承诺的AJAX

Question

How to use promises (ES6) and .then method in order to this code will work?

getGif: function (searchingText, callback) {
        var url = GIPHY_API_URL + '/v1/gifs/random?api_key=' + GIPHY_PUB_KEY + '&tag=' + searchingText;
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url);
        xhr.onload = function () {
            if (xhr.status === 200) {
                var data = JSON.parse(xhr.responseText).data;
                var gif = {
                    url: data.fixed_width_downsampled_url,
                    sourceUrl: data.url
                };
                callback(gif);
            }
        };
        xhr.send();
    },