function getId(element) {
        var state = getState(element);

        if (state && state.id !== undefined) {
            return state.id;
        }

        return null;
    }
Gets the resize detector id of the element.
@public
@param {element} element The target element to get the id of.
@returns {string|number|null} The id of the element. Null if it has no id.