variable in default constructor javascript code example

Example 1: javascript constructor default parameter

function greet(name, greeting, message = greeting + ' ' + name) {
  return [name, greeting, message]
}

greet('David', 'Hi')                     // ["David", "Hi", "Hi David"]
greet('David', 'Hi', 'Happy Birthday!')  // ["David", "Hi", "Happy Birthday!"]

Example 2: javascript class constructor

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

const person = new Person("John Doe", 23);

console.log(person.name); // expected output: "John Doe"

Example 3: constructor in js

A constructor is a function that creates an instance of a class
  which is typically called an “object. In JavaScript, a constructor gets 
  called when you declare an object using the new keyword.
  The purpose of a constructor is to create an object and set values if 
  there are any object properties present.