Constructor parameter types in Typescript - Mini Project: Build & Apply

Constructor Parameter Types in TypeScript

📖 Scenario: You are creating a simple program to manage a library's book records. Each book has a title and a number of pages.

🎯 Goal: Build a TypeScript class Book that uses constructor parameter types to store the title and page count of a book.

📋 What You'll Learn

Create a class called Book

Add a constructor with parameters title (string) and pages (number)

Store the parameters as public properties

Create an instance of Book with title 'The Hobbit' and pages 310

Print the book's title and pages

💡 Why This Matters

🌍 Real World

Classes with typed constructor parameters are common in real-world TypeScript programs to model data clearly and safely.

💼 Career

Understanding constructor parameter types helps you write clean, maintainable code in professional TypeScript projects.

1

Create the Book class with constructor parameters

Create a class called Book with a constructor that has two parameters: title of type string and pages of type number. Store these parameters as public properties directly in the constructor.

Typescript

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

Hint

Use public before the constructor parameters to automatically create properties.

2

Create a Book instance

Create a variable called myBook and assign it a new Book object with the title 'The Hobbit' and pages 310.

Typescript

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

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

Hint

Use new Book('The Hobbit', 310) to create the instance.

3

Access and print the book's properties

Use console.log to print the book's title and pages in the format: Title: The Hobbit, Pages: 310. Access the properties myBook.title and myBook.pages.

Typescript

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

const myBook = new Book('The Hobbit', 310);

// Print the book details
// Your code here

Hint

Use a template string with backticks to format the output.

4

Run the program to see the output

Run the program to display the book's title and pages exactly as: Title: The Hobbit, Pages: 310.

Typescript

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

const myBook = new Book('The Hobbit', 310);

console.log(`Title: ${myBook.title}, Pages: ${myBook.pages}`);

// Run the program to see the output

Hint

Check your console output matches exactly.

Practice

1. What does specifying parameter types in a TypeScript class constructor do?

easy

A. It changes the class name based on parameters.

B. It runs the constructor faster at runtime.

C. It automatically creates getter and setter methods.

D. It tells what kind of values the constructor expects when creating an object.

Solution

1. Step 1: Understand constructor parameter types

   Constructor parameter types define what type of values the constructor expects to receive when creating an instance of the class.

2. Step 2: Identify the purpose of parameter types

   They help TypeScript check for errors before running the code by ensuring the right types are passed.

3. Final Answer:

   It tells what kind of values the constructor expects when creating an object. -> Option D

4. Quick Check:

   Constructor parameter types = Define expected input types [OK]

Hint: Constructor types define expected inputs clearly [OK]

Common Mistakes:

• Thinking parameter types speed up runtime
• Confusing parameter types with method creation
• Believing parameter types rename classes

2. Which of the following is the correct way to declare a constructor with typed parameters in TypeScript?

easy

A. constructor(name: string, age: number) {}

B. constructor(name string, age number) {}

C. constructor(name, age) : string, number {}

D. constructor(name => string, age => number) {}

Solution

1. Step 1: Review TypeScript constructor syntax

   In TypeScript, parameter types are declared after the parameter name with a colon, like name: string.

2. Step 2: Check each option

   constructor(name: string, age: number) {} uses correct syntax with colons and types. Options A, C, and D use invalid syntax.

3. Final Answer:

   constructor(name: string, age: number) {} -> Option A

4. Quick Check:

   Parameter types use colon syntax [OK]

Hint: Use colon after parameter name for types [OK]

Common Mistakes:

• Omitting colons between name and type
• Using commas or arrows instead of colons
• Placing types after parentheses

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

class Person {
  constructor(public name: string, private age: number) {}
  getAge() { return this.age; }
}
const p = new Person('Alice', 30);
console.log(p.name);
console.log(p.getAge());

medium

A. Error: age is private

B. Alice
30

C. Alice
undefined

D. undefined
30

Solution

1. Step 1: Understand public and private in constructor

   The public name creates a public property accessible outside. The private age creates a private property accessible only inside the class.

2. Step 2: Check console.log outputs

   p.name prints 'Alice' because it's public. p.getAge() returns 30 because the method accesses private age internally.

3. Final Answer:

   Alice 30 -> Option B

4. Quick Check:

   Public accessible, private via method [OK]

Hint: Public props accessible directly, private via methods [OK]

Common Mistakes:

• Expecting private properties accessible outside
• Confusing undefined with private access
• Thinking private causes runtime error

4. Identify the error in this TypeScript class constructor:

class Car {
  constructor(make: string, year: number) {
    this.make = make;
    this.year = year;
  }
}

medium

A. Constructor must have return type declared

B. Cannot assign parameters to 'this' inside constructor

C. Class properties 'make' and 'year' are missing

D. Missing type for parameter 'year' in constructor

Solution

1. Step 1: Check class properties and constructor assignments

2. Step 2: Check class properties

   The class does not declare properties 'make' and 'year' before assigning them in the constructor, which causes an error: Property 'make' does not exist on type 'Car'.

3. Final Answer:

   Class properties 'make' and 'year' are missing -> Option C

4. Quick Check:

   Declare class properties before use [OK]

Hint: Declare properties before assigning in constructor [OK]

Common Mistakes:

• Ignoring missing property declarations
• Thinking constructor parameters auto-create properties
• Expecting return type in constructor

5. You want to create a class Book that takes a title (string), pages (number), and an optional author (string). Which constructor declaration correctly types these parameters and allows creating a Book by omitting the author argument (leaving author as undefined)?

hard

A. constructor(title: string, pages: number, author?: string) {}

B. constructor(title: string, pages: number, author: string = '') {}

C. constructor(title: string, pages: number, author: string) {}

D. constructor(title: string, pages: number, author?: string = '') {}

Solution

1. Step 1: Understand optional parameters in TypeScript

   Optional parameters are marked with a question mark ? after the parameter name, allowing them to be omitted when calling the constructor, leaving the parameter undefined.

2. Step 2: Evaluate each option

   constructor(title: string, pages: number, author?: string = '') {}: author?: string = '' -- omitting sets to '', not undefined. constructor(title: string, pages: number, author: string = '') {}: author: string = '' -- omitting sets to ''. constructor(title: string, pages: number, author: string) {} requires author. constructor(title: string, pages: number, author?: string) {}: author?: string -- omitting leaves undefined.

3. Final Answer:

   constructor(title: string, pages: number, author?: string) {} -> Option A

4. Quick Check:

   Optional params use ? after name [OK]

Hint: Use ? after parameter name for optional [OK]

Common Mistakes:

• Using default value without ? for optional
• Forgetting to mark optional parameters
• Mixing optional and default syntax incorrectly