Container.js

class Container {
  constructor () {
    this._definitions = {}
  }

  register (id, func, factory = false) {
    if (typeof func !== 'function') {
      throw new Error('Invalid service function.')
    }
    this._definitions[id] = {
      func,
      factory
    }
  }

  prime (id, instance) {
    this._definitions[id] = {instance}
  }

  get (id) {
    const definition = this._definitions[id]
    if (!definition) {
      throw new Error(`No service definition: ${id}`)
    }
    if (definition.factory) {
      return definition.func(this)
    }
    if (!definition.instance) {
      definition.instance = definition.func(this)
    }
    return definition.instance
  }
}