Creating interfaces, types aliases, and classes

In the Understanding basic types section, we introduced ourselves to objects, which are types that can have their own properties. Interfaces, type aliases, and classes are ways that we can define an object structure before we start using it.

Following here is the customer object we worked with, where we declared the customer variable with an initial object value:

const customer = {
  name: "Lamps Ltd", 
  turnover: 2000134, 
  active: true 
};

1. Let's try to declare the customer variable and set its value on a subsequent line:

let customer: object;
customer = {
  name: "Lamps Ltd", 
  turnover: 2000134, 
  active: true 
};

1. So far, so good. However, let's see what happens when we try to change the customers turnover value:

customer.turnover = 2000200;

1. The lack of IntelliSense when we type turnover isn't what we are used to. When we've finished typing the line, we get a compiler error:

The TypeScript compiler doesn't know about the properties in the customer object and so thinks there's a problem.

So, we need another way of defining an object structure with the ability to set property values later in the program. That's where interfaces, type aliases, and classes come in; they let us define the structure of an object by letting us define our own types.