javascript - How's file downloaded by the browser over HTTP - Stack Overflow

I'm currently trying to understand precisely what happens when a browser downloads a file to desktop, specifically how many HTTP requests are used, how the process is different to just getting a resource like an image or script, etc. I'd be grateful if anyone laid out a process here starting from A browser sends HTTP request.

I'm currently trying to understand precisely what happens when a browser downloads a file to desktop, specifically how many HTTP requests are used, how the process is different to just getting a resource like an image or script, etc. I'd be grateful if anyone laid out a process here starting from A browser sends HTTP request.

• javascript
• html
• http
• download

Share Improve this question Follow edited Oct 12, 2015 at 6:55 Max Koretskyi asked Oct 12, 2015 at 6:41 Max KoretskyiMax Koretskyi 106k6868 gold badges353353 silver badges516516 bronze badges

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1. A Browser sends an http request
2. The server responds with the file to be downloaded, and adds some HTTP headers (which say that it's a file to be downloaded) usually along these lines: Content-Type: application/octet-stream Content-Disposition: attachment; filename="picture.png"
3. The Browser then handles the response according to user settings (usually asks for permission before saving)

I know it's a bit of reading, but this is a good resource. Also, for a less intense introduction this helped me out quite a bit.

As to the number of HTTP requests used the answer is one. There is a TCP handshake done then the request is transmitted, after that the response is sent back. There is a little more involved if everything is over SSL/TLS, but that's all part of one HTTP request/response cycle.