Skip to main content

What is Functions in TypeScript? How many types you defined in TypeScript?

A function is a set of statements that perform a specific task and used to create readable, maintainable and re-usable code.

A function declaration tells to compiler about a function name, return type, and parameters.

TypeScript functions are almost similar to JavaScript functions but there are different ways of writing functions in TypeScript.

Stayed Informed – Learn Angular 2 with TypeScript

Different types of functions are available in the TypeScript i.e.

1.     Normal function
2.     Anonymous Function
3.     Named Function
5.     Class Function
7.     Rest Parameters
8.     Default Parameters

Anonymous Functions–
An anonymous function is a function that was declared without any named identifier to refer to it.

Example - Normal function
function printHello() {
    console.log('Hello Anil!');
}

printHello();

Examples - Anonymous function

JavaScript -
var hello = function () {
    console.log('Hello Anil!, I am Anonymous.');
};

hello();//Return - Hello Anil!, I am Anonymous.

OR 

setTimeout(function () {
    console.log('Hello Anil!, I am Anonymous.');
}, 2000); //Return - Hello Anil!, I am Anonymous.

TypeScript–
var anonymousFunc = function (num1: number, num2: number): number {
    return num1 + num2;
}

//RESULT
console.log(anonymousFunc(10, 20)); //Return is 30

//RESULT
console.log(anonymousFunc(10, "xyz")); 
// error: Argument of type 'number' is not assignable to parameter of type 'string'.
//because return type is number for anonymous function).

Named Function -
The named function is very similar to the JavaScript function and only one difference - we must declare the type on the passed parameters.

Example – JavaScript
function addTwoNumer(num1, num2) {
    return num1 + num2;
}

Example – TypeScript
function addTwoNumer(num1: number, num2: number): number {
    return num1 + num2;
}

Lambda Function/Arrow Function -
The arrow function is additional feature in typescript and it is also known as a lambda function.
A lambda function is a function without a name.

var addNum = (n1: number, n2: number) => n1 + n2;

In the above, the “=>” is a lambda operator and (n1 + n2) is the body of the function and (n1: number, n2: number) are inline parameters.

For example –
let addNum = (n1: number, n2: number): number => { return n1 + n2; }
let multiNum = (n1: number, n2: number): number => { return n1 * n2; }
let dividNum = (n1: number, n2: number): number => { return n1 / n2; }

addNum(10, 2);// Result - 12
multiNum(10, 2);// Result - 20
multiNum(10, 2);// Result - 5

Optional Parameters Function -
We can specify optional properties on interfaces and the property may be present or missing in the object.

In the below example, the address property is optional on the following “User” interface.

For Example as,
//USER INTERFACE 
interface User {
    name: string;
    age: number;
    address?: string //Optional
}

//FUNCTION USING USER INTERFACE 
let userInfo = function(user: User) {
   let info = "Hello, " + user.name + " Your Age is - " + user.age + " and Address is -" + user.address;

   return info;
}

//USER INFO JSON OBJECT
let info = {
    name: "Anil",
    age: 30
};

//RESULT
console.log(userInfo(info));

Rest Parameters –
The Rest parameters do not restrict the number of values that we can pass to a function and the passed values must be the same type otherwise throw the error.

For Example as,
//Rest Parameters
let addNumbers = function(...nums: number[]) {
    let p;
    let sum: number = 0;

    for (p = 0; p < nums.length; p++) {
        sum = sum + nums[p];
    }

    return sum;
}

//The Result
addNumbers(1, 2);
addNumbers(1, 2, 3);
addNumbers(1, 12, 10, 18, 17);

Default Parameters -
Function parameters can also be assigned values by default.
A parameter can't be declared as optional and default both at the same time.

