Slightly unsure regarding Gzip compression in relation to my web application

Apologies in advance for my beginner question, but I haven't been able to find a thorough explanation on this topic. I recently developed a web app using React and utilized an NPM plugin to generate standard and Gzip compressed versions of the main.js and main.css files. The original size of main.js was 1.2 Mb, but with Gzip compression, it reduced to around 331Kb.

Now, within my "dist" folder, I have:

  • Index.html (which loads the standard - uncompressed - main.js and main.css by default)
  • main.js
  • main.css
  • main.js.gz
  • main.css.gz

So, what should be my next steps?

  1. Initially, my assumption was that I had to upload all these files to my server and update the Index.html to load the .gz files instead of the standard ones. However, upon doing so, I encountered an error stating "Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of "application/x-gzip"

  2. To my surprise, when inspecting the Network tab in Chrome's developer console, I noticed in the response header of Main.js: "Content encoding: gzip", and its size is 343Kb. While this size is not exactly matching "my" compressed file, it's quite close.

Therefore, my main question: What's happening here? Is my server automatically generating gzipped versions of my .js and .css files (it certainly seems like it...)? If so, where are these versions stored? Are they created on-the-fly each time someone requests them? And if that's the case, why create gzipped versions of the Main.js and Main.css files? How should I proceed with them?

I appreciate any assistance in clarifying these queries for me!

Answer №1

An important distinction to understand here lies in the difference between the Content-Type and the Content-Encoding.


At the core of HTTP, when a browser sends a request for a URL, it is up to the server to determine how to respond to that request.


This response can vary in different ways, but at its most basic level, the server will look for a corresponding path on the hard disk matching the requested URL and provide that data to the browser along with a Content-Type specifying what it is.

For example, requesting /foo.css would return the content of the CSS file with a Content-Type indicating it as CSS. Whereas asking for /foo.css.gz would yield the file content labeled as GZip format.

If the browser receives a GZip file marked as CSS, it will not interpret it correctly since it does not expect CSS within a compressed file.


A more efficient approach involves compression. In this scenario, when the browser requests /foo.css, the server compresses the file before sending it back. The response now includes the compressed foo.css along with a Content-Type identifying it as CSS and a Content-Encoding indicating it is GZipped.

The browser can then decompress the file and extract the CSS content accordingly.

(There are additional intricacies involving browser support for compression which we won't delve into here).


Taking complexity to another level is the desire to minimize resources required for on-the-fly compression of CSS files (which may entail repeated compression or caching).

In this case, when the browser asks for /foo.css, the server detects both a regular foo.css file and a compressed foo.css.gz version alongside it. The server then sends back the latter with a specified Content-Type and Content-Encoding, treating it as GZipped CSS.


It appears that the server in use is configured to implement the second methodology among these options, rather than the most intricate one.

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