1. Instantiate the Vue function object, calling the built-in methods_init

src/core/instance/index.js

function Vue (options) {
  if(process.env.NODE_ENV ! = ='production' &&
    !(this instanceof Vue)
  ) {
    warn('Vue is a constructor and should be called with the `new` keyword')}this._init(options)
}
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  1. _initInitialize core functions

src/core/instance/init.js

Vue.prototype._init = function (options? :Object) {
  const vm: Component = this
  // a uid
  vm._uid = uid++

  let startTag, endTag
  /* istanbul ignore if */
  if(process.env.NODE_ENV ! = ='production' && config.performance && mark) {
    startTag = `vue-perf-start:${vm._uid}`
    endTag = `vue-perf-end:${vm._uid}`
    mark(startTag)
  }

  // a flag to avoid this being observed
  vm._isVue = true
  // merge options
  if (options && options._isComponent) {
    // optimize internal component instantiation
    // since dynamic options merging is pretty slow, and none of the
    // internal component options needs special treatment.
    initInternalComponent(vm, options)
  } else {
    vm.$options = mergeOptions(
      resolveConstructorOptions(vm.constructor),
      options || {},
      vm
    )
  }
  /* istanbul ignore else */
  if(process.env.NODE_ENV ! = ='production') {
    initProxy(vm)
  } else {
    vm._renderProxy = vm
  }
  // expose real self
  vm._self = vm
  initLifecycle(vm)
  initEvents(vm)
  initRender(vm)
  callHook(vm, 'beforeCreate')
  initInjections(vm) // resolve injections before data/props
  initState(vm)
  initProvide(vm) // resolve provide after data/props
  callHook(vm, 'created')

  /* istanbul ignore if */
  if(process.env.NODE_ENV ! = ='production' && config.performance && mark) {
    vm._name = formatComponentName(vm, false)
    mark(endTag)
    measure(`vue ${vm._name} init`, startTag, endTag)
  }

  if (vm.$options.el) {
    vm.$mount(vm.$options.el)
  }
}
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The core process of this method is mainly divided into: 1) merge options to $Option, bind options to vm object 2) initialize life cycle, event queue, state, etc. 3) obtain THE DOM node bound by EL string and load the DOM tips: InitState () is used to initialize data props methods computed watch, bind state to built-in objects, and obtain state through proxy and set get operation, but actually obtain the attributes of built-in objects. For example, this.data.message is obtained by this._data.message. After the proxy is completed, all states will be observed and responded