testA = function(){
this._data = [];
}

testA.prototype.add = function(data){
this._data.push(data) // Here i am facing this._data is undefined
}


testB = function(){}
testB.prototype = Object.create(testA);
var instance = new testB;

testB.prototype.pushData = function(data){
instance.prototype.add(data);
}

function (data){
  instance.prototype.add(data);
}
instance.pushData("someData")