Скачать или смотреть How to Use Promise.all() to Ensure Promises in Objects are Complete Before Outputting

Learn how to effectively wait for promises to resolve in JavaScript using `Promise.all()`, ensuring your output contains complete results instead of pending promises.
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Understanding the Problem

Asynchronous programming can be challenging, especially when dealing with promises in JavaScript. One common issue developers face is waiting for a promise within an object to fully resolve before using its value. If you attempt to log or return the results without handling the promise correctly, you may end up with a collection of unresolved promises, leading to confusion and unexpected behavior.

Imagine you want to process a list of IDs and for each ID, you want to retrieve a result that is computed asynchronously using a promise. For instance, taking an input ID and simulating a delay before resolving the result. However, you want to ensure that when you log or return the output, you receive fully resolved values instead of unresolved promise objects.

This is a typical scenario that many developers encounter when working with JavaScript's asynchronous capabilities. Fortunately, there is a straightforward solution that utilizes the power of Promise.all() effectively.

The Solution: Using Promise.all()

The Code Breakdown

Here's the original approach that resulted in unresolved promises:

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

In the above code, test(id) returns a promise, and as a result, each result object contains a promise rather than the expected resolved value. To fix this, we need to leverage the await keyword properly.

Correcting the Code

To ensure that you receive resolved values instead of promises, you can modify the code by introducing await:

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

This modification achieves the following:

By adding await, the program pauses execution until the promise from test(id) resolves, thereby returning the actual value instead of the promise itself.

Each object in the resulting array will have the resolved value included, ensuring your output is neatly structured without any pending promises.

Final Implementation

Here’s a complete example that incorporates this solution:

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

Explanation of the Final Code

Promise Handling: Each ID is processed using the test function, which simulates an asynchronous task using a promise that resolves after a timeout.

Awaiting Promises: The use of await ensures that the result will be fully retrieved before moving to the next promise.

Promise.all(): Finally, using Promise.all() helps in executing all promises concurrently while making sure we get a single array of resolved values.

Conclusion

Handling asynchronous operations correctly is crucial for efficient and bug-free programming in JavaScript. By making this small adjustment and incorporating await in the right places, you can easily ensure that you are working with complete data, thus avoiding the pitfalls of unresolved promises. This allows for cleaner, more expected outputs in your applications.

With this technique at your disposal, you can confidently manage asynchronous data and improve the quality of your JavaScript code.