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let IframeSrcs = [/* src URLs of iframe elements */];

let PendingReqs = 0;
let Stats = {}; // maps URL to its stats

let on_all_requests_complete = function() {
    //we're done!
};

let on_request_complete = function(Src, Data) {
    PendingReqs -= 1;
    Stats[Src] = get_stats_for_page(Data);
    if (PendingReqs == 0) {
        on_all_requests_complete();
    };
};

for (let Src of IframeSrcs) {
    makerequest(Src, (Data) => on_request_complete(Src, Data));
    PendingReqs += 1;
};