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import { isServerRendering } from 'core/util/env'
import { FunctionalRenderContext } from 'core/vdom/create-functional-component'

...

Object.defineProperty(Vue.prototype, '$isServer', {
  get: isServerRendering
})

...

Object.defineProperty(Vue, 'FunctionalRenderContext', {
  value: FunctionalRenderContext
})

...

Vue.prototype.$isServer = isServerRendering()

const a = {}

// Uncaught TypeError: Invalid property descriptor. Cannot both specify accessors and a value or writable attribute
Object.defineProperty(a, 'x', {
    get() {return 1},
    value: 2,
})

const obj1 = {}

Object.defineProperty(obj1, 'x', {
    value: 2,
    configurable: true,
    writable: false
})

obj1.x = 3
console.log(obj1.x) // 2 这个例子就是writeable的作用，控制是否能改变x的值，注意是x的值

const obj2 = {}
Object.defineProperty(obj2, 'x', {
    value: 2,
    configurable: false,
    writable: true
})

obj2.x = 3
console.log(obj2.x) // 3 可见configurable并不能控制x的值能否改变，往下看它的真实作用


const obj3 = {}
Object.defineProperty(obj3, 'x', {
    value: 2,
    configurable: false,
    writable: true
})

// Uncaught SyntaxError: Identifier 'obj3' has already been declared
Object.defineProperty(obj3, 'x', {
    configurable: true
})
// configurable其实去控制表中6个属性中除去两个数据描述符属性意外的4个属性 是否能被修改， 以及x这个属性可否被删除

Object.defineProperty(Vue.prototype, '$isServer', {
  get: isServerRendering
})

// 等价于

Object.defineProperty(Vue.prototype, '$isServer', {
  configurable: false,
  enumerable: false,
  get: isServerRendering
  set: undefined
})


Object.defineProperty(Vue, 'FunctionalRenderContext', {
  value: FunctionalRenderContext
})

// 等价于

Object.defineProperty(Vue, 'FunctionalRenderContext', {
  configurable: false,
  enumerable: false,
  value: FunctionalRenderContext,
  writable: false
})

...