<button id="button">点击按钮返回一张随机图片地址</button>
<p id="result">&nbsp;</p>

var button = document.getElementById('button');
var result = document.getElementById('result');

button.addEventListener('click', function () {
    var xhr = new XMLHttpRequest();
    xhr.open('post', 'http://www.zhangxinxu.com/study/201802/cros-ajax.php');
    xhr.onload = function () {
        result.innerHTML = xhr.responseText;
    };
    xhr.send(null);
});