What Is A Type System

This course breaks our learning into 2 parts:

• Syntax + features of TypeScript

  • What is an interface? What is the syntax for defining one?

• Design patterns with TypeScript

  • How do we use interfaces to write reusable code?

Types

Think of a type as a shorthand name that denotes all the properties and methods that a value has.

For example, if I say the value "red" is a string, I'm telling you that it has properties like length or methods like trim().

Similarly, in TypeScript, if I assigned an interface to an object, I'm telling TypeScript about that object's type and therefore its properties and methods!

interface Todo {
  id: number;
  title: string;
  completed: boolean;
}

axios.get('/todos/1').then(response => {
  const todo = response.data as Todo;
});

Categories of types

There are 2 major categories of types in TypeScript:

1. Primitive types: numbers, booleans, strings, null, undefined

2. Object types: functions, classes, arrays, objects

Pro tip: You'll see later on that this distinction is helpful because object types can be used to trick TypeScript into believing some value is an object type.