Understanding Why Promises Return undefined in Node.js and JavaScript

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Unpacking the Mystery of undefined Returns in Promises

The Problem: Why Does My Promise Return undefined?

Consider this code snippet: you have a function that creates a PDF download and utilizes promises. While running the code, an unexpected problem arises: the promise resolves to undefined. The code looks something like this:

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

As shown in the snippet above, the result ends up being undefined, leading to confusion. Let's delve into what's happening behind the scenes.

Examining the Code

In your scenario, you have two functions that involve promises: pdfDownload() and asdf(). Both functions, however, resolve their promises without any returned values, which leads to the undefined result. Here’s a breakdown of the two functions:

pdfDownload Function:

This function handles the downloading of a file and returns a promise.

The promise is resolved when a stream event (finish) is emitted.

The line of code responsible for resolving the promise is as follows:

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

The crux of the issue is that when resolve() is called, it does so without any arguments, resulting in an implicit undefined resolution.

asdf Function:

This is an async function where you're awaiting the result of the pdfDownload function.

The function doesn’t explicitly return a value, meaning its return will also resolve to undefined:

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

Crafting a Solution: How to Return a Desired Value

The good news is that you can resolve this issue by modifying your functions slightly to ensure they resolve with appropriate values! Here’s how:

Updating the pdfDownload Function

To provide a meaningful value when the PDF download is complete, you can change the resolution of the promise as follows:

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

Finalizing the asdf Function

You’ll want to add an explicit return statement in your asdf function. This ensures that the outcome of the promise reflects back to the caller:

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

Conclusion: Gaining Control Over Promise Values