'use strict';
  
  var toPosInt = require('../../number/to-pos-integer')
    , value    = require('../../object/valid-value')
  
    , hasOwnProperty = Object.prototype.hasOwnProperty;
  
  module.exports = function () {
  	var i, l;
  	if (!(l = toPosInt(value(this).length))) return null;
  	i = l - 1;
  	while (!hasOwnProperty.call(this, i)) {
  		if (--i === -1) return null;
  	}
  	return i;
  };