Where is jQuery.data() stored?

Question

Where does jQuery store the values of the data() that it sets to DOM objects?

Is there some kind of variable like jQuery.dataDb or somethin

Answer 1

jQuery gets or sets data in 3 different ways for 3 different type of object.

For DOM element, jQuery first get a unique id, than create a custom property for element called expando:

var counter = 0;
function uid() {
    // only example
    return 'jQuery' + counter;
}
function getExpando(element) {
    var expando = element['jQueryExpando'];
    // for those without expando, create one
    if (!expando) {
        expando = element['jQueryExpando'] = uid();
    }
    return expando;
}

On the other hand, jQuery has a $.cache object which stores data map for each element, jQuery searches $.cache by expando and get a data map for certain element, getting or setting data in that map:

function data(element, name, value) {
    var expando = getExpando(element);
    var map = $.cache[expando];

    // get data
    if (value === undefined) {
        return map && map[name];
    }
    // set data
    else {
        // for those without any data, create a pure map
        if (!map) {
            map = $.cache[expando] = {};
        }
        map[name] = value;
        return value;
    }
}

For custom object(which is not DOM element or window object), jQuery directly set or get a property from that object by name:

function data(obj, name, value) {
    if (!obj) {
        return obj;
    }
    // get data
    if (value === undefined) {
        return obj[name];
    }
    // set data
    else {
        obj[name] = value;
        return value;
    }
}

At last, for the special window object, jQuery has a special windowData variable in closure to store data for window:

function data(obj, name, value) {
    if ($.isWindow(obj)) {
        obj = windowData;
    }
    // same as data for custom object
}