Question

This is code:

chrome.tabs.getSelected(null, function (abcd) {
        alert(abcd.index));
        });

I wonder know how this tab object ‘abcd’ is transfered to callback function from parent ‘getSelected’ method?

Where is this ‘abcd’ generate from?

Answer 1

If understood correctly, getSelected() function might look like this
(the names of underlying functions are fictional)

chrome.tabs.getSelected = function(windowId, callback) {
    var selectedTab = GET_SELECTED_TAB(windowId || CURRENT_WINDOW_ID);
    callback(selectedTab); // this calls your function with the tab object as a parameter (abcd = selectedTab)
}