Creating an instance of a type class in JavaScript

instanceConstructor.js

const l = m => console.log( m );

function Maybe(v) {
	this.value = v
}

Maybe.prototype.typeName = 'Maybe'

function Monad() {
	this.value = null
}

Monad.prototype.mbind = function() { return this }
Monad.prototype.mreturn = function() { return this.value }

function Instance( BaseType, InstanceType, instanceMethods ) {
	Object.keys( BaseType.prototype )
		.filter( methodName => 'function' === typeof BaseType.prototype[ methodName ] )
		.forEach( methodName => {
			InstanceType.prototype[ methodName ] = instanceMethods[ methodName ]
		} )
}

Instance( Monad, Maybe, {
	mbind( f ) { return this.value ? new Maybe( f( this.value ) ) : this },
	mreturn() { return this.value }
} )

const just3 = new Maybe( 3 )
const nothing = new Maybe( null )

l( just3.mreturn() )
l( nothing.mreturn() )

l( just3.mbind( a => 2 * a ).mreturn() )
l( nothing.mbind( a => 2 * a ).mreturn() )