Use it like this:
var currentUserId = Xrm.Page.context.getUserId();
var req = new XMLHttpRequest();
req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/tasks?$select=_regardingobjectid_value&$filter=statecode eq 0 and _ownerid_value eq "+ currentUserId + "&$orderby=scheduledend desc", true);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.setRequestHeader("Prefer", "odata.include-annotations=\"*\"");
req.onreadystatechange = function() {
if (this.readyState === 4) {
req.onreadystatechange = null;
if (this.status === 200) {
var results = JSON.parse(this.response);
for (var i = 0; i < results.value.length; i++) {
var _regardingobjectid_value = results.value[i]["_regardingobjectid_value"];
var _regardingobjectid_value_formatted = results.value[i]["_regardingobjectid_value@OData.Community.Display.V1.FormattedValue"];
var _regardingobjectid_value_lookuplogicalname = results.value[i]["_regardingobjectid_value@Microsoft.Dynamics.CRM.lookuplogicalname"];
}
} else {
Xrm.Utility.alertDialog(this.statusText);
}
}
};
req.send();
