Polymorphism

You can override an inherited method which opens the door to polymorphism.

class CourseAssistant {
  getBaseSalary() {
    return 500.0; //dollars
  }

  getHourlyPayRate() {
    return 15.0; // dollars
  }
}

class ExperiencedCourseAssistant extends CourseAssistant {
  /* overrides */
  getHourlyPayRate() {
    return 1.1 * super.getHourlyPayRate();
  }
}

function calcPayment(courseAssistant, hoursWorked) {
  let wages =
    courseAssistant.getBaseSalary() +
    hoursWorked * courseAssistant.getHourlyPayRate(); /* dynamic dispatch */
  console.log(wages);
}

const tom = new CourseAssistant();
const mona = new ExperiencedCourseAssistant();

calcPayment(tom, 10);
calcPayment(mona, 10);

In the example above, getHourlyPayRate() is dispatched based on the actual "type" of the courseAssistant argument. It will decide on dispatching the overloaded getHourlyPayRate() during runtime which is a (dynamic) polymorphic behavior.

Method overriding in JavaScript simply means that you can have methods with the same name in parent and subclasses.

Aside: JavaScript does not method overloading since the number of arguments to a method can be fewer/more than declared parameters.