For Example as,

let discount = function (price: number, rate: number = 0.40) {
    return price * rate;
}

//CALCULATE DISCOUNT
discount(500); // Result - 200

//CALCULATE DISCOUNT
discount(500, 0.45); // Result - 225

I hope you are enjoying with this post! Please share with you friends. Thank you!!
By Anil Singh | Rating of this article (*****)

Popular posts from this blog

nullinjectorerror no provider for httpclient angular 17

In Angular 17 where the standalone true option is set by default, the app.config.ts file is generated in src/app/ and provideHttpClient(). We can be added to the list of providers in app.config.ts Step 1:   To provide HttpClient in a standalone app we could do this in the app.config.ts file, app.config.ts: import { ApplicationConfig } from '@angular/core'; import { provideRouter } from '@angular/router'; import { routes } from './app.routes'; import { provideClientHydration } from '@angular/platform-browser'; //This (provideHttpClient) will help us to resolve the issue  import {provideHttpClient} from '@angular/common/http'; export const appConfig: ApplicationConfig = {   providers: [ provideRouter(routes),  provideClientHydration(), provideHttpClient ()      ] }; The appConfig const is used in the main.ts file, see the code, main.ts : import { bootstrapApplication } from '@angular/platform-browser'; import { appConfig } from '

39 Best Object Oriented JavaScript Interview Questions and Answers

Most Popular 37 Key Questions for JavaScript Interviews. What is Object in JavaScript? What is the Prototype object in JavaScript and how it is used? What is "this"? What is its value? Explain why "self" is needed instead of "this". What is a Closure and why are they so useful to us? Explain how to write class methods vs. instance methods. Can you explain the difference between == and ===? Can you explain the difference between call and apply? Explain why Asynchronous code is important in JavaScript? Can you please tell me a story about JavaScript performance problems? Tell me your JavaScript Naming Convention? How do you define a class and its constructor? What is Hoisted in JavaScript? What is function overloadin

List of Countries, Nationalities and their Code In Excel File

Download JSON file for this List - Click on JSON file    Countries List, Nationalities and Code Excel ID Country Country Code Nationality Person 1 UNITED KINGDOM GB British a Briton 2 ARGENTINA AR Argentinian an Argentinian 3 AUSTRALIA AU Australian an Australian 4 BAHAMAS BS Bahamian a Bahamian 5 BELGIUM BE Belgian a Belgian 6 BRAZIL BR Brazilian a Brazilian 7 CANADA CA Canadian a Canadian 8 CHINA CN Chinese a Chinese 9 COLOMBIA CO Colombian a Colombian 10 CUBA CU Cuban a Cuban 11 DOMINICAN REPUBLIC DO Dominican a Dominican 12 ECUADOR EC Ecuadorean an Ecuadorean 13 EL SALVADOR

25 Best Vue.js 2 Interview Questions and Answers

What Is Vue.js? The Vue.js is a progressive JavaScript framework and used to building the interactive user interfaces and also it’s focused on the view layer only (front end). The Vue.js is easy to integrate with other libraries and others existing projects. Vue.js is very popular for Single Page Applications developments. The Vue.js is lighter, smaller in size and so faster. It also supports the MVVM ( Model-View-ViewModel ) pattern. The Vue.js is supporting to multiple Components and libraries like - ü   Tables and data grids ü   Notifications ü   Loader ü   Calendar ü   Display time, date and age ü   Progress Bar ü   Tooltip ü   Overlay ü   Icons ü   Menu ü   Charts ü   Map ü   Pdf viewer ü   And so on The Vue.js was developed by “ Evan You ”, an Ex Google software engineer. The latest version is Vue.js 2. The Vue.js 2 is very similar to Angular because Evan You was inspired by Angular and the Vue.js 2 components looks like -

55 Best TypeScript Interview Questions and Answers - JavaScript!

What Is TypeScript? By definition, "TypeScript is a typed superset of JavaScript that compiles to plain JavaScript." TypeScript is a superset of JavaScript which provides optional static typing, classes and interfaces. => The TypeScript was first made public in the year 2012. => Typescript is a modern age JavaScript development language. => TypeScript is a strongly typed, object oriented, compiled language. => TypeScript was designed by Anders Hejlsberg (designer of C#) at Microsoft. => TypeScript is both a language and a set of tools. As an Example of TypeScript, class Hello {     msg : string ;      constructor ( message : string ) {          this . msg = message ;      }       getMsg () {          return "Hello, " + this . msg ;      } } TypeScript introduced a great deal of syntax taken from object-oriented programming, including but not limited to: 1) Interfaces 2) Classes 3) Enumerated t