@helloanselm: simple off-dom storage added to dom nodes

gistfile1.js

(function() {
    var storage = {};
    var counter = 1;
    
    function nodeId(node) {
        return node._dataId || (node._dataId = counter++ );
    }
    
    function nodeData(node, key, value) {
        var id = nodeId(node);
        var data = storage[id] || {};
        if (key === undefined) {
            // read all data
            return data;
        } else if (typeof key === 'string') {
            if (value === undefined) {
                // read specific key
                return data[key];
            }
            
            // set specific key
            data[key] = value;
            storage[id] = data;
            return null;
        }
        
        // set value object
        storage[id] = value;
        return null;
    }
    
    // expose on dom nodes
    Node.prototype.myData = function(key, value) {
        return nodeData(this, key, value);
    };
})();

// USAGE:
document.body.myData('key', 'value');
console.log(document.body.myData('key'));

WARNING

This code is meant as an example to illustrate how data can be saved on a DOM element without storing the data using slow DOM API. This example is missing functions to removeData() and does not make use of Mutation Observers to clean the storage once an element was removed.