设置请求头前需先调用open方法打开一个url

xhr.open("post","/save");

设置数据格式

发送json格式数据

xhr.setRequestHeader("Content-type","application/json; charset=utf-8");

发送表单数据

xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded; charset=utf-8");

发送纯文本

不指定Content-type时，此是默认值值

xhr.setRequestHeader("Content-type","text/plain; charset=utf-8");

发送html文本

xhr.setRequestHeader("Content-type","text/html; charset=utf-8");

编码可带可不带

// 不带字符编码写法xhr.setRequestHeader("Content-type","application/json");

值对大小写不敏感

xhr.setRequestHeader("Content-type","Application/JSON; charset=utf-8");

虽然如此，但还是都保持小写