code: function getKeys(container) {
    function arrayIndex(value, i) {
      return i;
    }
    if (typeof container === 'object') {
      return Array.isArray(container) ? container.map(arrayIndex) : Object.keys(container);
    } else {
      return [];
    }
  }
docs: Get the keys of an object as strings or those of an array as integers.
  @param {object|Array} container A hash or array
  @returns {Array.<string|number>} The keys of the container