Two different ways to add a method to an object without mutating the object prototype

add-methods-to-object.js

"use strict"

class OriginalObj {
	sayHello() {
		console.log( 'hello' );
	}
}

const myOriginal = new OriginalObj();
myOriginal.sayHello();

class AdvancedObj extends OriginalObj {
	sayGoodbye() {
		console.log( 'bye' );
	}
}

const myAdvanced = new AdvancedObj();
myAdvanced.sayHello();
myAdvanced.sayGoodbye();

function wrapObj( original ) {
	original.sayGoodbye = function sayGoodbye() {
		console.log( 'bye' );
	}
	return original;
}

const myWrapper = wrapObj( new OriginalObj() );
myWrapper.sayHello();
myWrapper.sayGoodbye();