Скачать или смотреть Understanding Why a resolve Function Nested in an Event Callback Never Works

A deep dive into the common pitfalls of using Promises in JavaScript—specifically why your `resolve` function may not be behaving as expected in event callbacks when using Puppeteer.
---
This video is based on the question https://stackoverflow.com/q/69341134/ asked by the user 'Vector' ( https://stackoverflow.com/u/17012492/ ) and on the answer https://stackoverflow.com/a/69341149/ provided by the user 'jfriend00' ( https://stackoverflow.com/u/816620/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why a resolve function nested in event callback never works?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Why a resolve Function Nested in an Event Callback Never Works: A Deep Dive

When working with JavaScript, Promises can significantly improve your code's structure and maintainability. However, they can also lead to some tricky scenarios, especially when nested within event callbacks. A common problem faced by developers is related to how and when to correctly resolve a Promise. Let's explore this issue using a real-world scenario involving Puppeteer, a popular headless browser automation tool.

The Problem

Imagine you are automating a login process to a website using Puppeteer. You set up an event listener to detect a successful login by monitoring the page's navigation. However, despite your console logging messages confirming that the login was successful, your code remains in a pending state indefinitely. Here is a snippet illustrating this situation:

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

What's Going Wrong?

The mistake in the above code lies in how the resolve function is used within the event listener. You need to call the resolve function with parentheses, like this: resolve();. Instead, your current code is merely referencing resolve, which does nothing to change the Promise's state.

The Solution

To fix the issue, you should ensure that resolve is properly called when your condition for a successful login is met. Here's how you can modify your code:

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

Additional Considerations

Awaiting the Promise: After fixing the resolve function, remember that the newBrowser() function will return a pending Promise if you do not await it. To handle it correctly, you should call it like this:

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

Or using .then():

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

Debugging Promises: Make sure to always check the resolved state of your Promises. If it remains pending, review where and how you're resolving them.

Console Messages: While using console.log() is great for debugging, ensure that they lead to actual changes in the flow of your program.

Conclusion

Understanding how to properly use the resolve function in Promises is essential to avoiding issues like an unresponsive pending state in your asynchronous JavaScript code. By ensuring that functions are called correctly and anticipating the state of Promises, you can avoid common pitfalls and have smoother automation experiences with tools like Puppeteer. Happy coding!