initProxy

作用：主要作用是在实例上挂载一个 _renderProxy 指向 vm 实例自身， 这个代理的作用就是为了在开发阶段给我们一个友好而准确的提示
条件：当且仅当当前环境属于 非生产环境 并且支持 Proxy 的时候 拿取实例中不存在的值的时候会报错
场景：主要的使用场景是在 render 的时候会通过 _renderProxy 来获取实例中的内容，在上述条件中，如果获取的是不存在或者保留字符则会报错。

// core/isntance/init.js
if (process.env.NODE_ENV !== 'production') {
  initProxy(vm)
} else {
  vm._renderProxy = vm
}

let initProxy
if (process.env.NODE_ENV !== 'production') {
  const allowedGlobals = makeMap(
    'Infinity,undefined,NaN,isFinite,isNaN,' +
    'parseFloat,parseInt,decodeURI,decodeURIComponent,encodeURI,encodeURIComponent,' +
    'Math,Number,Date,Array,Object,Boolean,String,RegExp,Map,Set,JSON,Intl,' +
    'require' // for Webpack/Browserify
  )
  const warnNonPresent = (target, key) => {
    warn(
      `Property or method "${key}" is not defined on the instance but ` +
      'referenced during render. Make sure that this property is reactive, ' +
      'either in the data option, or for class-based components, by ' +
      'initializing the property. ' +
      'See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.',
      target
    )
  }
  const warnReservedPrefix = (target, key) => {
    warn(
      `Property "${key}" must be accessed with "$data.${key}" because ` +
      'properties starting with "$" or "_" are not proxied in the Vue instance to ' +
      'prevent conflicts with Vue internals. ' +
      'See: https://vuejs.org/v2/api/#data',
      target
    )
  }
  const hasProxy =
    typeof Proxy !== 'undefined' && isNative(Proxy)
  if (hasProxy) {
    const isBuiltInModifier = makeMap('stop,prevent,self,ctrl,shift,alt,meta,exact')
    config.keyCodes = new Proxy(config.keyCodes, {
      set (target, key, value) {
        if (isBuiltInModifier(key)) {
          warn(`Avoid overwriting built-in modifier in config.keyCodes: .${key}`)
          return false
        } else {
          target[key] = value
          return true
        }
      }
    })
  }
  const hasHandler = {
    has (target, key) {
      const has = key in target
      const isAllowed = allowedGlobals(key) ||
        (typeof key === 'string' && key.charAt(0) === '_' && !(key in target.$data))
      if (!has && !isAllowed) {
        if (key in target.$data) warnReservedPrefix(target, key)
        else warnNonPresent(target, key)
      }
      return has || !isAllowed
    }
  }
  const getHandler = {
    get (target, key) {
      if (typeof key === 'string' && !(key in target)) {
        if (key in target.$data) warnReservedPrefix(target, key)
        else warnNonPresent(target, key)
      }
      return target[key]
    }
  }
  initProxy = function initProxy (vm) {
    if (hasProxy) {
      // determine which proxy handler to use
      const options = vm.$options
      // options.render._withStripped 一般只有在测试的时候为 true，这也就一意味着，通常而言都是范式 hasHandler
      const handlers = options.render && options.render._withStripped
        ? getHandler
        : hasHandler
      vm._renderProxy = new Proxy(vm, handlers)
    } else {
      vm._renderProxy = vm
    }
  }
}