Resolving Invalid File Size Errors When Creating a File from Blob in JavaScript

Learn how to correctly create a file from a blob in JavaScript, avoiding issues like incorrect file sizes and potential image corruption.
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Resolving Invalid File Size Errors When Creating a File from Blob in JavaScript

When developing web applications, you might encounter situations where you need to manipulate images using a canvas and then save the modified image as a file. However, many developers face the frustrating issue of the output file having an invalid or incorrect size—sometimes as small as just 58 bytes! In this post, we’ll break down the problem, explore why it happens, and provide a clear solution to ensure your files are created correctly from blobs.

The Problem: Outputting an Invalid File Size

In this case, a developer was trying to create a file from a blob generated by a canvas element where an image had been manipulated (e.g., rotated). The code snippet they provided showed attempts to convert the blob into a file, yet the resulting file size was inexplicably small, indicating that something had gone wrong in the process.

Here’s a summarized version of the issue:

Generating Blob: The canvas toBlob function is used to create a blob from the canvas, which should contain the image data.

Creating File: The developer used the File constructor to convert the blob to a file, but the result was a file size of only 58 bytes, suggesting the image data was not correctly captured.

Corruption Concern: Despite testing with different blobs, the problem persisted, leading to concerns about potential corruption in the image or blob data.

The Solution: Avoiding Object URLs

After analyzing the situation, the solution lies in understanding the purpose of createObjectURL and how to work directly with the blob data. Here's a breakdown of the proper approach to creating a file from the blob:

Step 1: Create Blob from Canvas

Instead of relying on an object URL, directly resolve the blob generated by the canvas. Here’s the revised code snippet:

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

Step 2: Create a File from the Blob

To create a file from the blob, you can use the File constructor. Here's how to implement this:

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

Why This Works

Direct Access: By resolving the blob directly, we ensure that you have the actual image data rather than a temporary object URL that merely references the data without maintaining its integrity.

Correct File Size: This method produces a valid image file size, accurately reflecting the contents of the blob and preventing the issue of the file appearing to be just a few bytes large.

Conclusion

By following these steps to correctly create a file from a blob, you can effectively avoid errors related to invalid file sizes. Always remember to resolve the blob directly rather than through an object URL. This simple adjustment can save you a lot of headaches when working with images in your web applications. Happy coding!