A simple function for posting data

post.js

function post(url, data) {

    return new Promise(function (resolve, reject) {

        var xhr = new XMLHttpRequest();
        var params = new URLSearchParams();

        Object.entries(data).forEach(function (pair) {
            params.set(pair[0], pair[1]);
        });

        xhr.open("POST", url, true);
        xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

        xhr.onreadystatechange = function () {

            if (
                (
                    xhr.readyState === XMLHttpRequest.DONE
                    || xhr.readyState === "complete"
                )
                && xhr.status === 200
            ) {
                resolve(xhr.responseText, xhr);
            }
            
        };

        xhr.onerror = function () {
            reject(xhr);
        };

        xhr.send(params.toString());

    });

}