Get highest z-index in a given target (Javascript)

get_z_index.js

/**
 * @fileoverview Queries the computed styles for a given element to determine the highest z-index.
 * For example, GET_Z_INDEX(document) returns the highest z-index in the DOM. 
 */

/**
 * @function GET_Z_INDEX
 * @param {object} parent_node The target parent node. The function returns the highest z-index of parent_node's children.
 * @returns {number}
 */
function GET_Z_INDEX(parent_node) {
  var stored_z_index = 0;

  parent_node.querySelectorAll("*").forEach(function(element) {
    var z_index = parseInt(getComputedStyle(element).zIndex);

    if (isNaN(z_index) === false && z_index > stored_z_index) {
      stored_z_index = z_index;
    }
  });

  return stored_z_index;
}