TS Basic: Type Aliases

July 22, 2023

Type aliases are a way of creating a custom name for your type variables, functions and objects for reusability and readability.

Basic Type Aliases

type MyString = string;

const message : MyString = 'Hello world'

Object Type Aliases

type Point = {
	x: number,
	y: number,
}

const origin : Point = [ x: 0, y: 0 ];

Union Type Aliases

type NumberOrString = number | string;

const value : NumberOrString = 42;
const value : NumberOrString = 'hello';

Functional Type Aliases

type MathOperation = (x: number, y:number) => number;

const add : MathOperation = (a,b) => a + b;
const result = add(2,3); // 5

Union of Object Type Aliases

type Shape = Square | Circle | Triangle;

type Square = { type: 'square'; sideLength: number };
type Circle = { type: 'circle'; radius: number };
type Triangle = { type: 'triangle'; base: number; height: number; }; 

function calculateArea(shape: Shape): number {
  switch (shape.type) {
    case 'square':
      return shape.sideLength ** 2;
    case 'circle':
      return Math.PI * shape.radius ** 2;
    case 'triangle':
      return (shape.base * shape.height) / 2;
    default:
      throw new Error('Invalid shape type');
  }
}

const squareArea = calculateArea(square);
const circleArea = calculateArea(circle);
const triangleArea = calculateArea(triangle);

Type aliases can make your code more expressive and allow you to create reusable types that can simplify your type annotation throughout your codebase.

👌Note that aliases are only aliases - you cannot use type aliased to create different distinct "version" of the same type.