Parameter properties shorthand in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class with a constructor that uses parameter properties shorthand to create a public property.

Typescript

class Person {
  constructor(public [1]: string) {}
}

const p = new Person("Alice");
console.log(p.name);

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

Aage

Bname

Cvalue

Dtitle

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add public before the parameter.

Using a different parameter name than the property you want.

The public keyword before the constructor parameter name creates a public property automatically.

2fill in blank

medium

Complete the code to create a private property using parameter properties shorthand.

Typescript

class Car {
  constructor(private [1]: number) {}
  getSpeed() {
    return this.speed;
  }
}

const car = new Car(100);
console.log(car.getSpeed());

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

Aspeed

Bdistance

Ctime

Dpower

Attempts:

3 left

💡 Hint

Common Mistakes

Using a public or protected keyword instead of private.

Choosing a parameter name unrelated to the property.

The private keyword before the parameter speed creates a private property accessible inside the class.

3fill in blank

hard

Fix the error in the constructor by using parameter properties shorthand correctly.

Typescript

class Book {
  title: string;
  constructor([1]: string) {
    this.title = title;
  }
}

const b = new Book("My Book");
console.log(b.title);

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

Atitle

Bprivate title

Cpublic title

Dreadonly title

Attempts:

3 left

💡 Hint

Common Mistakes

Not adding any visibility keyword.

Trying to assign the parameter without declaring the property.

Adding public before the parameter title in the constructor automatically creates and initializes the property.

4fill in blank

hard

Fill both blanks to create a readonly property using parameter properties shorthand and access it in a method.

Typescript

class User {
  constructor([1] [2]: string) {}
  getUsername() {
    return this.username;
  }
}

const user = new User("john_doe");
console.log(user.getUsername());

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

Areadonly

Busername

Cpublic

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using private instead of readonly.

Using a different property name than the one accessed.

Using readonly and the parameter name username creates a readonly property accessible in the class.

5fill in blank

hard

Fill all three blanks to create a protected property with parameter properties shorthand, initialize it, and access it in a subclass.

Typescript

class Animal {
  constructor([1] [2]: string) {}
}

class Dog extends Animal {
  getType() {
    return this.[3];
  }
}

const dog = new Dog("mammal");
console.log(dog.getType());

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

Aprotected

Btype

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using private which blocks subclass access.

Using different names for the property in constructor and method.

The protected keyword creates a property accessible in subclasses. The property name type is used in both constructor and subclass method.

Practice

1. What does the parameter properties shorthand in TypeScript do inside a class constructor?

easy

A. It automatically generates getter and setter methods.

B. It declares global variables outside the class.

C. It creates and assigns class properties in one step using access modifiers.

D. It imports external modules into the class.

Solution

1. Step 1: Understand parameter properties shorthand

   Parameter properties shorthand uses access modifiers like public, private, or protected before constructor parameters to create and assign class properties automatically.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated features: global variables, getter/setter methods, and module imports, which are not related to parameter properties shorthand.

3. Final Answer:

   It creates and assigns class properties in one step using access modifiers. -> Option C

4. Quick Check:

   Parameter properties shorthand = create and assign properties [OK]

Hint: Look for access modifiers before constructor parameters [OK]

Common Mistakes:

• Confusing shorthand with getter/setter methods
• Thinking it declares global variables
• Assuming it imports modules

2. Which of the following is the correct syntax to use parameter properties shorthand in a TypeScript class constructor?

easy

A. constructor(public name: string) { }

B. constructor(name: string) { this.name = name; }

C. constructor(private name) { this.name = name; }

D. constructor(name: string public) { }

Solution

1. Step 1: Identify correct parameter properties syntax

   The correct syntax places an access modifier (public, private, protected) before the parameter name and type inside the constructor, like constructor(public name: string) { }.

2. Step 2: Check other options for errors

   constructor(name: string) { this.name = name; } lacks shorthand (no access modifier). constructor(private name) { this.name = name; } misses the type annotation. constructor(name: string public) { } has incorrect modifier placement after the type.

3. Final Answer:

   constructor(public name: string) { } -> Option A

4. Quick Check:

   Access modifier before parameter = correct syntax [OK]

Hint: Access modifier must come before parameter name and type [OK]

Common Mistakes:

• Placing access modifier after type
• Omitting type annotation
• Not using access modifier at all

3. What will be the output of this TypeScript code?

class Person {
  constructor(private name: string) {}
  greet() {
    return `Hello, ${this.name}!`;
  }
}
const p = new Person('Anna');
console.log(p.greet());

medium

A. Hello, !

B. Hello, undefined!

C. Error: Property 'name' does not exist

D. Hello, Anna!

Solution

1. Step 1: Understand private parameter property

   The constructor uses private name: string, which creates and assigns a private property name automatically.

2. Step 2: Check greet method output

   The greet() method accesses this.name and returns "Hello, Anna!" because the property was set correctly.

3. Final Answer:

   Hello, Anna! -> Option D

4. Quick Check:

   Private property set by constructor = correct output [OK]

Hint: Private properties set by constructor are accessible inside class [OK]

Common Mistakes:

• Assuming private means inaccessible inside class
• Expecting undefined because no explicit assignment
• Confusing property with parameter

4. Identify the error in this TypeScript class using parameter properties shorthand:

class Car {
  constructor(public model) {}
}
const c = new Car('Tesla');
console.log(c.model);

medium

A. Property 'model' is not accessible outside the class.

B. Missing type annotation for parameter 'model'.

C. Cannot use public modifier in constructor.

D. Constructor must have a return type.

Solution

1. Step 1: Check parameter properties syntax

   The constructor parameter model uses public but lacks a type annotation, which is required in TypeScript.

2. Step 2: Verify other options

   Cannot use public modifier in constructor. is wrong because public modifier is allowed. Property 'model' is not accessible outside the class. is wrong because public makes property accessible. Constructor must have a return type. is wrong because constructors do not have return types.

3. Final Answer:

   Missing type annotation for parameter 'model'. -> Option B

4. Quick Check:

   Parameter properties require type annotations [OK]

Hint: Always add type after parameter in constructor shorthand [OK]

Common Mistakes:

• Omitting type annotation
• Thinking public is invalid in constructor
• Expecting constructor to have return type

5. You want to create a TypeScript class Book that stores title and author as public properties using parameter properties shorthand. Which code correctly implements this and allows creating a Book instance with both properties set?

hard

A. class Book {
constructor(public title: string, public author: string) {}
}

B. class Book {
constructor(public title: string, author: string) {}
}

C. class Book {
constructor(title: string, author: string) {
this.title = title;
this.author = author;
}
}

D. class Book {
constructor(title: string, public author: string) {}
}

Solution

1. Step 1: Identify correct use of parameter properties shorthand

   Both title and author must have access modifiers (public) before their parameters to create public properties automatically.

2. Step 2: Evaluate each option

   class Book { constructor(public title: string, author: string) {} } only makes title public; author is just a parameter, not a property. class Book { constructor(public title: string, public author: string) {} } correctly uses public for both. class Book { constructor(title: string, author: string) { this.title = title; this.author = author; } } does not use shorthand but tries to assign properties without declaring them, causing errors. class Book { constructor(title: string, public author: string) {} } only makes author public, title is missing.

3. Final Answer:

   class Book { constructor(public title: string, public author: string) {} } -> Option A

4. Quick Check:

   Use public before all parameters to create properties [OK]

Hint: Add public before every parameter to create properties [OK]

Common Mistakes:

• Adding public to only some parameters
• Assigning properties without declaring them
• Mixing shorthand with manual assignments incorrectly