conditional-method.js

function Foo() {
  if (!this.bar) {
    this.bar = function () {
      console.log('instance.bar');
    };
  }
}

// no bar method
Foo.prototype = {};
var foo1 = new Foo();
foo1.bar(); //instance.bar

// with bar method
Foo.prototype = {
  bar: function () {
     console.log('prototype.bar');
   }
};

var foo2 = new Foo();
foo2.bar(); //prototype.bar