Dyn365CallWebApi.js

// Helper method for calling the WebApi
function callWebApi(method, collection, querystring, postData, successCallBack, errorCallBack) {

    // Get the client Url
    var clientUrl = Xrm.Page.context.getClientUrl(),
        // Create a XMLHttpRequest
        request = new XMLHttpRequest();

    // Build the request using the provided collection and querystring
    request.open(method, clientUrl + "/api/data/v8.1/" + collection + (querystring || ""), true);
    // Add the required headers for OData
    request.setRequestHeader("Accept", "application/json");
    request.setRequestHeader("Content-Type", "application/json; charset=utf-8");
    request.setRequestHeader("OData-MaxVersion", "4.0");
    request.setRequestHeader("OData-Version", "4.0");

    // Bind the onload, which only fires when a XHR requests finishes (regardless of status)
    request.onload = function () {
        // Parse any response data we may have
        var data = { };
        try {
            data = JSON.parse(this.response);
        } catch (err) { data = { }; }

        // Check if returned status code is within the OK range
        if (this.status >= 200 && this.status < 400) {
            // Success!
            // If status is 204, then a new record was created and we'll need to get it's ID
            if (this.status == 204) data.OData_EntityId = this.getResponseHeader("OData-EntityId");
            // Call the successCallBack with the data that was returned
            successCallBack(data);
        } else {
            // We reached our target server, but it returned an error
            // Call the errorCallBack with the statuscode and any error returned by the Api
            errorCallBack(this.status, typeof data.error != "undefined" ? data.error : null);
        }
    };

    request.onerror = function () {
        // There was a connection error of some sort, call the errorCallBack
        errorCallBack();
    };

    // If postdata was included, send that
    if (!!postData) request.send(postData);
    else request.send();
}