大概650-1600行

/*  */


var uid = 0;

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
//創(chuàng)建依賴镜硕，添加兩個(gè)屬性猫十，id為uid全局變量，累加,subs是數(shù)組浦妄，還有后面的target屬性
var Dep = function Dep () {
  this.id = uid++;
  this.subs = [];
};
// dep的原型鏈上添加方法addSub方法呢灶，觀察者數(shù)組添加觀察者
Dep.prototype.addSub = function addSub (sub) {
  this.subs.push(sub);
};
// dep的原型鏈上添加方法removeSub方法吴超，觀察者數(shù)組刪除觀察者
Dep.prototype.removeSub = function removeSub (sub) {
  remove(this.subs, sub);
};
// dep的原型鏈上添加方法depend方法，如果依賴已經(jīng)有target屬性鸯乃，target屬性調(diào)用addDep方法
Dep.prototype.depend = function depend () {
  if (Dep.target) {
    Dep.target.addDep(this);
  }
};
// dep的原型鏈上添加方法notify方法鲸阻，如果依賴已經(jīng)有target屬性，target屬性調(diào)用addDep方法
Dep.prototype.notify = function notify () {
  // stabilize the subscriber list first
  // 拷貝subs數(shù)組
  var subs = this.subs.slice();
  // 每個(gè)sub觀察對(duì)象調(diào)用update方法
  for (var i = 0, l = subs.length; i < l; i++) {
    subs[i].update();

  }
};

// the current target watcher being evaluated.
// this is globally unique because there could be only one
// watcher being evaluated at any time.
Dep.target = null;
// Dep的target組成的數(shù)組
var targetStack = [];
// dep的target更新缨睡，并將target屬性更換為新的watcher
function pushTarget (_target) {
  if (Dep.target) { targetStack.push(Dep.target); }
  Dep.target = _target;
}
// dep的target屬性更換為targetStack數(shù)組最后一個(gè)
function popTarget () {
  Dep.target = targetStack.pop();
}

/*  */
// 生成vNode函數(shù)鸟悴，綁定了多種屬性和方法
var VNode = function VNode (
  tag,
  data,
  children,
  text,
  elm,
  context,
  componentOptions,
  asyncFactory
) {
  this.tag = tag;
  this.data = data;
  this.children = children;
  this.text = text;
  this.elm = elm;
  this.ns = undefined;
  // watcher 監(jiān)聽(tīng)對(duì)象
  this.context = context;
  this.fnContext = undefined;
  this.fnOptions = undefined;
  this.fnScopeId = undefined;
  this.key = data && data.key;
  this.componentOptions = componentOptions;
  this.componentInstance = undefined;
  this.parent = undefined;
  this.raw = false;
  this.isStatic = false;
  this.isRootInsert = true;
  this.isComment = false;
  this.isCloned = false;
  this.isOnce = false;
  this.asyncFactory = asyncFactory;
  this.asyncMeta = undefined;
  this.isAsyncPlaceholder = false;
};
// 全局變量prototypeAccessors的child屬性是可枚舉的
var prototypeAccessors = { child: { configurable: true } };

// DEPRECATED: alias for componentInstance for backwards compat.
/* istanbul ignore next */
// 設(shè)置全局變量prototypeAccessors的child屬性的get方法
prototypeAccessors.child.get = function () {
  return this.componentInstance
};
// 定義了VNode的原型鏈上 child的屬性，包括get方法和枚舉屬性
Object.defineProperties( VNode.prototype, prototypeAccessors );
// 創(chuàng)建空文字節(jié)點(diǎn)
var createEmptyVNode = function (text) {
  if ( text === void 0 ) text = '';

  var node = new VNode();
  node.text = text;
  node.isComment = true;
  return node
};
// 生成文字節(jié)點(diǎn)的方法
function createTextVNode (val) {
  return new VNode(undefined, undefined, undefined, String(val))
}

// optimized shallow clone
// used for static nodes and slot nodes because they may be reused across
// multiple renders, cloning them avoids errors when DOM manipulations rely
// on their elm reference.
// 克隆節(jié)點(diǎn)
function cloneVNode (vnode) {
  var cloned = new VNode(
    vnode.tag,
    vnode.data,
    vnode.children,
    vnode.text,
    vnode.elm,
    vnode.context,
    vnode.componentOptions,
    vnode.asyncFactory
  );
  cloned.ns = vnode.ns;
  cloned.isStatic = vnode.isStatic;
  cloned.key = vnode.key;
  cloned.isComment = vnode.isComment;
  cloned.fnContext = vnode.fnContext;
  cloned.fnOptions = vnode.fnOptions;
  cloned.fnScopeId = vnode.fnScopeId;
  cloned.isCloned = true;
  return cloned
}

/*
 * not type checking this file because flow doesn't play well with
 * dynamically accessing methods on Array prototype
 */

var arrayProto = Array.prototype;
var arrayMethods = Object.create(arrayProto);

var methodsToPatch = [
  'push',
  'pop',
  'shift',
  'unshift',
  'splice',
  'sort',
  'reverse'
];

