Cheatsheet for TypeScript. Please make an issue if you encounter a problem and PR always welcome.
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- Go to TypeScript download from there or just type
npm install -g typescript. - Check the installed version
tsc -v.
There are some basic types available for TypeScript. Such as string, number, Array<number>, string[], boolean etc. We can do Type Declaration by them.
let x: number = 5;
let name: string = "John Doe";
let showErrorMsg: boolean = true;
let numbers: number[] = [1, 2, 3];
let numbers: Array<number> = [1, 2, 3];
let students: Array<string> = ["John Doe", "Michael"];
function showDetail(name: string, age: number): void {
console.log(name);
}This is something new in TypeScript 3. A Tuple is an array but the number of elements are fixed.
let product: [string, number];
product = ["John", 10];
let val: [string, ...number[]];
val = ["John", 5, 7, 9];Enum allows us to declare a set of collection in which we can assign them a perticular value such number, string by default TS compiler will assign them in number.
enum ProgrammingLanguage {
javascript,
python,
php,
}
// This will be changed as
enum ProgrammingLanguage {
javascript = 0,
python = 1,
php = 2,
}TypeScript allows us to assign more than one data type for a variable or for a function.
let address: string | number;
address = "New York";
// or
address = 3100;
function getValue(): string | number {
return "John Doe";
// or
return 123;
}If you don't know the actual data type or you don't want to assign a data type you can use any.
let detail: any;
detail = "Developer";
function getValue(): any {
return "Developer";
}unknown is the type-safe counterpart of any. Anything is assignable to unknown, but unknown isn’t assignable to anything but itself.
let value: unknown;
value.trim(); // Error
value(); // Error
new value(); // ErrorThis is useful when you are not returning anything from the function(this is not mandatory to include).
function getError(): void {
console.log("TypeScript is a SuperSet of JavaScript");
}Use never when a function doesn't return anything. This can be used when a function returns error.
function error(message: string): never {
throw new Error(message);
}Partial makes all properties of the type optional.
interface User {
id: number;
name: string;
email: string;
}
let user: Partial<User> = {
id: 1,
};When we use readonly, we can’t assign them with other variables, can’t update or delete. This avoids unexpected mutation.
let students: readonly Array<string> = ["John", "Michael", "Adam"];It allows you to create a new type from an existing interface.
interface User {
id: number
name: string
email: string
}
function foo(args: Pick<User, "name" | "email">) {
console.log(args);
}
foo({ name: "John Doe", email: "doe@gmail.com" });An Interface is a group of properties and methods that describe an object but neither does initialization nor implementation.
interface User {
name: string;
age: number;
getPoints(point: number): number;
}
let user: User = {
name: "John Doe",
age: 25,
getPoints(point: number): number {
return point * point;
},
};We can make properties and parameters optional.
interface User {
name?: string;
age: number;
getPoints(point?: number): number;
}
let user: User = {
age: 25,
getPoints(): number {
return 5 * 5;
},
};Type Alias creates a new name for a type.
type GetArea = (width: number, height: number) => number;
interface Rectangle {
getArea: GetArea;
}
let rectangle: Rectangle = {
getArea(width: number, height: number): number {
return width * height;
},
};Just like the other language TypeScript has a class feature.
class Product {
name: string;
price: number;
constructor(name: string, price: number) {
this.name = name;
this.price = price;
}
getTotalPrice(discount: number): number {
return this.price - discount;
}
}
const product = new Product("Milk", 250);We can add interface for Product class.
interface IProduct {
name: string;
price: number;
getTotalPrice(discount: number): number;
}
class Product implements IProduct {
name: string;
price: number;
constructor(name: string, price: number) {
this.name = name;
this.price = price;
}
getTotalPrice(discount: number): number {
return this.price - discount;
}
}
const product = new Product("Milk", 250);We can specifically pass the type to any function but what if when we don't know the type to pass, Generic can help us.
// Generic Function
function removeItem<T>(arr: Array<T>, item: T): Array<T> {
let index = arr.findIndex((i) => i === item);
arr.splice(index, 1);
return arr;
}
console.log(removeItem([1, 2, 3], 2));
console.log(removeItem(["John", "Michael"], "John"));// Generic Class
class Country {
protected countryName: string;
constructor(countryName: string) {
this.countryName = countryName;
}
getName() {
return this.countryName;
}
}
class Australia extends Country {}
class England extends Country {}
let australia = new Australia("Australia");
let england = new England("England");
function getName<T>(country: T): T {
return country;
}
console.log(getName(australia));
console.log(getName(england));Symbol is a primitive data type, which is also immutable and unique.
let sym1 = Symbol("key");
let sym2 = Symbol("key");
sym1 === sym2; // falseIn TypeScript keyof operator extracts the set of keys for a given type.
interface IUser {
name: string;
age: number;
}
type UserKeys = keyof IUser; // "name" | "age"Using the keyof operator we can do Indexed Access Types. The key parameter can get the type of each key of obj.
function getValue<T, K extends keyof T>(obj: T, key: K): T[K] {
return obj[key];
}
const user = {
id: 1,
name: "John Doe",
};
console.log(getValue(user, "id")); // 1In TypeScript Mapped Types allow us to create new types from existing types.
interface Person {
name: string;
age: number;
}
type readonlyPerson = {
readonly [P in keyof Person]: Person[p];
};In order to find specific type when we use union types, we can use the Type Guard. typeof, instanceof, in are the examples of Type Guard.
// typeof
function showMessage(message: string | object): void {
if (typeof message === "string") {
console.log("The type is string");
} else if (typeof message === "object") {
console.log("The type is object");
}
}
showMessage({ name: "John Doe" });// instanceof
class User {
id: number;
}
class Product {
name: string;
}
function showMessage(message: User | Product): void {
if (message instanceof User) {
console.log("Message is an instance of User");
} else if (message instanceof Product) {
console.log("Message is an instance of Product");
}
}
showMessage(new User());You might have come across the term namespace while working with TypeScript. And you might wonder what on earth is a namespace? We will be dealing with the basic concepts of namespaces and how we can use them to write efficient codes.
namespace AcmeCorp.Logging {
export class Logger {
static log(msg: string) : void {
console.log(msg);
};
}
}
/// <reference path="AcmeCorp.Logging.ts" />
//Alias
import logger = AcmeCorp.Logging.Logger;
namespace AcmeCorp.OnlineStore {
class OrderLogic {
calcOrder(): number {
logger.log("calculating order");
return 0;
}
}
}