Скачать или смотреть Understanding Promises: Why resolve Executes Before async Code

Explore the inner workings of JavaScript Promises and understand why `resolve` is executed before asynchronous code despite `setTimeout` having a 0ms delay.
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Understanding Promises: Why resolve Executes Before async Code

When working with JavaScript, especially in asynchronous programming, understanding how Promises function can be quite perplexing. There’s often a misinterpretation about the order of execution between synchronous and asynchronous code, particularly regarding the resolve method of Promises. In this post, we will take a closer look at this concept to clarify why resolve runs before an asynchronous call, including important insights into the event loop and call stack.

The Code Breakdown

Let's begin with the code that's at the heart of this discussion:

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

Expected Output

Based on the structure of the code, you might expect the output to be:

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

However, the actual output you’ll see is:

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

This unexpected behavior raises the question: Why does resolve execute before the asynchronous log from setTimeout?

Understanding the Execution Flow

To demystify this, we need to understand several concepts within JavaScript's concurrency model, including the call stack, the event loop, and how asynchronous code is handled.

1. Synchronous vs Asynchronous Code

Synchronous code runs line by line in the order it appears. This means that if there's code that doesn't rely on any asynchronous processes, it will execute first.

Asynchronous code, on the other hand, can run in the background. It uses mechanisms like callbacks, promises, and events to handle operations that take time (like network requests, file I/O, etc.) without blocking the main execution thread.

2. The Role of setTimeout

In the provided code, the line with setTimeout is registering an asynchronous function that will execute after a delay (even if it is set to 0ms).

However, registering this function does not mean executing it immediately. Instead, it places the function into the message queue to be executed once the call stack is clear.

3. The Execution Steps

Let's break down the important steps that occur after the promise is created:

setTimeout is called, scheduling the function to log # 2 (async call) in the background.

resolve is invoked with the string # 3 (async call?), fulfilling the promise immediately.

console.log outputs # 1 (sync call) synchronously to the console.

The execution of the promise constructor function completes.

The then method attached to the doWorkPromise is invoked because the promise has been resolved. This results in the output Success! # 3 (async call?).

After all synchronous code is done executing, the event loop checks the message queue, picks the setTimeout callback, and executes it, printing # 2 (async call).

Conclusion

In essence, the reason resolve runs before the asynchronous code in the setTimeout is due to the fact that synchronous code executes before any queued asynchronous callbacks. Once the current task (the promise creation) completes, the event loop checks the queue and executes the setTimeout callback.

Understanding this sequence is crucial for working effectively with asynchronous JavaScript. By mastering these concepts, you can write cleaner, more efficient code that behaves as expected.

If you're still curious about Promises and how asynchronous JavaScript operates, feel free to leave any questions or comments below!