Northborn Design Northborn Design - 3 months ago 30
Ajax Question

Ajax HEAD request via Javascript/jQuery

I seem to be having some issues with making

HEAD
requests, and preserving the integrity of data in an array.

Given this snippet:

var imageTemp = Array();

$('*')
.each(function(index){
if($(this).css('background-image') != 'none'){
imageTemp.push($(this).css('background-image').slice(5, -2));
}
});


I capture the URLs of all background-images on a given page. Now, trying to grab the size of each image via
HEAD
requests for
Content-Length
, I use this snippet:

var imageData = Array();

for(var i = 0; i < imageTemp.length; i++){
ajaxSizeRequest = $.ajax({
type: "HEAD",
async: true,
url: imageTemp[i],
success: function(message){
imageData.push([imageTemp[i], ajaxSizeRequest.getResponseHeader('Content-Length')]);
}
});
}


However, when I dump
imageData
via
console.log
, I each element (which should be an array containing the URL and the content-length) ends up as
[undefined, XXXX]
where
XXXX
is always the size of the last requested
Content-Length


I'm stumped, though it appears to be a timing/scoping issue. Do I have a sort of race-condition occuring here?

Answer

The problem is that the single variables i and ajaxSizeRequest being captured by the callback function are the same variables for all instances of the callback function. I think if you call a function and pass the index variable to it and, at the same time, scope the request variable locally to the function itself use the response parameter of the done handler, you should end up with independent variables captured by the callback. It should then reference each array element and each response variable correctly.

var imageData = Array();

for(var i = 0; i < imageTemp.length; i++){
    updateImageData( i );
}

function updateImageData( i )
    $.ajax({
        type: "HEAD",
        async: true,
        url: imageTemp[i],
    }).done(function(message,text,jqXHR){
        imageData.push([imageTemp[i], jqXHR.getResponseHeader('Content-Length')]);
    });
}