text: function getJSON(url, callback) {
      let xhr = new XMLHttpRequest();
      xhr.onload = function () { 
        callback(this.responseText) 
      };
      xhr.open("GET", url, true);
      xhr.send();
    }
    
    export function getUsefulContents(url, callback) {
      getJSON(url, data => callback(JSON.parse(data)));
    }