var A = function() { console.log("This is Constructor A"); };

A.prototype.x = function() { console.log("This is A.prototype.x"); };

var B = new A();

B.x(); //"This is A.prototype.x"

B.__proto__.x = function() { console.log("Edit B.__proto__.x"); };

B.x(); //"Edit B.__proto__.x"

var C = new A();

C.x(); //Edit B.__proto__.x

C.__proto__.x = function() { console.log("Edit C.__proto__.x"); };

C.x(); //Edit C.__proto__.x

B.x(); //Edit C.__proto__.x

A.prototype.x(); //Edit C.__proto__.x