup Form

hard

40 min

Create a builder that enforces a specific method call sequence—username and password must be set before optional fields or .build().

Problem statement

Goal

Implement a SignupFormBuilder with the following methods:

.withUsername(username: string): Sets the username (Required).
.withPassword(password: string): Sets the password (Required).
.withEmail(email: string): Adds an optional email.
.withPhone(phone: string): Adds an optional phone number.
.withAvatar(url: string): Adds an optional avatar URL.
.build(): Returns an object with shape: { username: string, password: string, email?: string, phone?: string, avatar?: string }.

Calling any optional method or .build() before both required fields are set must throw an error.

Constraints

Enforce the required sequence via internal state tracking—not static typing.
Do not allow .build() or optional setters to proceed until both required fields are set.
Avoid returning partial objects or silently accepting incomplete builds.

Sample output

The examples below illustrate what the output should look like:

// Valid
const form1 = new SignupFormBuilder()
  .withUsername('alice')
  .withPassword('secure123')
  .withEmail('alice@example.com')
  .withAvatar('https://cdn/avatar.png')
  .build();
console.log(form1);
/* Expected output:
{ username: 'alice',
  password: 'secure123',
  email: 'alice@example.com',
  avatar: 'https://cdn/avatar.png' } */
// Invalid: build too early
try {
  new SignupFormBuilder()
    .withUsername('bob')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call build before setting both username and password */
// Invalid: optional field before required
try {
  new SignupFormBuilder()
    .withEmail('x@x.com')
    .withUsername('x')
    .withPassword('y')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call withEmail before setting both username and password */

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Node.js

class SignupFormBuilder {
  // implement methods here
}
// Example usage:
// Valid
const form1 = new SignupFormBuilder()
  .withUsername('alice')
  .withPassword('secure123')
  .withEmail('alice@example.com')
  .withAvatar('https://cdn/avatar.png')
  .build();
console.log(form1);
/* Expected output:
{ username: 'alice',
  password: 'secure123',
  email: 'alice@example.com',
  avatar: 'https://cdn/avatar.png' } */
// Invalid: build too early
try {
  new SignupFormBuilder()
    .withUsername('bob')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call build before setting both username and password */
// Invalid: optional field before required
try {
  new SignupFormBuilder()
    .withEmail('x@x.com')
    .withUsername('x')
    .withPassword('y')
    .build();
} catch (err) {
  console.log('Error:', err.message);
}
/* Expected output:
Error: Cannot call withEmail before setting both username and password */