Callback in Node.js for Asynchronous Programming with Examples

What Are Callbacks in Node.JS

The callback is an asynchronous equivalent for a function. It is called following every task. In Node.js, callbacks are frequently used. All APIs of Node are drafted in a way that supports callbacks.  To illustrate, when a function commences reading a file, it instantly returns the control to the execution environment to execute the succeeding instruction.

In Node.js, once file I/O is concluded, it would call the callback function. There is no hindrance or waiting for File I/O. This presents Node.js as highly scalable, as it can process an extraordinary number of requests without pausing for any function to return results.

Understanding the callback pattern for asynchronous programming

In the asynchronous programming model, something happens one at a time. An asynchronous model acknowledges multiple things to occur at the same time. When you commence an action, your program proceeds to run. When the step completes, the program is acquainted and gets access to the result (for example, the data read from disk).

Node.js encourages an asynchronous coding method from the ground up, in contrast to several popular web frameworks. There are numerous vital things to be conscious of when learning to write asynchronous code – and at the same time, you will frequently find your code executing in highly unexpected ways.

Various functions in Node.js core have both synchronous and asynchronous versions. It will be far better to use asynchronous operations in most conditions; otherwise, why use Node.js?

Asynchronous continuation-passing technique

let's consider an example where a function is asynchronous, which is as follows:

function exampleAsync(a, b, callback) {
  setTimeout(function() {
    callback(a + b);
  }, 100);
}
console.log('Before asynchronous call’);
exampleAsync(2, 3, function(finalresult)  
{
    console.log('Result: ' + finalresult);
}); console.log('After asynchronous call');

OutPut:

Before asynchronous call 
After asynchronous call 
Result: 5

setTimeout() triggers an asynchronous process; it will not wait for the callback to get executed. It returns quickly, providing the control back to exampleAsync(), and then back to its caller.

The following image shows how this works:

Asynchronous continuation-passing technique

 

Examples of the callback pattern in Node.js

A callback is a function called when the task finishes, and a callback function allows other code to run in the meantime. Using the Callback concept, Node.js can process many requests without waiting for any function to return the result, making Node.js highly scalable. For example: In Node.js, when a function starts reading the file, it returns the control to the execution environment immediately to execute the next instruction. Once file I/O gets completed, the callback function will get called to avoid blocking or wait for File I/O.

Example 1: Reading a file synchronously in Node.js. Create a text file synch.txt with the following content:

Hello, this is my first testing of synchronous content.

Create a first.js file:

var fs = require("fs"); 
var datatxt = fs.readFileSync('synch.txt'); 
console.log(datatxt.toString()); 
console.log("Execution ends");

Output:

Hello, this is my first testing of synchronous content.
Execution ends

Information: The fs library is loaded to handle file-system associated operations. The readFileSync() function is synchronous and blocks execution till terminated. The function blocks the program until it reads the file, and then only it proceeds to end the program.

Example 2: Reading a file asynchronously in Node.js. Create a text file asynch.txt with the content as

Hello, this is my first testing of asynchronous content.

var fs = require("fs");     
fs.readFile('asynch.txt', function (ferr, dataasynch) {  
    if (ferr) return console.error(ferr);  
    console.log(dataasynch.toString());  
});  
console.log("Execution ends"); 

Output:

Execution ends
Hello, this is my first testing of asynchronous content.

Information: The fs library is loaded to handle file-system-related operations. The readFile() function is asynchronous, and the control returns immediately to the next instruction in the program while the function keeps running in the background. A callback function is relinquished, which gets called when the task running in the background is finished.

Getting trapped in callback hell

Callback Hell is an anti-pattern detected in the code of asynchronous programming. It is a slang phrase applied to define a cumbersome number of nested “if” statements or functions. If the application logic gets complex, a few callbacks appear harmless. But once your project demands grow, it is common to fall under piling layers of nested callbacks.

The callback is a function where “A” is passed to another function, “B,” as a parameter. The function “B” then executes the code “A” at some point. The invocation of “A” can be immediate, as in a synchronous callback, or it can occur later as in an asynchronous callback.

var callbackhell = require(‘fs’)
callbackhell.readFile(‘test.json’, function(‘err’, results){
if(err){
console.log(err);
}
console.log(JSON.parse(results).name)
});

In the code, we call readFile and pass it as a second parameter function (Callback Hell). readFile will execute the callback bypassing the procedure's results to parameters.

