server.js

_ = require('lodash')

_.mixin({
  acts: function(object, mixin) {
    _.extend(object.prototype, mixin.prototype);
  }
});

function Thing() {}

Thing.prototype.hello = function () {
  return 'hello world';
}

function Behavior() {}

Behavior.prototype.action = function () {
  return 'does action';
}

_(Thing).acts(Behavior)
// ...or a special method could be avoided entirely
// _(Thing.prototype).extend(Behavior.prototype)

t = new Thing

console.log(t.hello())
console.log(t.action())