Subclassing without using `class` keyword.

subclass.js

const mixins = {
  mixinMethod () {
    console.log(`This is a mixin method.`)
    return this
  }
}

// Constructor definitions.
function Class () { console.log(`This is the constructor.`) }
function Subclass () { Class.apply(this, arguments) }

Subclass.prototype = Object.assign(Object.create(Class.prototype), {
  subclassMethod () {
    console.log(`This is a method on the subclass.`)
    return this
  }
}, mixins)

new Subclass().subclassMethod().mixinMethod()

subclass_es6.js

// Compared to the ES6 class version:
class Class {
  constructor () { console.log(`This is the constructor.`) }
}
class Subclass extends Class {
  subclassMethod () { console.log(`This is a method on the subclass.`) }
}
// There is not really mixin support when using `extends`, so it's not really
// a fair comparison.
new Subclass().subclassMethod()