The use of callbacks makes the code tedious to write and manage. It increases the difficulty of identifying the application's flow, which is an obstacle, hence the popular name of Callback Hell.

Using Promises to write asynchronous code

Promises are the function that a value would be returned at a later time. Instead of returning concrete values, these asynchronous functions return a Promise object, which will at some point either be fulfilled or not.

A promise represents an asynchronous operation. It means a process that has not been completed yet but is expected to in the future. Let's have a look at a simple file read example without using promises:

fs.readFile(filePath, (err, result) => {
     if (err) { console.log(err); }
     console.log(data);
});
if the readFile function returned a promise, the logic would be  written as below
var fileReadandPromise = fs.readFile(filePath);
fileReadandPromise.then(console.log, console.error)

The fileReadandPromise is passed multiple times in a code where you need to read a file. This helps in writing robust unit tests for your code since you now only have to write a single test for a promise. And more readable code!

Promise.all()

The Promise. all() method accepts an iterable of promises as an input and returns a single Promise that fixes to an array of the input promises' results.

const promise1 = Promise.resolve(5);
const promise2 = 54;
const promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
});
Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values);
});

Promise.any()

Promise.any() takes an iterable of Promise objects and, as soon as one of the promises in the iterable fulfills, returns a single promise that resolves with the value from that promise. If there are no promises in the iterable fulfill, then the returned promise is rejected with an AggregateError, a new subclass of Error that groups together individual errors. This method is the opposite of Promise.all().

const promiseand1 = Promise.reject(0);
const promiseand2 = new Promise((resolve) => setTimeout(resolve, 100, 'Large'));
const promiseand3 = new Promise((resolve) => setTimeout(resolve, 500, 'Small'));
const promises = [promiseand1, promiseand2, promiseand3];
Promise.any(promises).then((value) => console.log(value));

Using Async / Await for handling asynchronous code

Initial versions of Node didn't have the Node architecture single-threaded and asynchronous. The intricacy with this kind of code is that this kind of position can create many problems, and the code can get messy when there are several functions, and this situation is called callback hell.

Promises and function chaining were introduced to overcome this situation.

By Node v8, the async/await feature was finally wheeled out to deal with Promises and function chaining. The functions were not necessarily required to be chained after another; they simply await the function that returns the Promise. But the async function is needed to be declared before awaiting a function returning a Promise.

Examples of Async / Await

The code looks like the following.

async function testfun1(req, res){
  let resp1 = await request.get('http://localhost:8080');
    if (resp1.err) { console.log('error');}
    else { console.log('Response Fetched');
}

Explanation

The code above essentially demands the JavaScript engine driving the code to wait for the request.get() function to accomplish before moving on to the next line to execute it. The request.get() function returns a Promise for which the user will await. Before async/await, if it is required to check that the functions are running in the desired sequence, i.e. one after the another, chain them one after the another or register callbacks. 

Code review and understanding become comfortable with async/await, as observed from the above example.

Error handling in the case of async / await

Formulating exceptions
An exception is built using the throw keyword:
throw value
As soon as the above line executes, the standard program flow stops and the control is held back to the most imminent exception handler.

Typically, in client-side code, a value can be any JavaScript value, including a string, a number or an object.

In Node.js, we don't throw strings; we throw Error objects.

Error objects
An error object is an object that is either an instance of the Error object or extends the Error class provided in the Error core module:

throw new Error('Out of Mall')
OR
class NoPersoninMallError extends Error {
  //...
}
throw new NoPersoninMallError()

Exception handling

An exception handler is a try/catch statement.
Any exception created in the lines of code in the try block is as below:

try { 
//code would be written here. 
} catch (e) {} 
//e is the exception value. 

Error handling with async/await

Using async/await, the errors can be caught as below:

async function AnyFunction() { 
  try { 
    await anotherFunction() 
  } catch (err) { 
    console.error(err.message) 
  } 
}

Conclusion

Node.js is more beneficial to the developers in association with its disadvantages. What’s more important is that it has extended the JavaScript applications area and can be used for both front-end and back-end servers.

