Accessing HAR files from web page activity and request-response size data from HAR files using pure JavaScript or web APIs

Question

How can I access HAR files generated from the activity of a web page in a browser tab and access the request-response size data from the HAR files using pure JavaScript or web APIs? I referred the following source but don't know the exact way: http://www.softwareishard.com/blog/har-12-spec/#postData.
How can I use the following line of code mentioned there?
var totalSize = entry.response.headersSize + entry.response.bodySize;

Answer 1

There is no Web API for that. You can, however, construct a HAR file by logging calls to fetch() (and XHR) in the main thread, but that would not include fetches initiated by other means (e.g. <img>). The way to solve that is to register a service worker and logging every "fetch" event instead.

If you just want to get a HAR file programmatically, you can use something like Puppeteer and puppeteer-har.