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Explore the execution order in JavaScript when using callbacks inside promises. Learn why certain outputs occur in a step-by-step explanation suitable for beginners!
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Understanding the Order of Execution with Callbacks Inside Promises in JavaScript

JavaScript is a powerful programming language, but it can be a bit challenging, especially when dealing with asynchronous operations. If you’ve ever worked with promises and callbacks, you might have come across some unexpected behavior in the order of execution.

In this guide, we’ll break down the code provided in a recent question regarding the order of execution when using a callback inside a promise. We’ll explain why the output might not be what you expect and clarify a fundamental concept of JavaScript that is crucial for understanding asynchronous behavior.

The Code and Its Output

Consider the following piece of JavaScript code:

[[See Video to Reveal this Text or Code Snippet]]

When this code runs, the output generated is:

[[See Video to Reveal this Text or Code Snippet]]

This sequence might seem confusing at first glance. Let’s dive deeper to understand why it behaves this way.

Clarifying the Behavior: Synchronous vs Asynchronous

Synchronous Code

The code execution starts in a sequential manner, executing each line one after the other.

The logs for "sync programming in progress" and "sync program done" are printed immediately because they are synchronous calls and are executed as soon as they are encountered.

Asynchronous Code

The Promise constructor takes two parameters: res and rej, used to resolve or reject the promise.

Although the setTimeout function is called to print "promise executed" after 10 seconds, it doesn’t block the execution of subsequent code. This is crucial: setTimeout does not delay the executing code that comes afterward; it schedules the callback (printing "promise executed") to run later.

The Promise Resolution

The line res("async executed"); is called immediately after the setTimeout setup. This resolves the promise and triggers the .then() method, which prints "async executed."

The promise’s resolution happens without waiting for the setTimeout callback to execute. This is what makes JavaScript non-blocking and allows it to handle asynchronous tasks efficiently.

The Final Order of Execution

Thus, when you run the code, the printed output follows this order:

"sync programming in progress" - executed immediately.

"sync program done" - executed next, immediately.

"async executed" - this is printed as soon as the promise resolves.

"promise executed" - this is the delayed message that appears after 10 seconds due to setTimeout.

Conclusion

Understanding the asynchronous nature of JavaScript, particularly with promises and callbacks, is key to mastering the language. The behavior witnessed in this code is a clear demonstration of how JavaScript handles execution order, allowing for a smooth user experience even when operations may take time to complete.

As a novice, it’s a valuable lesson: callbacks given to functions like setTimeout execute later, while code beneath them continues to run immediately. By grasping these principles, you’ll be better equipped to tackle more advanced programming challenges!

Happy coding!