Node.js is, without a doubt, one of the more exciting technologies in use today, and it has grown into one of the most popular platforms used for web applications, services, and desktop apps. With time, more and more business organizations have opted to use Node.js and are consistently getting positive results.

#node #nodejs #javascript #programming 

What is GEEK

Buddha Community

Callback in Node.js for Asynchronous Programming with Examples

NBB: Ad-hoc CLJS Scripting on Node.js

Nbb

Not babashka. Node.js babashka!?

Ad-hoc CLJS scripting on Node.js.

Status

Experimental. Please report issues here.

Goals and features

Nbb's main goal is to make it easy to get started with ad hoc CLJS scripting on Node.js.

Additional goals and features are:

  • Fast startup without relying on a custom version of Node.js.
  • Small artifact (current size is around 1.2MB).
  • First class macros.
  • Support building small TUI apps using Reagent.
  • Complement babashka with libraries from the Node.js ecosystem.

Requirements

Nbb requires Node.js v12 or newer.

How does this tool work?

CLJS code is evaluated through SCI, the same interpreter that powers babashka. Because SCI works with advanced compilation, the bundle size, especially when combined with other dependencies, is smaller than what you get with self-hosted CLJS. That makes startup faster. The trade-off is that execution is less performant and that only a subset of CLJS is available (e.g. no deftype, yet).

Usage

Install nbb from NPM:

$ npm install nbb -g

Omit -g for a local install.

Try out an expression:

$ nbb -e '(+ 1 2 3)'
6

And then install some other NPM libraries to use in the script. E.g.:

$ npm install csv-parse shelljs zx

Create a script which uses the NPM libraries:

(ns script
  (:require ["csv-parse/lib/sync$default" :as csv-parse]
            ["fs" :as fs]
            ["path" :as path]
            ["shelljs$default" :as sh]
            ["term-size$default" :as term-size]
            ["zx$default" :as zx]
            ["zx$fs" :as zxfs]
            [nbb.core :refer [*file*]]))

(prn (path/resolve "."))

(prn (term-size))

(println (count (str (fs/readFileSync *file*))))

(prn (sh/ls "."))

(prn (csv-parse "foo,bar"))

(prn (zxfs/existsSync *file*))

(zx/$ #js ["ls"])

Call the script:

$ nbb script.cljs
"/private/tmp/test-script"
#js {:columns 216, :rows 47}
510
#js ["node_modules" "package-lock.json" "package.json" "script.cljs"]
#js [#js ["foo" "bar"]]
true
$ ls
node_modules
package-lock.json
package.json
script.cljs

Macros

Nbb has first class support for macros: you can define them right inside your .cljs file, like you are used to from JVM Clojure. Consider the plet macro to make working with promises more palatable:

(defmacro plet
  [bindings & body]
  (let [binding-pairs (reverse (partition 2 bindings))
        body (cons 'do body)]
    (reduce (fn [body [sym expr]]
              (let [expr (list '.resolve 'js/Promise expr)]
                (list '.then expr (list 'clojure.core/fn (vector sym)
                                        body))))
            body
            binding-pairs)))

Using this macro we can look async code more like sync code. Consider this puppeteer example:

(-> (.launch puppeteer)
      (.then (fn [browser]
               (-> (.newPage browser)
                   (.then (fn [page]
                            (-> (.goto page "https://clojure.org")
                                (.then #(.screenshot page #js{:path "screenshot.png"}))
                                (.catch #(js/console.log %))
                                (.then #(.close browser)))))))))

Using plet this becomes:

(plet [browser (.launch puppeteer)
       page (.newPage browser)
       _ (.goto page "https://clojure.org")
       _ (-> (.screenshot page #js{:path "screenshot.png"})
             (.catch #(js/console.log %)))]
      (.close browser))

See the puppeteer example for the full code.

Since v0.0.36, nbb includes promesa which is a library to deal with promises. The above plet macro is similar to promesa.core/let.

Startup time

$ time nbb -e '(+ 1 2 3)'
6
nbb -e '(+ 1 2 3)'   0.17s  user 0.02s system 109% cpu 0.168 total

The baseline startup time for a script is about 170ms seconds on my laptop. When invoked via npx this adds another 300ms or so, so for faster startup, either use a globally installed nbb or use $(npm bin)/nbb script.cljs to bypass npx.

