Since the settings object is tied to that ajax call, you can simply add in the indexer as a custom property, which you can then access using this in the success callback:
//preloader for images on gallery pages
window.onload = function() {
var urls = ["./img/party/","./img/wedding/","./img/wedding/tree/"];
setTimeout(function() {
for ( var i = 0; i < urls.length; i++ ) {
$.ajax({
url: urls[i],
indexValue: i,
success: function(data) {
image_link(data , this.indexValue);
function image_link(data, i) {
$(data).find("a:contains(.jpg)").each(function(){
console.log(i);
new Image().src = urls[i] + $(this).attr("href");
});
}
}
});
};
}, 1000);
};
Edit: Adding in an updated JSFiddle example, as they seem to have changed how their ECHO endpoints work: https://jsfiddle.net/djujx97n/26/.
To understand how this works see the "context" field on the ajaxSettings object: http://api.jquery.com/jquery.ajax/, specifically this note:
"The
thisreference within all callbacks is the object in the context option passed to $.ajax in the settings; if context is not specified, this is a reference to the Ajax settings themselves."