Скачать или смотреть Solving the Problem of Function Gets Triggered Before Promise is Resolved in JavaScript

Learn how to properly handle asynchronous functions in JavaScript using promises and avoid common pitfalls in your code.
---
This video is based on the question https://stackoverflow.com/q/64687212/ asked by the user 'BlowFish' ( https://stackoverflow.com/u/13631122/ ) and on the answer https://stackoverflow.com/a/64687258/ provided by the user 'CertainPerformance' ( https://stackoverflow.com/u/9515207/ ) 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: Function gets trigger before promise is resolved

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.
---
Understanding the Issue: Asynchronous Function Execution

When writing JavaScript code, particularly when dealing with asynchronous operations like file uploads, developers often encounter a common pitfall: functions executing before promises have been resolved. This is the case in the scenario presented where a function writedb is intended to execute only after image uploads have completed. In practice, it runs prematurely, causing unintended behavior.

The Code Breakdown

Let's take a closer look at the provided code to understand the issue:

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

What's Going Wrong?

The key problem arises in this function:

The for loop is calling fs.readFile, but the log statement and resolved promise (resolve function) executes before any of the read operations finish. As a result, this.imageURL appears null during the resolve, as the asynchronous file read hasn't completed.

This leads to console.log statements potentially running before the image uploads have completed, thus providing incorrect timings and outcomes.

The Solution: Properly Handling Promises

To solve this problem, we need to ensure that the uploadImages function doesn't resolve until all images are processed. This can be achieved by leveraging Promise.all, which allows us to wait for multiple promises to resolve.

Revised Implementation

Here’s how you can refactor your uploadImages function to properly handle the promises and avoid the ant-pattern seen in the previous version:

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

Explanation of Changes

Using Promise.all: This ensures that the uploadImages function only resolves once all individual image reads and uploads are complete.

Map Function: The images array is transformed into an array of promises using map, providing a more functional and cleaner approach.

Chain with then(): This allows us to handle the returned URLs once the promise resolves, ensuring that the URLs are available for later operations.

Integrating with writedb

Now that uploadImages will correctly resolve after all operations are completed, you can simply call it in writedb:

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

Takeaways

Always ensure that your asynchronous operations complete (resolve) before proceeding with dependent operations.

Utilizing Promise.all not only helps manage multiple asynchronous actions but also leads to cleaner code.

Error handling is critical; always handle potential promise rejections to avoid unhandled promise errors.

By implementing these changes, you will have a more reliable and predictable code behavior when handling file uploads in JavaScript.