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TypeScript allows types to be defined separately from the variables that use them.

Aliases and Interfaces allows types to be easily shared between different variables/objects.

Type Aliases

Type Aliases allow defining types with a custom name (an Alias).

Type Aliases can be used for primitives like string or more complex types such as objects and arrays:

Example

type CarYear = number
type CarType = string
type CarModel = string
type Car = {
  year: CarYear,
  type: CarType,
  model: CarModel
}

const carYear: CarYear = 2001
const carType: CarType = "Toyota"
const carModel: CarModel = "Corolla"
const car: Car = {
  year: carYear,
  type: carType,
  model: carModel
};

Interfaces

Interfaces are similar to type aliases, except they only apply to object types.

Example

interface Rectangle {
  height: number,
  width: number
}

const rectangle: Rectangle = {
  height: 20,
  width: 10
};

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Extending Interfaces

Interfaces can extend each other's definition.

Extending an interface means you are creating a new interface with the same properties as the original, plus something new.

Example

interface Rectangle {
  height: number,
  width: number
}

interface ColoredRectangle extends Rectangle {
  color: string
}

const coloredRectangle: ColoredRectangle = {
  height: 20,
  width: 10,
  color: "red"
};

TypeScript Exercises

Test Yourself With Exercises

Exercise:

Create a Type Alias for a string, called carType:

 carType =

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