let jsonParser = (function() {
    let that = {
        jsonResponse: null,
        parsedJSON: null,
        load: function(filepath, callback) {
            let jsonObject = new XMLHttpRequest();
            jsonObject.overrideMimeType("application/json");
            jsonObject.open('GET', filepath, true);
            jsonObject.onreadystatechange = function() {
                if (jsonObject.readyState == 4 && jsonObject.status == "200") {
                    callback(jsonObject.responseText);
                }
            }
            jsonObject.send(null);
        },
        parse: function() {
            let parsedJSON = JSON.parse(that.jsonResponse);
            return that.parsedJSON;
        },
    };
    return {
        load: that.load,
        parse: that.parse,
    };
})();

let response;
jsonParser.load('data.json', () => {response = jsonParser.parse});
console.log(response);