Ajax - Function to check if Url exists

问题

I'm building a building a website using the Reddit API to display images from Reddit. I'm getting the data via JSON then use it to build the page, using URLs provided as sources for the images.

Some of the URLs that I get don't go to images directly, but instead to image hosting sites (like imgur.com). Usually, adding '.jpg' at the end of the URL takes me to the right image.

So, doing that, I would like to check if the URL + '.jpg' exists before using it.

I tried to build a function to check the url.

function checkUrl(url){
    var request = new XMLHttpRequest;
    request.open('GET', url, true);
    request.send();
    request.onreadystatechange = function(){
        if(request.readyState==4){
            console.log(request.readyState);
            return true;
        }else{
            return false;
        }
    }
};

//Then I use the function to check and append the data to the page
var url = url+'.jpg';
if(checkUrl(url)){
    //work with the data
}else{
    //do nothing
}

Nothing happens to the page, still I get the readyState logged into the console, so the checkUrl() function seems to be returning true.

What I am doing wrong ? I am pretty new to the whole Ajax thing, so some help would very appreciated.

Thank you

回答1:

Your problem is that when request.readyState == 4 this means the request has completed, regardless of what the result of that request was. So even if the URL you request returns a "404 not found", you'll still see the XHR resolving itself to readyState 4.

To address what you're trying to do, I'd recommend checking the status code of the response. For example, using your code:

if(request.status==200){
   return true;
}

回答2:

Why your code won't work:

Your AJAX request is asynchronous, and if(checkUrl(url)){ will return null (false) as the function checkUrl() sends the AJAX request and immediately returns, before the AJAX call has completed.

Change

request.open('GET', url, true);

request.open('GET', url, false);

Also, move your request.send() to after the request.onreadystatechange() as now it is a non-async request.

request.onreadystatechange = function(){
    if(request.readyState==4){
        console.log(request.readyState);
        return true;
    }else{
        return false;
    }
}
request.send();

Or, you could simply place your check logic into the onreadystatechange function:

request.onreadystatechange = function(){

    var url = url+'.jpg';
    if(request.readyState==4){
        //work with the data
    }else{
        //do nothing
    }
}

回答3:

I believe your problem is misunderstanding of AJAX; AJAX is basically asynchronous, and that's why the behavior you are describing.
I've used the following to get a simple true/false indication whether a URL is valid, in a synchronous manner:

function isValidURL(url) {
    var encodedURL = encodeURIComponent(url);
    var isValid = false;

    $.ajax({
      url: "http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20html%20where%20url%3D%22" + encodedURL + "%22&format=json",
      type: "get",
      async: false,
      dataType: "json",
      success: function(data) {
        isValid = data.query.results != null;
      },
      error: function(){
        isValid = false;
      }
    });

    return isValid;
}

The usage is then trivial:

var isValid = isValidURL("http://www.wix.com");
alert(isValid ? "Valid URL!!!" : "Damn...");

Hope this helps

回答4:

The return value of request.reonreadystatechange is not the return value of checkUrl. Try this:

function checkUrl(url){
    var request = new XMLHttpRequest;
    request.open('GET', url, true);
    request.send();
    request.onreadystatechange = function(){
        if(request.readyState==4){
            console.log(request.readyState);
            // perform your data manipulation here
            // don't separate it with the request
        }
    }
};

回答5:

On Safari, ActiveXObject will work fine, compared to XMLHttpRequest:

function isThere(url)
{
    var req= new AJ(); // XMLHttpRequest object
    try {
        req.open("HEAD", url, false);
        req.send(null);
        //alert(req.status); //un-comment if need alert for status code

        return req.status== 200 ? true : false;
    }
    catch (er) {
        //alert ('ERROR:'); // un-comment if need alert for error
        return false;
    }
}

function AJAX()
{
    var obj;
    if (window.XMLHttpRequest) obj= new XMLHttpRequest();
    else if (window.ActiveXObject)
    {
        try
        {
            obj= new ActiveXObject('MSXML2.XMLHTTP.3.0');
        }
        catch(er)
        {
            try
            {
                obj= new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch(er)
            {
                obj= false;
            }
        }
    }
    return obj;
}

来源：https://stackoverflow.com/questions/14764115/ajax-function-to-check-if-url-exists

标签

javascript

ajax

readystate