判断是否ajax请求完毕 (可作为第三方插件引入)

;(function () {
    if ( typeof window.CustomEvent === "function" ) return false;

    function CustomEvent ( event, params ) {
        params = params || { bubbles: false, cancelable: false, detail: undefined };
        var evt = document.createEvent( 'CustomEvent' );
        evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
        return evt;
    }

    CustomEvent.prototype = window.Event.prototype;

    window.CustomEvent = CustomEvent;
})();
;(function () {
    function ajaxEventTrigger(event) {
        var ajaxEvent = new CustomEvent(event, { detail: this });
        window.dispatchEvent(ajaxEvent);
    }
    
    var oldXHR = window.XMLHttpRequest;

    function newXHR() {
        var realXHR = new oldXHR();

        realXHR.addEventListener('abort', function () { ajaxEventTrigger.call(this, 'ajaxAbort'); }, false);

        realXHR.addEventListener('error', function () { ajaxEventTrigger.call(this, 'ajaxError'); }, false);

        realXHR.addEventListener('load', function () { ajaxEventTrigger.call(this, 'ajaxLoad'); }, false);

        realXHR.addEventListener('loadstart', function () { ajaxEventTrigger.call(this, 'ajaxLoadStart'); }, false);

        realXHR.addEventListener('progress', function () { ajaxEventTrigger.call(this, 'ajaxProgress'); }, false);

        realXHR.addEventListener('timeout', function () { ajaxEventTrigger.call(this, 'ajaxTimeout'); }, false);

        realXHR.addEventListener('loadend', function () { ajaxEventTrigger.call(this, 'ajaxLoadEnd'); }, false);

        realXHR.addEventListener('readystatechange', function() { ajaxEventTrigger.call(this, 'ajaxReadyStateChange'); }, false);

        return realXHR;
    }

    window.XMLHttpRequest = newXHR;
})();

// 这样，就为 XHR 添加了自定义事件。如何调用？
var xhr = new XMLHttpRequest();

window.addEventListener('ajaxProgress', function (e) {
    console.log('请求中 可以添加loader动画');
});

window.addEventListener('ajaxError', function (e) {
    console.log('请求出错');
});

window.addEventListener('ajaxLoadEnd', function (e) {
    console.log('请求结束');
});

window.addEventListener('ajaxTimeout', function (e) {
    console.log('请求超时');
});