Understanding Asynchronous JavaScript

1. Introduction

Brief explanation of the tutorial's goal

In this tutorial, we will be diving into the world of asynchronous JavaScript. We'll unravel the mystery behind callbacks, promises, and the Async/Await syntax, and we'll learn how to use these tools to handle asynchronous operations effectively in our applications.

What the user will learn

• Understand how asynchronous programming works in JavaScript
• Learn how to use callbacks, promises and async/await
• Write asynchronous JavaScript code effectively

Prerequisites (if any)

Basic understanding of JavaScript is required. Knowledge of ES6 syntax will be beneficial but not necessary.

2. Step-by-Step Guide

Callbacks

In JavaScript, a callback is a function passed into another function as an argument to be executed later. Callbacks are often used to handle asynchronous operations such as reading files or making HTTP requests.

fs.readFile('example.txt', 'utf8', function(err, data) {
  if (err) throw err;
  console.log(data);
});

In the above example, we are reading a file asynchronously. The callback function will be invoked after the file is read.

Promises

Promises are objects that represent the eventual completion or failure of an asynchronous operation. Promises can be in one of three states: pending, fulfilled, or rejected.

let promise = new Promise(function(resolve, reject) {
  // asynchronous operation
});

Async/Await

Async/await is a modern way of handling promises. An async function is a function that knows how to expect the possibility of the await keyword being used to invoke asynchronous code.

async function exampleFunction() {
  const data = await asyncFunction();
  console.log(data);
}

3. Code Examples

Callbacks

function greeting(name, callback) {
  let message = `Hello, ${name}!`;
  callback(message);
}

// Use the function
greeting("John", (message) => {
  console.log(message); // Outputs: Hello, John!
});

Promises

let promise = new Promise((resolve, reject) => {
  let operationCompleted = true;
  if (operationCompleted) {
    resolve("Operation completed");
  } else {
    reject("Operation not completed");
  }
});

// Use the promise
promise
  .then((message) => {
    console.log(message); // Outputs: Operation completed
  })
  .catch((message) => {
    console.log(message);
  });

Async/Await

async function asyncFunction() {
  try {
    let response = await fetch('https://api.example.com/data');
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.log(error);
  }
}

// Use the function
asyncFunction();

4. Summary

In this tutorial, we learned about asynchronous JavaScript, including callbacks, promises, and async/await. We saw how to use these features to handle asynchronous operations in our applications.

5. Practice Exercises

1. Write a function that uses a callback to asynchronously add two numbers after a delay of two seconds.

2. Convert the above function to return a promise.

3. Use async/await to call the function created in the second exercise.

Solutions and further practice can be found at MDN.