在a.js用XMLHttpRequest 封装一个HttpGet请求函数,在a.html 引用这个函数发送请求,并打印返回值，直接给出代码示例

// a.js
function httpGet(url){
let xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.onreadystatechange = function(){
if(xhr.readyState === 4){
if(xhr.status === 200){
console.log(xhr.responseText);
}
}
}
xhr.send();
}// a.html
<script src="a.js"></script>
<script>
httpGet('http://example.com/api')
</script>