Run time lookup

a.js

function A() {
    this.name = 'A';
}
A.prototype.log = function () {
    console.log( 'A', this );
};

var a = new A();

a.log();

A.prototype.log = function () {
    console.log( 'B', this );
};

a.log();

a.constructor.prototype.log = function () {
    console.log( 'C', this );
};

a.log();