HTTP Post request to be used with SharePoint Online REST API to do the post activities

spPostRequest.js

// POST request Call
function RestRequest(url, params) {
    var req = new XMLHttpRequest();
    return new Promise(function(resolve, reject) {
        req.onreadystatechange = function() {
            if (req.readyState != 4) // Loaded
                return;
            if (req.status >= 200 && req.status < 300) {
                resolve(req);
            } else {
                reject({
                    status: req.status,
                    statusText: req.statusText
                });
            }
        };
        // Prepend web URL to url and remove duplicated slashes.
        var webBasedUrl = (_spPageContextInfo.webServerRelativeUrl + "//" + url).replace(/\/{2,}/, "/");
        req.open("POST", webBasedUrl, true);
        req.setRequestHeader("Content-Type", "application/json;odata=verbose");
        req.setRequestHeader("ACCEPT", "application/json;odata=verbose");
        req.setRequestHeader("x-requestdigest", _spPageContextInfo.formDigestValue);
        req.send(params ? JSON.stringify(params) : void 0);
    });
}