12/21/2018 - 8:38 AM

01 - Basic Types

01 - Basic Types

TypeScript Basic Types


let isDone: boolean = false;

Number (can also use hex, binary, octal)

  • can also use hex, binary, octal
let decimal: number = 6;


  • can use single or double quotes and template strings
let color: string = 'blue';


First way of declaring array type

let list: number[] = [1, 2, 3];

Second way of declaring array type

  • uses a generic array type, Array<elemType>
let list: Array<number> = [1, 2, 3];


  • allows you to express an array where the type of a fixed number of elements is known, but need not be the same
  • when accessing an element outside the set of known indices, a union type is used instead, this means that the third element can be assigned to a string or number
let x: [string, number];
x = ['hello', 10];  // ok
x = [10, 'hello'];  // error


  • a way of giving more friendly names to sets of numeric values
  • by default numbering begins at 0, can be changed with {Red = 1}
enum Color {Red, Green, Blue}
let c: Color = Color.Green;


  • a way of describing the type of variables that we don't know the values of, may come from dynamic content i.e. user-input
let notSure: any = 4;


  • the absence of having any type at all, could be a return type of functions
  • declaring variables of type void is not useful because you can only assign undefined or null to them


let n: null = null;


let u: undefined = undefined;


  • represents the type of values that never occur e.g. never is the return type for a function expression or arrow function that always throws an exception
function error(message: string) : never {
  throw new Error(message);


  • represents the non-primitive type
declare function create (o: object | null): void;

Type Assertions

  • a way to tell the compiler "trust me, I know what I'm doing"
  • when using TypeScript with JSX, only as-style assertions are allowed

First form:

let someValue: any = "this is a string";
let strLength: number = (<string>someValue).length;

Second form:

let someValue: any = "this is a string";
let strLength: number = (someValue as string).length;