LESSON 285 OF 675 · FUNCTIONS & CLASSES

Constructors in TypeScript

Lesson

TL ; DR:

A constructor is a special method that runs automatically when you create an object with new. It's used to set up the object's properties.
In TypeScript, the constructor is a method literally named constructor, defined inside the class. The fields it sets are declared first, with their types, and the parameters are typed too:
```
class Dog {
    name: string;
    age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }
}
```
Then create objects with arguments: new Dog("Rex", 4) — the constructor fills in the properties for you.

Full lesson:

In the previous lesson you set properties one at a time after creating the object:

let dog = new Dog();
dog.name = "Rex";
dog.age = 4;

That works, but it's verbose and easy to forget. Constructors let you initialize the properties the moment new Dog(...) is called.

Defining a constructor

In TypeScript, the constructor is a method named exactly constructor:

class Dog {
    name: string;
    age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }
}

The this keyword

this is a built-in reference to the object being created. this.name = name means "set the name property on this new object to the value of the name parameter".

Calling the constructor

let rex  = new Dog("Rex", 4);
let luna = new Dog("Luna", 7);

By the time these lines finish, rex.name is already "Rex", rex.age is 4, and the same for Luna.

Your task

Inside the Dog class, declare the fields name: string and age: number, then define a constructor that takes name and age and assigns them with this. Then below the class, create two dogs:

let rex: Dog  = new Dog("Rex", 4);
let luna: Dog = new Dog("Luna", 7);

Expected output:

Rex is 4
Luna is 7

STEP 1 OF 4

Guided

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