Parameter properties shorthand in Typescript - Mini Project: Build & Apply

Using Parameter Properties Shorthand in TypeScript

📖 Scenario: Imagine you are creating a simple app to manage books in a library. Each book has a title and an author.

🎯 Goal: You will create a Book class using TypeScript's parameter properties shorthand to store the title and author. Then, you will create an instance and display its details.

📋 What You'll Learn

Create a Book class with a constructor using parameter properties shorthand

The constructor should have two parameters: title and author, both strings

Create an instance of Book with title 'The Hobbit' and author 'J.R.R. Tolkien'

Print the book's title and author using the instance properties

💡 Why This Matters

🌍 Real World

Parameter properties shorthand is commonly used in TypeScript to quickly create classes that hold data, such as models in apps, configuration objects, or simple data containers.

💼 Career

Understanding this shorthand helps you write cleaner and more maintainable TypeScript code, a valuable skill for frontend and backend developers working with modern frameworks and libraries.

1

Create the Book class with a constructor

Create a class called Book with a constructor that takes two parameters: title and author, both of type string. Do not add any code inside the constructor yet.

Typescript

class Book {
  constructor(title: string, author: string) {
    // Your code here
  }
}

Hint

Define a class with a constructor that accepts two string parameters named title and author.

2

Use parameter properties shorthand in the constructor

Modify the Book class constructor to use parameter properties shorthand by adding public before both parameters title and author.

Typescript

class Book {
  constructor(public title: string, public author: string) {
    // Your code here
  }
}

Hint

Add the public keyword before each parameter to automatically create and assign class properties.

3

Create an instance of Book

Create a variable called myBook and assign it a new Book instance with the title 'The Hobbit' and author 'J.R.R. Tolkien'.

Typescript

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

// Create an instance of Book
// Your code here

Hint

Use the new keyword to create an instance and assign it to myBook.

4

Print the book's title and author

Write a console.log statement to print the book's title and author from the myBook instance in the format: Title: The Hobbit, Author: J.R.R. Tolkien.

Typescript

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

const myBook = new Book('The Hobbit', 'J.R.R. Tolkien');

// Print the book details
// Your code here

Hint

Use a template string to include myBook.title and myBook.author in the output.

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