如何使用javascript（和AJAX？）调用外部对象

Question

First off, thanks to this community. I'm slowly starting to understand this stuff and I appreciate all the feedback. That said, I come with another question.

THE SITUATION: I'm trying to build a new site and I'm trying to use it as a learning experience to use some php and teach myself javascript (note, I'm wanting to stick with js right now and not jump to jquery until I get a better handle on js). I've created a gallery that will have a series of thumbnails at the top that when clicked will populate a div below with artwork, its title and a description.

THE SETUP: Because I want to have these called by names and not numbers (such as an array) so they can be interchangeable, I built each as an id's div containing all the components and then I'm calling it to the target location via the script:

function changeDiv(id) {
var target = document.getElementById('generic');
var id = document.getElementById(id);
target.innerHTML = id.innerHTML; 
}

In the body it looks like:

<img src="images/thumbs/digital-art/thumb-space-oddity.jpg" id="thumb-space-odity" onclick="callDiv('space-oddity')"/>
...
<div class="gallery-frame" id="generic"></div>

Currently this works but since I have to have a place for all of the divs to be called from I've added a php include in the footer so it loads last but contains all of the images I could possibly need:

<div style="display:none;">
<?php include("gallery.php"); ?>
</div>

... but from what I understand since this will still mean the page will load all the content that will really slow down the page. So I need a new solution.

MY CHALLENGE: First off, is there a way to do this exclusively in javascript? If so I'd gladly do that!

Assuming I cannot, I have started looking into AJAX to see how it works and while it's starting to make nominal sense (still new to javascript so I only get half of it) and between w3schools.com and the stackoverflow topic I think i have a general idea of what I need to do but it's still not working.

THE PROBLEM: Based on the above stackoverflow comment I attempted to recreate the example function in my own instance and I have set it up as the following:

<script type="text/javascript">
function createXMLHttpRequest() {
    try {
        return new XMLHttpRequest();
    }
    catch (e)
    { alert('XMLHttpRequest not working'); }
    try {
        return new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e)
    { alert('Msxml2.XMLHTT not working'); }
    try {
        return new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (e)
    { alert('Microsoft.XMLHTTP not working'); }
    alert("XMLHttpRequest not supported");
    return null;
}
function callDiv(id) {
    var xmlHttpReq = createXMLHttpRequest();
    var url = "gallery.asp?ID=" + id;
    xmlHttpReq.open("GET", url, true);
    xmlHttpReq.send();
}
</script>

From what I've read I left the first section alone as that sets up the function (removing the null and prompts as I don't need them) and trying to have the callDiv function run the same as the previous script I was using. I have also saved out the gallery.php page as an .asp page but should I have left it as php?

Did it have to be .ashx?

And since I don't know the file it was being called from I don't know what the '?REASON' that was used in the original example meant. I tried ?ID thinking I was telling it to look for an id. Is that wrong, or unnecessary?

Thanks!

-Chris

Answer 1

Ok, good (and indeed brave) to want do do this in pure JS. It's a quirky, but fun language, honestly. The easiest way to achieve your goals will be using ajax calls, though it isn't a necessity.
Looking at your code, your calls will probably work just fine but you've left out the most important part of it all: a callback for the onreadystatechange event

As your script sends a request to the server, the state of that request is checked. Once the server has finished processing the sent request and the has finished responding the readyState should be 4. (more info can be found here ).
Basically, try adding this to your ajax object:

function callDiv(id)
{
    var xmlHttpReq = createXMLHttpRequest();
    xmlHttpReq.onreadystatechange = function()
    {
        if (this.readyState === 4 && this.status === 200)
        {
            //this === xmlHttpReq, "this" can be tricky, it refers to the calling obj.
            console.log(this.responseText);//check your console, see what that tells you
        }
    };
    var url = "gallery.asp?ID=" + id;
    xmlHttpReq.open("GET", url, true);
    xmlHttpReq.send();
}

Personally, seeing as you're new to JS, I'd also suggest just preloading your images in a regular way:

var nextImg = document.createElement('img');
nextImg.src = 'images/nextImage.jpg';
nextImg.onload = function()
{
    //targetId
    document.getElementById('wherever').appendChild(this);//this === nextImg
};

The last scenario creates a new DOM element, img, the source is set, so that the image is being loaded, when the loading has completed, there's an event handler that appends the new image to whatever container element (might as well be the document.body element) of your choice.
the advantage here is that you don't have to wrestle with AJAX. It's asynchronous nature can be tricky. On a smal scale, this is also easily implemented and easy to debug. The downside: it's a nightmare to maintain, and shifts all functionality to the client side. There's also a couple of more issues, but I'm a bit pissed and tired. read more here . If needs must, I'll have another look at this tomorrow.

Oh, yes: welcome to SO BTW.

Answer 2

Ok, see the follow code, you execute the request inside callDiv , and the result go inside .foo element:

function callDiv(id) {
    var request;  // Store AJAX request

    // Here you verify browsers support, to create new Ajax request
    try { // Opera, Firefox and Safari support
        request = new XMLHttpRequest();
    } catch (e) { // IE support
        try {
            request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try{
                request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                // Hide error, something goes wrong
            }
        }
    }

    // Show result inside .foo div
    request.onreadystatechange = function() {
        if(request.readyState == 4) { // Success
            var element = document.getElementByClass('foo');
            element.innerHTML = request.responseText;
        }
    }

    // Here you send the request
    request.open("GET", "gallery.php?id=" + id, true);
    request.send(null); 
}

<div class='foo'><!-- HERE WILL SHOW THE AJAX RESULT --></div>