TypeScript Cheat Sheet

Last updated on 7 Feb, 2022

TypeScript is language that brings static type check to JavaScript. Here are few TypeScript points for quick reference.


Variable type can be inferred from the initialized value.

let age = 10; // number type

Re-assigning a value to a const variable throws error.

const a = 10; // literal type
a = 10; // Error

Type of a variable that is declared, but not initialized is any

let a; // any type

Annotation can be used to define type explicitly.

let a: string;


Types for function arguments and return value can be specified using annotations.

function add(a: number, b: number): number {
  return a + b;


An object student contains 2 properties, name and age. name needs to be of string type and age needs to be a number.

let student: {
  name: string;
  age: number;

We can mark a property as optional by using ?

let person: {
  name: string;
  status: string;
  spouse?: string;
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Joby Joseph
Web Architect