The old $/_.extend, the old Mootools options, the iconic Object.prototype.extend... Yes merging defaults with configuration using Object.assign.

gistfile1.js

class Person {
  constructor(config = {}) {
    var defaults = {
      name: 'Unknown',
      age: 23
    };
    
    [defaults, config].reduce(Object.assign, this);
  }
}

var me = new Person({ name: 'Merrick' });

console.log('The person is named %s and is %d years old.', me.name, me.age);
// Logs The person is named Merrick and is 23 years old.

If the defaults will never change you can avoid the redeclaration and define it outside of class Person :)

So true. Thanks man. :-)

@rwaldron This gist was inspired by your code review of our Diary.js code :-)

I must give proper credit where it's due: @ericf came up with this pattern while sitting next to me during a TC39 meeting last year: https://gist.github.com/ericf/3751340 :)