Dependencies

NPM dependencies

Nbb does not depend on any NPM dependencies. All NPM libraries loaded by a script are resolved relative to that script. When using the Reagent module, React is resolved in the same way as any other NPM library.

Classpath

To load .cljs files from local paths or dependencies, you can use the --classpath argument. The current dir is added to the classpath automatically. So if there is a file foo/bar.cljs relative to your current dir, then you can load it via (:require [foo.bar :as fb]). Note that nbb uses the same naming conventions for namespaces and directories as other Clojure tools: foo-bar in the namespace name becomes foo_bar in the directory name.

To load dependencies from the Clojure ecosystem, you can use the Clojure CLI or babashka to download them and produce a classpath:

$ classpath="$(clojure -A:nbb -Spath -Sdeps '{:aliases {:nbb {:replace-deps {com.github.seancorfield/honeysql {:git/tag "v2.0.0-rc5" :git/sha "01c3a55"}}}}}')"

and then feed it to the --classpath argument:

$ nbb --classpath "$classpath" -e "(require '[honey.sql :as sql]) (sql/format {:select :foo :from :bar :where [:= :baz 2]})"
["SELECT foo FROM bar WHERE baz = ?" 2]

Currently nbb only reads from directories, not jar files, so you are encouraged to use git libs. Support for .jar files will be added later.

Current file

The name of the file that is currently being executed is available via nbb.core/*file* or on the metadata of vars:

(ns foo
  (:require [nbb.core :refer [*file*]]))

(prn *file*) ;; "/private/tmp/foo.cljs"

(defn f [])
(prn (:file (meta #'f))) ;; "/private/tmp/foo.cljs"

Reagent

Nbb includes reagent.core which will be lazily loaded when required. You can use this together with ink to create a TUI application:

$ npm install ink

ink-demo.cljs:

(ns ink-demo
  (:require ["ink" :refer [render Text]]
            [reagent.core :as r]))

(defonce state (r/atom 0))

(doseq [n (range 1 11)]
  (js/setTimeout #(swap! state inc) (* n 500)))

(defn hello []
  [:> Text {:color "green"} "Hello, world! " @state])

(render (r/as-element [hello]))

Promesa

Working with callbacks and promises can become tedious. Since nbb v0.0.36 the promesa.core namespace is included with the let and do! macros. An example:

(ns prom
  (:require [promesa.core :as p]))

(defn sleep [ms]
  (js/Promise.
   (fn [resolve _]
     (js/setTimeout resolve ms))))

(defn do-stuff
  []
  (p/do!
   (println "Doing stuff which takes a while")
   (sleep 1000)
   1))

(p/let [a (do-stuff)
        b (inc a)
        c (do-stuff)
        d (+ b c)]
  (prn d))
$ nbb prom.cljs
Doing stuff which takes a while
Doing stuff which takes a while
3

Also see API docs.

Js-interop

Since nbb v0.0.75 applied-science/js-interop is available:

(ns example
  (:require [applied-science.js-interop :as j]))

(def o (j/lit {:a 1 :b 2 :c {:d 1}}))

(prn (j/select-keys o [:a :b])) ;; #js {:a 1, :b 2}
(prn (j/get-in o [:c :d])) ;; 1

Most of this library is supported in nbb, except the following:

  • destructuring using :syms
  • property access using .-x notation. In nbb, you must use keywords.

See the example of what is currently supported.

Examples

See the examples directory for small examples.

Also check out these projects built with nbb:

API

See API documentation.

Migrating to shadow-cljs

See this gist on how to convert an nbb script or project to shadow-cljs.

Build

Prequisites:

  • babashka >= 0.4.0
  • Clojure CLI >= 1.10.3.933
  • Node.js 16.5.0 (lower version may work, but this is the one I used to build)

To build:

  • Clone and cd into this repo
  • bb release

Run bb tasks for more project-related tasks.

Download Details:
Author: borkdude
Download Link: Download The Source Code
Official Website: https://github.com/borkdude/nbb 
License: EPL-1.0

#node #javascript

Aria Barnes

Aria Barnes

1622719015

Why use Node.js for Web Development? Benefits and Examples of Apps

Front-end web development has been overwhelmed by JavaScript highlights for quite a long time. Google, Facebook, Wikipedia, and most of all online pages use JS for customer side activities. As of late, it additionally made a shift to cross-platform mobile development as a main technology in React Native, Nativescript, Apache Cordova, and other crossover devices. 

