Скачать или смотреть How to Make Your JS Promise Wait for Nested Asynchronous Functions

Discover how to handle nested asynchronous operations in JavaScript using Promises. Learn to control the flow of your async code with a clear solution.
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Understanding Asynchronous Operations in JavaScript

As developers, we often encounter situations where we need to manage multiple asynchronous operations that depend on each other. A common dilemma is ensuring that an outer function does not proceed until an inner asynchronous function has completed its task. This issue can cause a lot of confusion, especially for those newer to using callbacks, Promises, or async/await syntax.

In this guide, we’ll explore a practical solution to manage nested asynchronous functions using Promises, especially when working with JavaScript’s Image.onload events. Let's break it down step by step.

The Problem

Consider the following scenario: you have a class structure consisting of Main, Invoker, and Inner. The Main class executes a command from the Inner class and needs to wait for an image to be fully loaded before proceeding.

Initially, you might set up your function like this:

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

Here, even though you attempt to return a promise, you're not actually resolving it after the image loads and thus cannot wait for that process to complete.

The Solution

To solve this problem, you need to return a Promise from the setSize function that resolves once the image has loaded. Below, I'll walk you through the changes required to make it work.

Step 1: Update setSize to Return a Promise

Create a new Promise: In your setSize method, declare a new Promise.

Resolve or Reject within the Callbacks: Inside the onload callback, call resolve with the newImage once the loading is successfully completed. Add an error handler with onerror to call reject in case of loading errors.

Here’s the revised setSize function:

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

Step 2: Utilizing the Updated setSize in Your Main Class

Now that setSize properly returns a promise, you can use the then method in your Main class to wait for the image data:

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

Conclusion

By wrapping your asynchronous operations within Promises, you can effectively manage their execution flow. This approach avoids common pitfalls experienced with simple callbacks and ensures that your code remains clean and error-free.

With these adjustments, your outer function will wait for the inner asynchronous operations to complete before proceeding, ensuring a more robust application.

If you have any questions or further scenarios in your coding journey where you've faced similar issues, feel free to share them in the comments!