liushuping

/*
A JavaScript object notition is by nature a map, however there are limitations of it: the map is not empty, 
the Object prototype properties are mapped keys which are unexpected. For below example, the "toString" is by
default a key in the map.

var x = {};
x["toString"]; // function toString()

But with the Object.create method, we can create a real empty map object. What needed is just to pass null to 
the method.