Throughout the most recent couple of years, Node.js moved to backend development as well. Designers need to utilize a similar tech stack for the whole web project without learning another language for server-side development. Node.js is a device that adjusts JS usefulness and syntax to the backend. 

What is Node.js? 

Node.js isn’t a language, or library, or system. It’s a runtime situation: commonly JavaScript needs a program to work, however Node.js makes appropriate settings for JS to run outside of the program. It’s based on a JavaScript V8 motor that can run in Chrome, different programs, or independently. 

The extent of V8 is to change JS program situated code into machine code — so JS turns into a broadly useful language and can be perceived by servers. This is one of the advantages of utilizing Node.js in web application development: it expands the usefulness of JavaScript, permitting designers to coordinate the language with APIs, different languages, and outside libraries.

What Are the Advantages of Node.js Web Application Development? 

Of late, organizations have been effectively changing from their backend tech stacks to Node.js. LinkedIn picked Node.js over Ruby on Rails since it took care of expanding responsibility better and decreased the quantity of servers by multiple times. PayPal and Netflix did something comparative, just they had a goal to change their design to microservices. We should investigate the motivations to pick Node.JS for web application development and when we are planning to hire node js developers. 

Amazing Tech Stack for Web Development 

The principal thing that makes Node.js a go-to environment for web development is its JavaScript legacy. It’s the most well known language right now with a great many free devices and a functioning local area. Node.js, because of its association with JS, immediately rose in ubiquity — presently it has in excess of 368 million downloads and a great many free tools in the bundle module. 

Alongside prevalence, Node.js additionally acquired the fundamental JS benefits: 

  • quick execution and information preparing; 
  • exceptionally reusable code; 
  • the code is not difficult to learn, compose, read, and keep up; 
  • tremendous asset library, a huge number of free aides, and a functioning local area. 

In addition, it’s a piece of a well known MEAN tech stack (the blend of MongoDB, Express.js, Angular, and Node.js — four tools that handle all vital parts of web application development). 

Designers Can Utilize JavaScript for the Whole Undertaking 

This is perhaps the most clear advantage of Node.js web application development. JavaScript is an unquestionable requirement for web development. Regardless of whether you construct a multi-page or single-page application, you need to know JS well. On the off chance that you are now OK with JavaScript, learning Node.js won’t be an issue. Grammar, fundamental usefulness, primary standards — every one of these things are comparable. 

In the event that you have JS designers in your group, it will be simpler for them to learn JS-based Node than a totally new dialect. What’s more, the front-end and back-end codebase will be basically the same, simple to peruse, and keep up — in light of the fact that they are both JS-based. 

A Quick Environment for Microservice Development 

There’s another motivation behind why Node.js got famous so rapidly. The environment suits well the idea of microservice development (spilling stone monument usefulness into handfuls or many more modest administrations). 

Microservices need to speak with one another rapidly — and Node.js is probably the quickest device in information handling. Among the fundamental Node.js benefits for programming development are its non-obstructing algorithms.

Node.js measures a few demands all at once without trusting that the first will be concluded. Many microservices can send messages to one another, and they will be gotten and addressed all the while. 

Versatile Web Application Development 

Node.js was worked in view of adaptability — its name really says it. The environment permits numerous hubs to run all the while and speak with one another. Here’s the reason Node.js adaptability is better than other web backend development arrangements. 

Node.js has a module that is liable for load adjusting for each running CPU center. This is one of numerous Node.js module benefits: you can run various hubs all at once, and the environment will naturally adjust the responsibility. 

Node.js permits even apportioning: you can part your application into various situations. You show various forms of the application to different clients, in light of their age, interests, area, language, and so on. This builds personalization and diminishes responsibility. Hub accomplishes this with kid measures — tasks that rapidly speak with one another and share a similar root. 

What’s more, Node’s non-hindering solicitation handling framework adds to fast, letting applications measure a great many solicitations. 

Control Stream Highlights

Numerous designers consider nonconcurrent to be one of the two impediments and benefits of Node.js web application development. In Node, at whatever point the capacity is executed, the code consequently sends a callback. As the quantity of capacities develops, so does the number of callbacks — and you end up in a circumstance known as the callback damnation. 

