Synchronous and asynchronous XMLHttpRequest in Javascript from Microsoft Dynamics CRM perspective

Below are the examples of Synchronous and Asynchronous XMLHttpRequest.

Asynchronous request

       
  
var req = new XMLHttpRequest();
    req.open(requestType, serverURL + "/api/data/v8.2/" + query, false);
    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 /* complete */) {
            req.onreadystatechange = null;
            if (this.status === 200) {
                var data = JSON.parse(this.response);
                fnSuccessCallback(data);
            } else {
                var error = JSON.parse(this.response).error;
                fnErrorCallBack(error.message);
            }
        }
    };
    if (data == null) {
        req.send();
    } else {
        req.send(JSON.stringify(data));
    }
} 

Synchronous Request

       
  
var req = new XMLHttpRequest();
    req.open(requestType, serverURL + "/api/data/v8.2/" + query, false);
    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=\"*\"");
 if (data == null) {
        req.send();
    } else {
        req.send(JSON.stringify(data));
    }
    req.onreadystatechange = function () {
        if (this.readyState == 4 /* complete */) {
            req.onreadystatechange = null;
            if (this.status === 200) {
                var data = JSON.parse(this.response);
                fnSuccessCallback(data);
            } else {
                var error = JSON.parse(this.response).error;
                fnErrorCallBack(error.message);
            }
        }
    };
}
  
 

Here,
requestType can be 'GET' or 'POST'
serverUrL is CRM client url
query is oData query


thanks
p.