Array type with known and possibly different types:

let day: [number, string];
day = [0, 'Monday']; // valid
day = ['zero', 'Monday']; // invalid: 'zero' is not numeric
console.log(day[0]); // 0
console.log(day[1]); // Monday
day[2] = 'Saturday'; // valid: [0, 'Saturday']
day[3] = false; // invalid: must be union type of 'number | string'

Boolean in TypeScript

A boolean represents the most basic datatype in TypeScript, with the purpose of assigning true/false values.

// set with initial value (either true or false)
let isTrue: boolean = true;
// defaults to 'undefined', when not explicitly set
let unsetBool: boolean;
// can also be set to 'null' as well
let nullableBool: boolean = null;

Intersection Types

A Intersection Type combines the member of two or more types.

interface Knife {
interface BottleOpener{
interface Screwdriver{
type SwissArmyKnife = Knife & BottleOpener & Screwdriver;
function use(tool: SwissArmyKnife){
console.log("I can do anything!");

Types in function arguments and return value. Number

When you create a function in TypeScript you can specify the data type of the function’s arguments and the data type for the return value


function sum(x: number, y: number): number {
return x + y;

Here the syntax x: number, y: number means that the function can accept two arguments x and y and they can only be numbers and (…): number { means that the return value can only be a number.


sum(84 + 76) // will be return 160

You can not do so

function sum(x: string, y: string): number {
return x + y;


function sum(x: number, y: number): string {
return x + y;

it will receive the following errors:

error TS2322: Type ‘string’ is not assignable to type ‘number’ and error TS2322:

Type ‘number’ is
not assignable to type ‘string’ respectively

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