In any case, Node.js offers an exit plan. You can utilize systems that will plan capacities and sort through callbacks. Systems will associate comparable capacities consequently — so you can track down an essential component via search or in an envelope. At that point, there’s no compelling reason to look through callbacks.

 

Final Words

So, these are some of the top benefits of Nodejs in web application development. This is how Nodejs is contributing a lot to the field of web application development. 

I hope now you are totally aware of the whole process of how Nodejs is really important for your web project. If you are looking to hire a node js development company in India then I would suggest that you take a little consultancy too whenever you call. 

Good Luck!

Original Source

#node.js development company in india #node js development company #hire node js developers #hire node.js developers in india #node.js development services #node.js development

Hire Dedicated Node.js Developers - Hire Node.js Developers

If you look at the backend technology used by today’s most popular apps there is one thing you would find common among them and that is the use of NodeJS Framework. Yes, the NodeJS framework is that effective and successful.

If you wish to have a strong backend for efficient app performance then have NodeJS at the backend.

WebClues Infotech offers different levels of experienced and expert professionals for your app development needs. So hire a dedicated NodeJS developer from WebClues Infotech with your experience requirement and expertise.

So what are you waiting for? Get your app developed with strong performance parameters from WebClues Infotech

For inquiry click here: https://www.webcluesinfotech.com/hire-nodejs-developer/

Book Free Interview: https://bit.ly/3dDShFg

#hire dedicated node.js developers #hire node.js developers #hire top dedicated node.js developers #hire node.js developers in usa & india #hire node js development company #hire the best node.js developers & programmers

Node JS Development Company| Node JS Web Developers-SISGAIN

Top organizations and start-ups hire Node.js developers from SISGAIN for their strategic software development projects in Illinois, USA. On the off chance that you are searching for a first rate innovation to assemble a constant Node.js web application development or a module, Node.js applications are the most appropriate alternative to pick. As Leading Node.js development company, we leverage our profound information on its segments and convey solutions that bring noteworthy business results. For more information email us at hello@sisgain.com

#node.js development services #hire node.js developers #node.js web application development #node.js development company #node js application

sophia tondon

sophia tondon

1625114985

Top 10 NodeJs app Development Companies- ValueCoders

Node.js is a prominent tech trend in the space of web and mobile application development. It has been proven very efficient and useful for a variety of application development. Thus, all business owners are eager to leverage this technology for creating their applications.

Are you striving to develop an application using Node.js? But can’t decide which company to hire for NodeJS app development? Well! Don’t stress over it, as the following list of NodeJS app development companies is going to help you find the best partner.

Let’s take a glance at top NodeJS application development companies to hire developers in 2021 for developing a mind-blowing application solution.

Before enlisting companies, I would like to say that every company has a foundation on which they thrive. Their end goals, qualities, and excellence define their competence. Thus, I prepared this list by considering a number of aspects. While making this list, I have considered the following aspects:

  • Review and rating
  • Enlisted by software peer & forums
  • Hourly price
  • Offered services
  • Year of experience (Average 8+ years)
  • Credibility & Excellence
  • Served clients and more

I believe this list will help you out in choosing the best NodeJS service provider company. So, now let’s explore the top NodeJS developer companies to choose from in 2021.

#1. JSGuru

JSGuru is a top-rated NodeJS app development company with an innovative team of dedicated NodeJS developers engaged in catering best-class UI/UX design, software products, and AWS professional services.

It is a team of one of the most talented developers to hire for all types of innovative solution development, including social media, dating, enterprise, and business-oriented solutions. The company has worked for years with a number of startups and launched a variety of products by collaborating with big-name corporations like T-systems.

If you want to hire NodeJS developers to secure an outstanding application, I would definitely suggest them. They serve in the area of eLearning, FinTech, eCommerce, Telecommunications, Mobile Device Management, and more.

  • Ratings: 4.9/5.0

  • Founded: 2006

  • Headquarters: Banja Luka, Bosnia, and Herzegovina

  • Price: Starting from $50/hour

Visit Website - https://www.valuecoders.com/blog/technology-and-apps/top-node-js-app-development-companies

#node js developer #hire node js developer #hiring node js developers #node js development company #node.js development company #node js development services