Union Type
let tax: number | string = 10;
Literal Value Type
let requestStatus: 'pending' | 'success' | 'error' = 'pending';
Undefined
let foundBook: string | undefined;
foundBook?.length; // only run if foundBook is a string
let prices: number[] = [100, 74, 23];
Be Careful with Empty Square Brackets
let randomValues:[] = []; // it will assume it is always an empty array
Default to any, but not a good practice
let randomValues: = []; // type any
Union Type
let array: (string | boolean)[] = ['apple', true, 'orange', false];