用promise封装一个简单的ajax啊

 function getJSON(url) {    
        return new Promise(function(resolve, reject) {      
              var XHR = new XMLHttpRequest();   
              XHR.open('GET', url, true);        
              XHR.send();        
              XHR.onreadystatechange = function() {           
                if (XHR.readyState == 4) {                
                    if (XHR.status == 200) {                    
                        try {                        
                            var response = JSON.parse(XHR.responseText);                        
                            resolve(response);                    
                            } 
                        catch (e) {                        
                                reject(e);                    
                        }                
                    } 
                    else {                    
                        reject(new Error(XHR.statusText));                
                    }            
                }        
            }    
    })} 
    
    getJSON(url).then(resp => console.log(resp));