What do the keys in the '_performanceLogger' object of an XMLHttpRequest response mean? - reactjs

Question

I am trying to measure download (and later upload) speed using Axios in React Native. My question's relate to the most appropriate way to do so, and understanding if the _performanceLogger in the response from XMLHttpRequest is accurate for this purpose. There surprisingly doesn't seem to be much out there about _performanceLogger ... Appreciate the help!
A MWE is:
import React from "react";
import axios from 'axios';
// Dummy API endpoint
let requestURL = 'https://jsonplaceholder.typicode.com/users/2'
// Getting timestamp immediately before request is started
var startTimestamp = (new Date()).getTime()
// Make request through Axios
// Make a request for a user with a given ID
axios.get(requestURL)
.then(function (response) {
// Handle success
console.log("And I am burdened with glorious purpose.")
// Getting timestamp immediately after request is completed
let endTimestamp = (new Date()).getTime()
console.log("Request start time stamp: " + startTimestamp)
console.log("Request completed time stamp: " + endTimestamp)
// Logging the whole _performanceLogger object
let responsePerformanceLogger = response.request._performanceLogger
console.log('Perf Logger:')
console.log(responsePerformanceLogger)
// Logging the key/value specific to the actual request within the _performanceLogger object
let requestPerfString = 'network_XMLHttpRequest_' + requestURL
let responseTimespans = responsePerformanceLogger._timespans[requestPerfString]
console.log('Specific Request Timings:')
console.log(responseTimespans)
})
.catch(function (error) {
// Handle error
console.log("This is a mistake! I shouldn't be here!")
console.log(error);
})
.then(function () {
// always executed
console.log('For all time! Always!')
});
I use Expo to run this code on my device, and I get the following output to the console:
And I am burdened with glorious purpose.
Request start time stamp: 1640229022039
Request completed time stamp: 1640229022156
Perf Logger:
PerformanceLogger {
"_closed": false,
"_extras": Object {},
"_pointExtras": Object {},
"_points": Object {
"initializeCore_end": 194691080.25150004,
"initializeCore_start": 194691027.19495836,
},
"_timespans": Object {
"network_XMLHttpRequest_http://192.168.1.40:19000/logs": Object {
"endExtras": undefined,
"endTime": 194692352.89145836,
"startExtras": undefined,
"startTime": 194691383.22187504,
"totalTime": 969.6695833206177,
},
"network_XMLHttpRequest_http://192.168.1.40:19001/symbolicate": Object {
"endExtras": undefined,
"endTime": 194694267.96945837,
"startExtras": undefined,
"startTime": 194694100.6875417,
"totalTime": 167.2819166779518,
},
"network_XMLHttpRequest_https://jsonplaceholder.typicode.com/users/2": Object {
"endExtras": undefined,
"endTime": 194699846.2774167,
"startExtras": undefined,
"startTime": 194699738.0606667,
"totalTime": 108.21674999594688,
},
},
}
Specific Request Timings:
Object {
"endExtras": undefined,
"endTime": 194699846.2774167,
"startExtras": undefined,
"startTime": 194699738.0606667,
"totalTime": 108.21674999594688,
}
For all time! Always!
My specific questions are:
What are the different objects under the _timespans key? There are two (log, symbolicate) to the local IP for Expo ... what's happening here? Presumably these won't there when I deploy outside of Expo.
Is the third, the one that contains the actual URL made in the request, the actual timing of the download? (I've extracted this one as Specific Request Timings.) This is total download time (latency + file download), correct?
Is relying on the XMLHttpRequest timing less/more accurate than just using the simple getTime() timestamps that I have called before the request and after successful completion? I would expect that they should be close, but the getTime()'s require some finite execution time that could be variable by device/load.