我制作了一个 ajax 调用拦截器来监听 XMLHttpRequests。
我想修改 XHR 的请求标头。出于某种原因，我只能将它们更改为XMLHttpRequest.send(). 修改它们是否为时过早XMLHttpRequest.open()？它抛出了我在下面的代码中粘贴的异常。
那么，为什么我会收到错误消息？
(function (open) {
XMLHttpRequest.prototype.open = function () {
this.setRequestHeader('foo', 'bar');
// InvalidStateError: An attempt was made to
// use an object that is not, or is no longer, usable
open.apply(this, arguments);
};
})(XMLHttpRequest.prototype.open);
(function (send) {
XMLHttpRequest.prototype.send = function () {
this.setRequestHeader('foo', 'bar'); // OK
send.apply(this, arguments);
};
})(XMLHttpRequest.prototype.send);
for(var i = 0; i < 3; i++){
var rnd = Math.round(Math.random()*3+1),
httpRequest = new XMLHttpRequest();
httpRequest.onreadystatechange = function () { };
httpRequest.open('GET', '/echo/json?delay=' + rnd);
httpRequest.send();
}