code: function normalize (v, instance) {
    const type = typof(v)
  
    switch (type) {
      case 'undefined':
      case 'null':
        return ''
      case 'regexp':
        return v.toString()
      case 'date':
        return v.toISOString()
      case 'number':
      case 'string':
      case 'boolean':
      case 'array':
      case 'object':
        if (v instanceof renderer.Element) {
          return v.ref
        }
        return v
      case 'function':
        instance.callbacks[++instance.callbackId] = v
        return instance.callbackId.toString()
      default:
        return JSON.stringify(v)
    }
  }
docs: Convert all type of values into "safe" format to send to native.
  1. A `function` will be converted into callback id.
  2. An `Element` object will be converted into `ref`.
  The `instance` param is used to generate callback id and store
  function if necessary.
  @param  {any}    v
  @param  {object} instance
  @return {any}