TypeScript Cheat Sheet

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

Variables

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;

Functions

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

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

Objects

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;
};