如果没有jQuery,javascript如何检查多个ajax调用是否已经完成,一旦有了,就执行一段代码？

只需命名您传递给的函数,并将其xmlhttp.onreadystatechange分配给onreadystatechange每个请求的属性.

以下是我将如何概括解决此问题的方法:

function requestsAreComplete(requests) {
    return requests.every(function (request) {
        return request.readyState == 4;
    });
} 

function unsuccessfulRequests(requests) {
    var unsuccessful = requests.filter(function (request) {
         return request.status != 200;
    };
    return unsuccessful.length ? unsuccessful : null;
}

function onRequestsComplete(requests, callback) {   
    // Wrap callback into a function that checks for all requests completion     
    function sharedCallback() {
        if (requestsAreComplete(requests)) {
            callback(requests, unsuccessfulRequests(requests));
        }
    }

    // Assign the shared callback to each request's `onreadystatechange`
    requests.forEach(function (request) {
        request.onreadystatechange = sharedCallback;
    });
}

而且你会这样使用它:

onRequestsComplete([xmlhttp, xmlhttp2], function(requests, unsuccessful) {
    if (unsuccessful) { return; } // Abort if some requests failed

    document.getElementById("login_panel").innerHTML=requests[1].responseText;
    document.getElementById("login_panel_settings").innerHTML=requests[0].responseText;
});