The XMLHttpRequestEventTarget.onloadstart is the function called when an XMLHttpRequest transaction starts transferring data. 
XMLHttpRequest.onloadstart = callback;
callback is the function to be called when the transaction begins to transfer data. var xmlhttp = new XMLHttpRequest(), method = 'GET', url = 'https://developer.mozilla.org/'; xmlhttp.open(method, url, true); xmlhttp.onloadstart = function () { console.log("Download underway"); }; xmlhttp.send();
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
onloadstart | 
1  | 
12  | 
3.5  | 
10  | 
≤12.1  | 
4  | 
1  | 
18  | 
4  | 
≤12.1  | 
3  | 
1.0  | 
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    https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequestEventTarget/onloadstart