Callbacks

JavaScript is a single-threaded programming language, which means only one thing can happen at a time. While a single thread simplifies writing and reasoning about code, this also has some drawbacks.

Imagine we perform a long-running task like fetching a resource over the network. Now, we block the browser until the resource is downloaded. This can make for a bad user experience and might result in the user leaving our page.

That’s where asynchronous JavaScript comes into play. Using asynchronous JavaScript, we can perform long-lasting tasks without blocking the main thread.

There are three ways of doing asynchronous JavaScript:

• Callbacks
• Promises
• Async/Await

Before we look into these techniques, let’s try to understand the difference between synchronous and asynchronous code.

Synchronous code

When we execute code synchronously, we wait for it to finish before moving ...