Скачать или смотреть Understanding function returns and callbacks in JavaScript

A comprehensive guide on how to handle function returns and callbacks in JavaScript, addressing common issues and offering clear solutions.
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Understanding Function Returns and Callbacks in JavaScript

If you've been working with JavaScript (or Node.js, especially with libraries like Discord.js), you may have encountered issues around function returns and the use of callbacks. One common problem involves understanding how JavaScript handles asynchronous operations—specifically when working with promises and callbacks.

In this guide, we will dissect a scenario where the order of functions causes confusion, ultimately leading to undefined being returned when you expected a value. We will provide clarity on how to properly return values from callbacks using promises and how to structure your asynchronous functions effectively.

The Problem

Consider the following excerpt that portrays the issue:

When hovering over the call for function_2 in VSCode, you get Promise<void> as the return type.

The console unexpectedly outputs undefined when running your code before function_2() even completes.

You are unsure how returns work in promises and callbacks, causing confusion about the expected output of your functions.

This issue is rooted in misunderstanding how JavaScript returns values from asynchronous functions and how callback functions work.

Understanding the Callback Function Behavior

The root cause of the problem lies in how callbacks are structured:

Async Functions: When you define a function as async, it always returns a promise. This means that:

If you don't explicitly return a value from an async function, it implicitly returns Promise<void>, which appears as undefined in the console.

Callbacks: In the code snippet provided, function_2() has a callback provided to the request() method. Here's the critical point:

The return value of the callback is used by the request() function itself, not by function_2.

Thus, any return within the callback does not affect the return value of function_2.

Example Structure of Incorrect Code

Here's a simplified version of the problem code:

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

The Solution: Using Promises Correctly

To resolve issues with asynchronous function returns, you can use Promise resolution effectively. Here is the revised and organized solution:

Step-by-Step Breakdown

Remove the Async Modifier from function_2:

function_2 should not be declared as async since we're returning a new Promise.

Return a Promise from function_2:

Wrap the request call in a Promise constructor, allowing you to resolve or reject the promise based on the callback's result.

Use Resolve to Pass Values:

Call resolve() with the desired value inside the callback once the necessary conditions are met.

Example of Correct Code

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

Conclusion

In summary, the key to managing asynchronous operations in JavaScript is understanding the distinction between promises and callbacks. By correctly structuring functions that use promises, you can ensure that you return values as expected. This simple adjustment can save you a lot of debugging time and frustration while writing asynchronous code.

Feel free to ask for any clarifications or further examples if needed!