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Understanding Asynchronous JavaScript

Understanding Asynchronous JavaScript

Learn how JavaScript’s event loop handles asynchronous tasks with callbacks, promises, and async/await.

Asynchronous programming is the backbone of modern JavaScript, especially in back-end development with Node.js. In Express.js applications, handling asynchronous tasks—like reading files, querying databases, and making API calls—is crucial to keeping the main execution thread unblocked and responsive.

JavaScript achieves this with its event-driven, non-blocking architecture. But what does that look like in practice? Before we get into the details, let’s explore a simple example of JavaScript’s non-blocking behavior.

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console.log("Start");
setTimeout(() => {
  console.log("Inside setTimeout");
}, 1000);
console.log("End");

Explanation:

  • Line 1: This logs "Start".

  • Line 3: This calls setTimeout(), which schedules a task to run after 1000 milliseconds (one second) and moves on.

  • Line 7: This logs "End" immediately before the timeout completes.

  • After one second: "Inside setTimeout" is logged.

Even though setTimeout() has a delay, JavaScript doesn’t wait—it moves on to the next line. This non-blocking behavior is crucial for handling high performance server applications. Let’s now explore the underlying mechanisms that make this possible.

The event loop and non-blocking operations

As JavaScript is single-threaded, it executes one task at a time. By design, it runs synchronous code in the order it appears, and defers asynchronous tasks until all synchronous operations have completed. Here’s how it manages execution:

  • Call stack: JavaScript maintains a call stack, which keeps track of currently executing functions and follows a last in, first out (LIFO) order. It executes synchronous code sequentially and only processes asynchronous tasks when the call stack is empty.

  • Web APIs: For asynchronous operations (like timers, HTTP requests, and event listeners), JavaScript delegates these tasks to built-in browser or Node.js APIs. These APIs handle the tasks independently, allowing JavaScript to continue executing other synchronous code.

  • Event queue: The event queue holds tasks (called callbacks) that are scheduled to run after an asynchronous operation completes—for example, a function that processes the contents of a file after it has been read asynchronously. These tasks wait to be moved to the call stack for execution when it is empty.

  • Event loop: The event loop is a mechanism within the JavaScript engine that continuously checks whether the call stack is empty. If it is, the event loop moves the next pending task from the event queue to the call stack for execution. This allows JavaScript to manage ...

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