Скачать или смотреть Understanding JavaScript Promises: Why Console Logs Appear During Push but Not with Promise.all

Explore the behavior of JavaScript promises in array manipulation. Discover why console logs show during push but remain silent with Promise.all in this engaging guide.
---
This video is based on the question https://stackoverflow.com/q/70830615/ asked by the user 'Mike K' ( https://stackoverflow.com/u/2891356/ ) and on the answer https://stackoverflow.com/a/70831183/ provided by the user 'FZs' ( https://stackoverflow.com/u/8376184/ ) 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: Log message printed for promises during a `push` to an array but not a call to `Promise.all`

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 JavaScript Promises: Why Console Logs Appear During Push but Not with Promise.all

JavaScript promises are a fundamental part of asynchronous programming, enabling developers to handle asynchronous operations more efficiently. However, understanding their behavior can sometimes lead to confusion. In this guide, we'll address a common scenario in JavaScript involving promises, console logs, and the Promise.all method.

The Problem: Confusion with Promises and Console Logs

Imagine you have two asynchronous functions that return promises. When you push these promises to an array, you notice that the console logs from within the promises are printed. However, when you attempt to use Promise.all on that array, you observe silence—no console logs appear, even though you receive the resolved values correctly.

Here’s a quick example of what’s happening:

You create promises using two asynchronous functions, promiseA and promiseB.

You push these promises into an array.

You call Promise.all with that array, expecting to see the console logs, but nothing is printed.

So, why does this happen?

The Explanation: Promises Are Not Executed on Push

Promises in JavaScript do not execute when they are pushed to an array. Instead, they are instantiated and executed when created. Understanding this difference is crucial.

The Nature of Promises

When you create a new promise using the Promise constructor, the function you provide is called immediately. This function initiates the actions that will determine the result of the promise, such as resolving or rejecting it. Here’s a breakdown:

Creating a Promise: The given function runs right away.

Pushing to the Array: When you push a promise to an array, you are just referencing an already-created promise. No new logs from within those promises will trigger at this point.

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

This log appears during the creation of the promise, not during the push operation.

Understanding the Push Operation

When you invoke promiseA({}), you are calling the function that creates the promise. Therefore, the console logs inside promiseA are executed at the moment of function execution, not when the promise is pushed:

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

To visualize this further, let’s clarify through restructured functions:

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

Using Promise.all

When you call Promise.all(promises), it waits for all promises in the array to resolve but does not trigger additional logs since they are already created.

You will only see logs from inside each promise when that specific promise is initially created—not during aggregation with Promise.all.

What Can You Do?

If you want to ensure you see logs reflecting the resolution of each promise:

Destructure the results returned by Promise.all to access the resolved values.

Example for clarity:

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

Conclusion

Understanding how promises operate in JavaScript can clear up a lot of misunderstandings, especially concerning console logs and the behavior of methods like Promise.all. Remember, promises represent the results of asynchronous actions that have already begun when created. Manipulating them afterward does not trigger their execution again, hence the lack of logs.

By breaking down the structure of promises and their lifecycle, we can better navigate the complexities of JavaScript’s asynchronous nature.

Now that you have a better understanding, you can explore the power of promises and async programming more effectively. Happy coding!