Скачать или смотреть Understanding Promise.all and Nested Promises in JavaScript

Learn how to effectively use `Promise.all` with nested promises in JavaScript and ensure your asynchronous operations complete successfully.
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Understanding Promise.all and Nested Promises in JavaScript

JavaScript promises are an essential part of asynchronous programming. However, managing multiple asynchronous operations can be tricky, especially when dealing with nested promises. A common problem developers face is ensuring that Promise.all() waits for all nested promises to resolve. In this post, we will explore the issue and provide a straightforward solution.

The Problem: Promise.all Not Waiting for Nested Promises

As a developer new to promises, you may find yourself running into issues when trying to use Promise.all(). This function is designed to take an array of promises and return a single promise that resolves when all of the promises in the array have resolved. However, if any of the promises you pass to it are actually not returning promises, it can lead to unexpected outcomes.

A Scenario to Illustrate

Consider the following code that uses MySQL queries wrapped in promises:

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

In this example, the function scanCountWrapper is expected to return a promise. However, the promise returned by the database.query() call is not being returned out of scanCountWrapper(). This issue leads to Promise.all() receiving undefined values, causing it not to wait for the actual nested promises to complete.

The Solution: Returning Promises Correctly

To resolve the issue of Promise.all() not waiting for nested promises, you must ensure that your promise-returning functions actually return the promise. In this case, we need to modify scanCountWrapper to return the promise from the database.query() call directly, thus allowing Promise.all() to function correctly.

Here’s the Correct Implementation

You should modify your scanCountWrapper function like this:

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

Key Changes Made:

Return the Promise: By adding return before database.query(...), the function now returns a promise that Promise.all can wait on.

Remove Unnecessary Nesting: The creation of an additional promise inside the .then() handler is unnecessary since resProjectDict[resname] can be set directly, and the return statement effectively resolves the value needed.

Putting it All Together

Now that we have adjusted scanCountWrapper, let’s look at how this integrates with the rest of your code:

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

Summary

Always return the promise in any function that is expected to return a promise.

Avoid nesting new promises unnecessarily; use the existing promise chain effectively.

With these changes, your use of Promise.all will correctly handle nested promises, allowing your asynchronous operations to run as intended. This method will help you manage more complex asynchronous workflows in JavaScript with ease.

Now you have the tools to utilize promises effectively, ensuring your applications work smoothly and without hiccups in asynchronous execution. Happy coding!