Constructors

Constructors are functions that perform the initialization of new instances of a class. In order to implement a constructor, we implement a function called constructor. It's common to set property values in the constructor to simplify consumption of the class.

Let's look at the following example:

1. Let's create a constructor in the OrderDetail class that allows us to set the product and quantity:

class OrderDetail implements IOrderDetail {
  product: Product;
  quantity: number;

  constructor(product: Product, quantity: number) {
    this.product = product;
    this.quantity = quantity;
  }

  getTotal(discount: number): number {
    ...
  }
}

1. If we create an instance of the class, we are forced to pass in the product and quantity:

const orderDetail = new OrderDetail(table, 2);

1. This is nice because we've reduced three lines of code to one line. However, we can make our class even nicer to work with by making the default quantity parameter 1 if nothing is passed in:

constructor(product: Product, quantity: number = 1) {
  this.product = product;
  this.quantity = quantity;
}

1. We now don't have to pass in a quantity if it is 1:

const orderDetail = new OrderDetail(table);

1. We can save ourselves a few keystrokes and let the TypeScript compiler implement the product and quantity properties by using the public keyword before the parameters in the constructor:

class OrderDetail implements IOrderDetail {
  constructor(public product: Product, public quantity: number = 1) {
    this.product = product;
    this.quantity = quantity;
  }

  getTotal(discount: number): number {
    ...
  }
}