Ajax 两种请求方式的区别onload和onreadystatechange

<button id="btn">请求纯文本</button>
    <script>
        let btn = document.getElementById(‘btn‘);
        btn.addEventListener("click",loadText);
        function loadText(){
            let xhr = new XMLHttpRequest();
            xhr.open(‘GET‘,‘sample.txt‘,true);
            xhr.onreadystatechange = function(){
                console.log("READYSTATE"+ xhr.readyState);
                console.log(this.responseText);
            }
            xhr.send();
        }
    </script>

function loadText(){
            let xhr = new XMLHttpRequest();
            xhr.open(‘GET‘,‘sample.txt‘,true);
            console.log("READYSTATE"+ xhr.readyState);
            xhr.onreadystatechange = function(){
                console.log("READYSTATE"+ xhr.readyState);
                if(this.status == 200 && this.readyState == 4){
                    console.log(this.responseText);
                }else if(this.status == 404){
                    console.log("网页不存在");
                }
                
            }
            xhr.send();
        }

 function loadText(){
            let xhr = new XMLHttpRequest();
            xhr.open(‘GET‘,‘sample.txt‘,true);
            console.log("READYSTATE"+ xhr.readyState);
            //两种请求方式onload和onreadystatechange
            xhr.onload = function(){
                console.log("READYSTATE"+ xhr.readyState);
                console.log(this.responseText);
            }
            xhr.send();
}

function loadText(){
            let xhr = new XMLHttpRequest();
            xhr.open(‘GET‘,‘sample.txt‘,true);
            console.log("READYSTATE"+ xhr.readyState);
            //两种请求方式onload和onreadystatechange
            xhr.onprogress = function(){
                console.log("测试加载状态READYSTATE"+ xhr.readyState);
            }
            xhr.onload = function(){
                console.log("READYSTATE"+ xhr.readyState);
                console.log(this.responseText);
            }
            xhr.send();
        }

xhr.onload = function(){
   document.getElementById(‘text‘).innerHTML = this.responseText;
}