Javascript 是否可以在 Http 请求之前检查文件是否存在？

Question

我使用简单的 AJAX 并使用谷歌调试然后发现该 url 不存在...

代码很简单:

var http;

if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
  http=new XMLHttpRequest();
} else {
  http=new ActiveXObject("Microsoft.XMLHTTP");
}

try {
  http.open("GET", 'http://'+ip+':5000/test.html', true);
  http.onreadystatechange = onRcvData;
  if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
    http.send(null);
  } else {// code for IE6, IE5
    http.send();
  }
} catch(e) {
  http.abort();
}

function onRcvData() {
  if (http.readyState==4) {
    if (http.status==404) {

    } else if(http.status==200) {

    } else {

    }
  }
}

如果文件 test.html 存在就可以了。当文件不存在时，错误显示在部分:

if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
  http.send(null);
} else { // code for IE6, IE5
  http.send();
}

所以，即使我使用 onreadystatechange 方法也无法避免错误...

该文件在我的网页旁边的目录中。

那我应该怎么结合httprequest呢？

感谢任何建议。

添加:

我使用了“Head”方法，但返回 404...(无论 jQuery 插件/javascript)

点赞图片:

我该怎么办...

我发现的错误方向是不是被误导了？

Answer 1

试试这个功能

function urlExists(testUrl) {
    var http = jQuery.ajax({
        type:"HEAD", //Not get
        url: testUrl,
        async: false
    })
    return http.status!=404;
}


//Usage
if(!urlExists('http://www.mysite.com/somefileOrImage.ext')) {
   alert('File not found');
}

头

The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.

阅读head here