vanilla-ajax.js

/**
 * Usage:
 * makeRequest("GET", "URL_GET_JSON", { postId: 1})
 *  .then(data => {
 *    console.log(data);
 *  });
 */
function makeRequest(method, url, qs_params) {
    return new Promise((resolve, reject) => {
        const q = buildQueryString(qs_params);
        const qs = qs_params ? '?' + q : '';
        const xhr = new XMLHttpRequest();

        if (method === 'GET') {
            xhr.open('GET', `${url}${qs}`);
        } else {
            xhr.open('POST', `${url}`);
        }

        xhr.onload = function() {
            if (xhr.status >= 200 && xhr.status < 400) {
                resolve(JSON.parse(xhr.responseText));
            } else {
                resolve(xhr.responseText);
            }
        };
        xhr.onerror = () => reject(xhr.statusText);
        if (method == "POST" && q) {
            xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
            xhr.send(q);
        } else {
            xhr.send();
        }
    });
}

function buildQueryString(params) {
    return Object.entries(params).map(d => `${d[0]}=${d[1]}`).join('&');
}