hey In this blog I am going to talk about how Asynchronous Program works in Js which is a synchronous programming language.

JavaScript is synchronous by default and is single-threaded. This means that code cannot create new threads and run in parallel. Find out what asynchronous code means and how it looks like

Computers are asynchronous by design.

Asynchronous means that things can happen independently of the main program flow.

In the current consumer computers, every program runs for a specific time slot and then it stops its execution to let another program continue its execution. This thing runs in a cycle so fast that it's impossible to notice. We think our computers run many programs simultaneously, but this is an illusion

Programs internally use interrupts, a signal that's emitted to the processor to gain the attention of the system.

Let's not go into the internals of this now, but just keep in mind that it's normal for programs to be asynchronous and halt their execution until they need attention, allowing the computer to execute other things in the meantime. When a program is waiting for a response from the network, it cannot halt the processor until the request finishes.

Normally, programming languages are synchronous and some provide a way to manage asynchronicity in the language or through libraries. C, Java, C#, PHP, Go, Ruby, Swift, and Python are all synchronous by default. Some of them handle async operations by using threads, spawning a new process.

But JavaScript was born inside the browser, its main job, in the beginning, was to respond to user actions, like onClick, onMouseOver, onChange, onSubmit and so on. How could it do this with a synchronous programming model?

The answer was in its environment. The browser provides a way to do it by providing a set of APIs that can handle this kind of functionality.

More recently, Node.js introduced a non-blocking I/O environment to extend this concept to file access, network calls, and so on.

Callbacks

You can't know when a user is going to click a button. So, you define an event handler for the click event. This event handler accepts a function, which will be called when the event is triggered:

document.getElementById('button').addEventListener('click', () => {
  // item clicked
});

This is the so-called callback.

it uses everywhere not just in DOM but also in xhr requests.

Handling errors in callbacks

const fs = require('fs');

fs.readFile('/file.json', (err, data) => {
  if (err) {
    // handle error
    console.log(err);
    return;
  }

  // no errors, process data
  console.log(data);
});

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