dfcreative/mucss

/**
 * Detect whether element is placed to fixed container or is fixed itself.
 *
 * @module mucss/is-fixed
 *
 * @param {(Element|Object)} el Element to detect fixedness.
 *
 * @return {boolean} Whether element is nested.
 */
module.exports = function (el) {
    var parentEl = el;

    //window is fixed, btw
    if (el === window) return true;

    //unlike the doc
    if (el === document) return false;

    while (parentEl) {
        if (getComputedStyle(parentEl).position === 'fixed') return true;
        parentEl = parentEl.offsetParent;
    }
    return false;
};