polymorphism.js

// NOT polymorphism

const obj1 = { 
  a: 'a',
}

const obj2 = {
  a: 'b'
}

let objArray = [obj1, obj2];

objArray.forEach(obj => {
  if (obj.a === 'a') {
    console.log('a is equal to a!');
  } else if (obj.a === 'b') {
      console.log('a is equal to b!');
    }
});

//Polymorphism

let BaseObj = function(a) {
  this.a = a;
}

BaseObj.prototype.showA = function() {
  console.log(`a is an ${this.a}`);
}

let DerivedObj1 = function(b) {
  this.b = b;
}

DerivedObj1.prototype = new BaseObj();

DerivedObj1.prototype.showA = function() {
  console.log('show some stuff about object 1');
}


let DerivedObj2 = function(b) {
 this.b = b;
}

DerivedObj2.prototype = new BaseObj();

DerivedObj2.prototype.showA = function() {
 console.log('show some stuff about object 2');
}

const obj1 = new DerivedObj1('c', 'd');
const obj2 = new DerivedObj2('e', 'f');

let objArray = [obj1, obj2];

objArray.forEach(obj => {
  obj.showA();
});

// In the polymorphism example, we could create an arbitrary amount of new DerivedObject's and we don't ever have to go 
// back into our forEach and alter the existing code to now check for another condition. It may not seem like such a big 
// deal with this trivial example, but suppose you needed to create 10,000 objects each with their own unique showA() method 
// - that would be one hell of an if-else block.

// It should be noted that Javascript's type system has no notion of polymorphism.