Скачать или смотреть Resolving fs.createReadStream Issues in Node.js: Ensuring Accurate Byte Reads

Discover how to reliably read specific byte ranges from files using Node.js fs.createReadStream with practical solutions and best practices.
---
This video is based on the question https://stackoverflow.com/q/64099688/ asked by the user 'Astral Cloud' ( https://stackoverflow.com/u/11813718/ ) and on the answer https://stackoverflow.com/a/64100313/ provided by the user 'Tawfik Nasser' ( https://stackoverflow.com/u/11450174/ ) 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: NodeJS: fs.createReadStream sometimes does not read the desired "start" and "end" bytes

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 Node.js ReadStream and Byte Reading Issues

When working with file systems in Node.js, you might encounter scenarios where you're required to read specific bytes from a file. Using fs.createReadStream can be a great way to do this. However, you may find that your attempts sometimes yield unexpected results, like receiving fewer bytes than anticipated. This guide will delve into this common problem and offer a solid solution to ensure you get the desired data every time.

The Problem

Let's break down a typical issue one might face when using fs.createReadStream. Imagine you've written a function to read a specified number of bytes from a file, and it works fine for the first few iterations. However, as you start running this function in a loop, an odd behavior emerges: some reads return fewer bytes than expected.

For instance, when setting the start position to 0 and the end bytes to 5000, you might log outputs revealing that, during multiple executions, the results vary significantly:

Expected: 5000 bytes

Actual: Can be as low as 432 bytes

Explanation of the Code Implementation

Initially, one may set up a basic implementation like this:

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

In this setup, the promise resolves as soon as the first data chunk is received, leading to inconsistencies.

The Solution

Key Insight: Resolve on 'end'

The messy output primarily results from resolving the promise on the first data event. Instead, you should resolve it when the stream has completed reading all data, which occurs in the end event. Here's an updated approach that utilizes this concept:

Initialize a Buffer: Start by creating an empty buffer to store the data chunks.

Concatenate Chunks: As data comes in, append it to the buffer.

Resolve on 'end': Only resolve the promise when the stream indicates it has completed reading all the data.

This leads us to the improved function:

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

Key Takeaways

Resolve Logic: Always resolve your promises on the end event to ensure you capture complete data.

Data Management: Using a buffer to concatenate data chunks helps retain all bytes instead of just the first chunk received.

Conclusion

By following the adjustment to your stream handling as shown above, you can overcome the unpredictability of fs.createReadStream. This will allow for accurate readings of the specified byte ranges from your files, regardless of the number of iterations or the specific file data.

Big thanks to Tawfik Nasser for the insight that led to this solution! Happy coding, and may your file reading endeavors be smooth and successful!