Builder pattern with generics in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic Builder class.

Typescript

class Builder<[1]> {
  private value: T;
  constructor(initialValue: T) {
    this.value = initialValue;
  }
}

Drag options to blanks, or click blank then click option'

AK

BV

CT

DU

Attempts:

3 left

💡 Hint

Common Mistakes

Using a lowercase letter for the generic type parameter.

Not matching the generic parameter with the constructor argument.

The generic type parameter T is used to define the type for the Builder class.

2fill in blank

medium

Complete the method to set a new value in the Builder class.

Typescript

setValue(newValue: T): this {
  this.value = [1];
  return this;
}

Drag options to blanks, or click blank then click option'

AnewValue

Bvalue

Cthis.value

DnewVal

Attempts:

3 left

💡 Hint

Common Mistakes

Using an undefined variable like value instead of newValue.

Trying to assign this.value to itself.

The method should assign the parameter newValue to the internal value property.

3fill in blank

hard

Fix the error in the build method to return the stored value.

Typescript

build(): [1] {
  return this.value;
}

Drag options to blanks, or click blank then click option'

Avoid

BT

Cany

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using void which means no return value.

Using a concrete type like number which may not match T.

The build method should return the generic type T which is the type of the stored value.

4fill in blank

hard

Fill both blanks to create a generic Builder method that updates a property and returns the builder.

Typescript

setProperty<K extends keyof T>(key: [1], value: T[K]): this {
  this.value[key] = [2];
  return this;
}

Drag options to blanks, or click blank then click option'

AK

Bkey

Cvalue

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using key as the type instead of the parameter name.

Assigning key instead of value to the property.

The method uses K as the key type and assigns value to the property.

5fill in blank

hard

Fill all three blanks to create a generic Builder class with a static create method.

Typescript

class Builder<[1]> {
  private value: T;
  private constructor(initialValue: T) {
    this.value = initialValue;
  }

  static create<[2]>(initialValue: [3]): Builder<U> {
    return new Builder<U>(initialValue);
  }
}

Drag options to blanks, or click blank then click option'

AT

BU

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same generic parameter name for both class and static method.

Mismatching the generic parameter in the static method signature.

The class uses T as the generic type. The static method defines its own generic U and uses it for the parameter and return type.

Practice

1.

What is the main advantage of using the builder pattern with generics in TypeScript?

easy

A. It automatically generates UI components from objects.

B. It makes the code run faster by skipping type checks.

C. It allows creating different object types step-by-step with type safety.

D. It replaces all functions with classes.

Solution

1. Step 1: Understand builder pattern purpose

   The builder pattern helps create complex objects step-by-step, improving clarity and flexibility.

2. Step 2: Role of generics in TypeScript

   Generics allow the builder to work with any object type while keeping type safety, ensuring keys and values match.

3. Final Answer:

   It allows creating different object types step-by-step with type safety. -> Option C

4. Quick Check:

   Builder + generics = flexible, safe object creation [OK]

Hint: Generics + builder = reusable and type-safe object creation [OK]

Common Mistakes:

• Thinking builder auto-generates UI
• Believing it skips type checks
• Confusing builder with replacing functions

2.

Which of the following is the correct syntax to declare a generic builder class in TypeScript?

class Builder<T> {
  private obj: T;
  constructor() {
    this.obj = {} as T;
  }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T {
    return this.obj;
  }
}

easy

A. class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } }

B. class Builder<T> { private obj: T; constructor() { this.obj = new T(); } }

C. class Builder { private obj: T; constructor() { this.obj = {} as T; } }

D. class Builder<T> { private obj: any; constructor() { this.obj = {}; } }

Solution

1. Step 1: Check generic class declaration

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } correctly declares a generic class with <T> and initializes obj as {} as T.

2. Step 2: Identify incorrect initializations

   class Builder<T> { private obj: T; constructor() { this.obj = new T(); } } tries new T() which is invalid in TypeScript. class Builder { private obj: T; constructor() { this.obj = {} as T; } } misses generic <T>. class Builder<T> { private obj: any; constructor() { this.obj = {}; } } uses any losing type safety.

3. Final Answer:

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } -> Option A

4. Quick Check:

   Generic class with {} as T init = class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } [OK]

Hint: Use {} as T to initialize generic object safely [OK]

Common Mistakes:

• Trying to instantiate generic type with new
• Forgetting to declare generic parameter <T>
• Using any instead of generic type

3.

What will be the output of this code?

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {} as T; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

interface Person { name: string; age: number; }
const person = new Builder<Person>()
  .set('name', 'Alice')
  .set('age', 30)
  .build();
console.log(person);

medium

A. { name: 'Alice', age: 30 }

B. { name: 'Alice' }

C. Error: Cannot assign to 'name' of type 'T'

D. {}

Solution

1. Step 1: Understand builder usage

   The builder sets 'name' to 'Alice' and 'age' to 30 on an empty object typed as Person.

2. Step 2: Check final build output

   The build() method returns the object with both properties set, so output is { name: 'Alice', age: 30 }.

3. Final Answer:

   { name: 'Alice', age: 30 } -> Option A

4. Quick Check:

   Builder sets keys correctly = { name: 'Alice', age: 30 } [OK]

Hint: Builder sets keys stepwise, final object has all properties [OK]

Common Mistakes:

• Expecting partial object missing keys
• Thinking assignment causes error
• Assuming empty object returned

4.

Identify the error in this builder class and select the fix:

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {}; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

medium

A. Use any type for obj instead of T.

B. Change set method to accept key: string.

C. Remove generic <T> from class declaration.

D. Initialize obj as {} as T in constructor.

Solution

1. Step 1: Identify initialization error

   Assigning {} directly to obj: T causes a type error because {} is not guaranteed to be T.

2. Step 2: Correct initialization with type assertion

   Using {} as T tells TypeScript to treat empty object as type T safely during construction.

3. Final Answer:

   Initialize obj as {} as T in constructor. -> Option D

4. Quick Check:

   Use {} as T to fix generic object init [OK]

Hint: Use type assertion {} as T to initialize generic objects [OK]

Common Mistakes:

• Assigning {} directly without assertion
• Changing key type to string loses type safety
• Removing generics breaks flexibility

5.

You want to build a generic builder that only allows setting properties defined in the object type and prevents setting undefined keys. Which TypeScript feature helps enforce this at compile time?

hard

A. Using any type for keys to allow flexibility.

B. Using keyof T as the key type in the set method.

C. Using unknown type for values to accept any value.

D. Using Partial<T> for the object storage.

Solution

1. Step 1: Understand key restriction need

   To prevent setting keys not in the object type, the key parameter must be limited to valid keys.

2. Step 2: Use keyof T for key type

   keyof T represents all valid keys of type T, so using it restricts keys to only those defined in T.

3. Final Answer:

   Using keyof T as the key type in the set method. -> Option B

4. Quick Check:

   keyof T restricts keys to valid properties [OK]

Hint: Use keyof T to limit keys to object properties [OK]

Common Mistakes:

• Using any loses key restriction
• Using unknown does not restrict keys
• Partial<T> affects values, not keys