To learn more, see our tips on writing great answers. Is cycling an aerobic or anaerobic exercise? XMLHttpRequest: readystatechange event The readystatechange event is fired whenever the readyState property of the XMLHttpRequest changes. You can't use synchronous requests outside web workers as it freezes the main interface. Hi, I'm trying to do a HTTP Call and tried fetch api did not work so I followed the documentation which uses XMLHttpRequest which also did not work.