createStore.js

const { extendObservable, action } = require('mobx')

class Store {
  constructor(observables = {}, actions = {}, props = {}) {
    extendObservable(this, props)

    Object.keys(actions).forEach((key) => {
      this[key] = action(actions[key])
    })

    Object.assign(this, props)
  }
}

function createStore(observables, actions, props) {
  return new Store(observables, actions, props)
}

module.exports = createStore

Store.js

const observables = {
  count: 1,
  computedFunc: () => {
    return this.count + 1
  }
}

const actions = {
  increment: () => {
    this.count += 1
  }
}

const props = {
  staticProperty: 'I am static',
  dynamicFunc: (randomNumber) => {
    return this.count + randomNumber
  }
}

const store = createStore(observables, actions, props)