XMLHttpRequest（）在函数中返回响应[重复]

Question

This question already has an answer here:

• How do I return the response from an asynchronous call? 35 answers

I have a function isOnline(), which looks like this:

function isOnline() {
    var request=new XMLHttpRequest(); 

    request.onreadystatechange=function() {
        if(request.readyState==4) {
            if(request.responseText=="online")
                return true;    
        }    
    }
    request.open("GET","onlinecheck.php?user=user",false);
    request.send();

    return false;
}

If I run document.write(isOnline()); for testing , I ALWAYS get false, (undefined if i dont write return false; , I get undefined.

How do I 'wait' before the readyState is 4 and return true after that ?

</div>

Answer 1

You need to focus on few things:

• onreadystatechange is used when sending an asynchronous request - more here.

• So if you are trying to send an asynchronous request, use async=true in the call - read more here.

  request.open("GET", "onlinecheck.php?user=user", true);
  

• finally, if you are going to use asynchronous call, you can't return a value. Check these two links

  1. Returning values from the event onreadystatechange in AJAX
  2. How to return value from an asynchronous callback function?

---

In case you decide to do it synchronously:

function isOnline() {
    var request=new XMLHttpRequest(); 

    request.open("GET","www.google.com/",false);
    request.send();

    /* check the readyState, you can also check for status code */
    if (request.readyState === 4)
        // put other conditions here if you need to    
        return true;
    else
        return false;
}

// It returns true