Call Action from JavaScript

function Callcustomaction(actionName, param1Value) {
    var result2 = null;
    var result = {};
    result.Output1 = {};
    result.Output2 = {};
    var data = {
        "Param1Name": param1Value,
        "Param2Name": ""
    };

    var req = new XMLHttpRequest();
    var uri = Xrm.Page.context.getClientUrl() + "/api/data/v8.0/";
    try {
        req.open("POST", encodeURI(uri + actionName), false);
        req.setRequestHeader("Accept", "application/json");
        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
        req.setRequestHeader("OData-MaxVersion", "4.0");
        req.setRequestHeader("OData-Version", "4.0");
        req.onreadystatechange = function () {
            if (this.readyState == 4) {
                req.onreadystatechange = null;
                if (this.status == 200) {
                    result2 = JSON.parse(this.response);
                } else {
                    var err = JSON.parse(this.response).error;
                    alert(err.message);
                }
            }
        };
        req.send(JSON.stringify(data));
        result.Output1 = result2.Output1;
        result.Output2 = result2.Output2;
        return result;

    } catch (err) {
        alert(err.message);
    }
    return result;
}