Override valueOf method of a custom object to covert it to a primitive type

Question | May 24, 2017 | nextptr 

All JavaScript objects, either custom or built-ins, inherit the valueOf method from Object. JavaScript automatically calls the valueOf method when an object is required to be converted to a primitive type. Although the default implementation of valueOf returns the object itself, it can be overridden to return a primitive value.

Let's take an example. The Percentage is a constructor function and its valueOf method is overridden:

function Percentage( v ) {
   this.value = v;
// Override valueOf method
Percentage.prototype.valueOf = function() {
   return this.value/100;

What is the result of adding an instance of Percentage to a primitive number as shown below?

console.log( new Percentage( 40 ) + 1 );
2015 nextptr