What is polymorphism in Javascript?

Polymorphism is one of the tenets of Object Oriented Programming (OOP). It is the practice of designing objects to share behaviors and to be able to override shared behaviors with specific ones. Polymorphism takes advantage of inheritance in order to make this happen.

In OOP everything is considered to be modeled as an object. This abstraction can be taken all the way down to nuts and bolts for a car, or as broad as simply a car type with a year, make, and model.

To have a polymorphic car scenario there would be the base car type, and then there would subclasses which would inherit from car and provide their own behaviors on top of the basic behaviors a car would have. For example, a subclass could be TowTruck which would still have a year make and model, but might also have some extra behaviors and properties which could be as basic as a flag for IsTowing to as complicated as the specifics of the lift.

Getting back to the example of people and employees, all employees are people, but all people are not employees. Which is to say that people will be the super class, and employee the sub class. People may have ages and weights, but they do not have salaries. Employees are people so they will inherently have an age and weight, but also because they are employees they will have a salary.

So in order to facilitate this, we will first write out the super class (Person)

function Person(age,weight){
 this.age = age;
 this.weight = weight;
}

And we will give Person the ability to share their information

Person.prototype.getInfo = function(){
 return "I am " + this.age + " years old " +
    "and weighs " + this.weight +" kilo.";
};

Next we wish to have a subclass of Person, Employee

function Employee(age,weight,salary){
 this.age = age;
 this.weight = weight;
 this.salary = salary;
}
Employee.prototype = new Person();

And we will override the behavior of getInfo by defining one which is more fitting to an Employee

Employee.prototype.getInfo = function(){
 return "I am " + this.age + " years old " +
    "and weighs " + this.weight +" kilo " +
    "and earns " + this.salary + " dollar.";  
};

These can be used similar to your original code use

var person = new Person(50,90);
var employee = new Employee(43,80,50000);

console.log(person.getInfo());
console.log(employee.getInfo());

However, there isn’t much gained using inheritance here as Employee’s constructor is so similar to person’s, and the only function in the prototype is being overridden. The power in polymorphic design is to share behaviors.