// ES7 code, with async/await

async-2.js

// ES7 code, with async/await
function httpGet(url) {
    return new Promise(function (resolve, reject) {
        // do the usual Http request
        let request = new XMLHttpRequest();
        request.open('GET', url);

        request.onload = function () {
            if (request.status == 200) {
                resolve(request.response);
            } else {
                reject(Error(request.statusText));
            }
        };

        request.onerror = function () {
            reject(Error('Network Error'));
        };

        request.send();
    });
}

async function httpGetJson(url) {
    // check if the URL looks like a JSON file and call httpGet.
    let regex = /\.(json)$/i;

    if (regex.test(url)) {
        // call the async function, wait for the result
        return await httpGet(url);
    } else {
        throw Error('Bad Url Format');
    }
}

httpGetJson('file.json').then(function (response) {
    console.log(response);
}).catch(function (error) {
    console.log(error);
});