Script only works while alert box is in the script_问答_开发者

If I take the alert call out of the following script firebug says that results is undefined?`

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

<html>
    <head>
        <title>Inventory Management</title>
        <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
        <title>Untitled Document</title>
        <script src="json.js" type="text/javascript"></script>
        <script src="prototype.js" type="text/javascript"></script>
    </head>
    <body>
        <div id="content">
            <div id="header">   
            </div>

            <script type="text/javascript">
                var xhr;
                var results=getPlants(xhr,results);
                var plants=[];

                function getPlants(xhr,results){
                    try {
                        xhr=new XMLHttpRequest();   
                    }catch(microsoft){
                        try{
                            xhr=new ActiveXObject("Msxml2.XMLHTTP");                
                        }catch(othermicrosoft){
                            try{
                                xhr = new ActiveXObject("Microsoft.XMLHTTP");               
                            }catch(failed){
                                xhr=false;
                                alert("ajax not supported");
                            }
                        }               
                    }

                    xhr.onreadystatechange= function () {
                        if(xhr.readyState==4 && xhr.status==200) {
                            results = xhr.responseText;                     
                        }
                    }

                    xhr.open("GET","db_interactions.php",true);     
                    xhr.send(null);
                    alert("sent");
                    return results;
                }

                plants.inv=[];
 开发者_如何学C               plants.split = results.split("~");

                for (var x=0;x<=plants.split.length-2;x++){
                    plants.inv[x]=plants.split[x].evalJSON();
                }

                document.write(plants.inv[1].scientific_name);
            </script>
        </div>
    </body>
</html>

The reason it's not working is because the XmlHttpRequest is asynchronous. What this means is that you cannot do return results;

Here's a timeline, with your alert box:

Create XmlHttpRequest
Send it
Alert box pops up and sits there for a while
XmlHttpRequest result comes back and sets the results variable in your onreadystatechange function
You click OK on the alert box
the function returns (the return results line)
Other code runs and does things with the results variable.

Here's the timeline without the alert box:

Create XmlHttpRequest
Send it
the function returns (the return results line)
Other code runs and does things with the results variable. This fails because it hasn't been set yet
XmlHttpRequest result comes back and sets the results variable in your onreadystatechange function

Instead you have to put the plants code (splitting, etc) in the onreadystatechange callback, or put it in it's own function, and call that function from the onreadystatechange

You should modify the plants variable in the callback function assigned to the onreadystatechange property.

xhr.onreadystatechange = function(){
    if(xhr.readyState==4 && xhr.status==200) {
        results = xhr.responseText;
        // Populate the "plants" variable here, then call a function
        // that does something with the plants variable.
    }
}

For more information: https://developer.mozilla.org/en/AJAX/Getting_Started

When you call

xhr.open("GET","db_interactions.php",true);

you are telling the XMLHttpRequest to operate asynchronously. You are telling it to go off and do its work, calling the callback function at some unspecified time in the future, when the request has been fully processed. Change that true to false, and it will work as you expect. On the other hand, that function might hang indefinitely in the event of a network slowdown, etc., so caveat programmor.

The callback function that sets the 'results' variable is asynchronous -- meaning that it will likely get set after the return statement, unless there's something to delay the return statement (e.g. an alert() call).

XMLHttpRequest.send() is asynchronous. Without the alert the function runs faster than the ajax call and results is never initialized. You can fix this by changing the true in your open call to false to make it synchronous or set your global variable in your inline function