TypeScript Constructors
Initializing objects with custom values
🔧 What are Constructors?
Constructors are special methods that automatically run when creating a new object. They initialize object properties with values, making it easy to set up objects with specific data right from the start.
// Constructor example
class Person {
name: string;
age: number;
constructor(name: string, age: number) {
this.name = name;
this.age = age;
}
}
const person = new Person("Alice", 30);
Output:
Person { name: "Alice", age: 30 }
Constructor Features
Auto-Execution
Runs automatically on object creation
constructor() {
console.log("Created!");
}Parameters
Accept values during initialization
constructor(name: string) {
this.name = name;
}Initialization
Set up initial property values
constructor() {
this.count = 0;
this.active = true;
}Shorthand
Quick property declaration
constructor(
public name: string
) {}🔹 Basic Constructor
Create a constructor to initialize object properties:
class Car {
brand: string;
model: string;
year: number;
constructor(brand: string, model: string, year: number) {
this.brand = brand;
this.model = model;
this.year = year;
}
getInfo() {
return `${this.year} ${this.brand} ${this.model}`;
}
}
const myCar = new Car("Toyota", "Camry", 2023);
console.log(myCar.getInfo());Output:
2023 Toyota Camry
🔹 Constructor with Default Values
Provide default values for optional parameters:
class Product {
name: string;
price: number;
inStock: boolean;
constructor(name: string, price: number, inStock: boolean = true) {
this.name = name;
this.price = price;
this.inStock = inStock;
}
display() {
return `${this.name}: $${this.price} - ${this.inStock ? 'Available' : 'Out of Stock'}`;
}
}
const product1 = new Product("Laptop", 999);
const product2 = new Product("Phone", 599, false);
console.log(product1.display());
console.log(product2.display());Output:
Laptop: $999 - Available
Phone: $599 - Out of Stock
🔹 Parameter Properties Shorthand
TypeScript allows you to declare and initialize properties directly in the constructor:
// Traditional way
class User {
name: string;
email: string;
constructor(name: string, email: string) {
this.name = name;
this.email = email;
}
}
// Shorthand way (recommended)
class UserShort {
constructor(
public name: string,
public email: string,
private password: string
) {}
getInfo() {
return `${this.name} (${this.email})`;
}
}
const user = new UserShort("Bob", "[email protected]", "secret123");
console.log(user.getInfo());Output:
Bob ([email protected])
🔹 Constructor with Validation
Add logic to validate data during object creation:
class Employee {
constructor(
public name: string,
public age: number,
public salary: number
) {
if (age < 18) {
throw new Error("Employee must be at least 18 years old");
}
if (salary < 0) {
throw new Error("Salary cannot be negative");
}
}
getDetails() {
return `${this.name}, Age: ${this.age}, Salary: $${this.salary}`;
}
}
const emp = new Employee("Sarah", 25, 50000);
console.log(emp.getDetails());
// This would throw an error:
// const invalidEmp = new Employee("Tom", 16, 30000);Output:
Sarah, Age: 25, Salary: $50000
🔹 Multiple Initialization Patterns
Use optional parameters for flexible object creation:
class Rectangle {
constructor(
public width: number,
public height: number = width // Default to square
) {}
getArea() {
return this.width * this.height;
}
getPerimeter() {
return 2 * (this.width + this.height);
}
}
const square = new Rectangle(5);
const rect = new Rectangle(5, 10);
console.log(`Square area: ${square.getArea()}`);
console.log(`Rectangle area: ${rect.getArea()}`);Output:
Square area: 25
Rectangle area: 50