How to get data via XMLHttpRequest using javascript

function getSearchResults(searchString) {

    debugger;

    var req = new XMLHttpRequest();

    var results;

    req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v9.1/EntitysPlurualName?$select=Field1,Field2&$filter=contains(productid/disc_codicepanthera,'" + searchString + "') or contains(productid/description,'" + searchString + "') or contains(productid/name,'" + searchString + "') or contains(productid/productnumber,'" + searchString + "')", 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) {

            req.onreadystatechange = null;

            if (this.status === 200) {

                results = JSON.parse(this.response);

            }

            else {

            }

        }

    };

    req.send();

    return results;

}