/**
 * Intercept mutating methods and emit events
 */
methodsToPatch.forEach(function (method) {
  // cache original method
  var original = arrayProto[method];
  def(arrayMethods, method, function mutator () {
    var args = [], len = arguments.length;
    while ( len-- ) args[ len ] = arguments[ len ];
    var result = original.apply(this, args);
    var ob = this.__ob__;
    var inserted;
    switch (method) {
      case 'push':
      case 'unshift':
        inserted = args;
        break
      case 'splice':
        inserted = args.slice(2);
        break
    }
    if (inserted) { ob.observeArray(inserted); }
    // notify change
    ob.dep.notify();
    return result
  });
});

/*  */
// 獲取arrayMethods數(shù)組的每個(gè)方法名
var arrayKeys = Object.getOwnPropertyNames(arrayMethods);

/**
 * In some cases we may want to disable observation inside a component's
 * update computation.
 */
// shouldObserve屬性設(shè)置為true,表示應(yīng)該監(jiān)聽(tīng)
var shouldObserve = true;
// 切換是否監(jiān)聽(tīng)的函數(shù)
function toggleObserving (value) {
  shouldObserve = value;
}

/**
 * Observer class that is attached to each observed
 * object. Once attached, the observer converts the target
 * object's property keys into getter/setters that
 * collect dependencies and dispatch updates.
 */
// 觀察者
var Observer = function Observer (value) {
  // 設(shè)置自身value
  this.value = value;
  // 生成自身
  this.dep = new Dep();
  this.vmCount = 0;
  // 定義傳參的‘__ob__'屬性設(shè)置為本身
  def(value, '__ob__', this);
  // 如果參數(shù)是數(shù)組
  if (Array.isArray(value)) {
    // 將arrMethods的屬性復(fù)制到傳參上
    var augment = hasProto
      ? protoAugment
      : copyAugment;
    augment(value, arrayMethods, arrayKeys);
    //并監(jiān)聽(tīng)數(shù)組的形式監(jiān)聽(tīng)傳參
    this.observeArray(value);
  } else {
    // 否者以對(duì)象的形式監(jiān)聽(tīng)
    this.walk(value);
  }
};

/**
 * Walk through each property and convert them into
 * getter/setters. This method should only be called when
 * value type is Object.
 */
Observer.prototype.walk = function walk (obj) {
  var keys = Object.keys(obj);
  for (var i = 0; i < keys.length; i++) {
    defineReactive(obj, keys[i]);
  }
};

/**
 * Observe a list of Array items.
 */
Observer.prototype.observeArray = function observeArray (items) {
  for (var i = 0, l = items.length; i < l; i++) {
    observe(items[i]);
  }
};

// helpers

/**
 * Augment an target Object or Array by intercepting
 * the prototype chain using __proto__
 */
// 設(shè)置__proto__原型鏈
function protoAugment (target, src, keys) {
  /* eslint-disable no-proto */
  target.__proto__ = src;
  /* eslint-enable no-proto */
}

/**
 * Augment an target Object or Array by defining
 * hidden properties.
 */
/* istanbul ignore next */
// 將src的屬性復(fù)制到target對(duì)象上
function copyAugment (target, src, keys) {
  for (var i = 0, l = keys.length; i < l; i++) {
    var key = keys[i];
    def(target, key, src[key]);
  }
}

/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
 */
// 重新設(shè)置監(jiān)聽(tīng)
function observe (value, asRootData) {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  var ob;
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__;
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    ob = new Observer(value);
  }
  if (asRootData && ob) {
    ob.vmCount++;
  }
  return ob
}

/**
 * Define a reactive property on an Object.
 */
// 監(jiān)聽(tīng)對(duì)象屬性
function defineReactive (
  obj,
  key,
  val,
  customSetter,
  shallow
) {
  var dep = new Dep();

  var property = Object.getOwnPropertyDescriptor(obj, key);
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  var getter = property && property.get;
  // 如果沒(méi)有改屬性的get方法宏蛉，并沒(méi)有val參數(shù)遣臼，就設(shè)置val為該屬性對(duì)應(yīng)的值
  if (!getter && arguments.length === 2) {
    val = obj[key];
  }
  var setter = property && property.set;
  // 如果不是淺監(jiān)聽(tīng)，開(kāi)始監(jiān)聽(tīng)屬性
  var childOb = !shallow && observe(val);
  // 設(shè)置改屬性的get方法和set方法拾并，重置改屬性和修改該屬性
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      // 如果有g(shù)et方法揍堰，調(diào)用get方法，沒(méi)有設(shè)置改屬性的值
      var value = getter ? getter.call(obj) : val;
      // 如果有set嗅义，添加觀察者
      if (Dep.target) {
        dep.depend();
        // 如果是深度監(jiān)聽(tīng)屏歹，value也設(shè)置監(jiān)聽(tīng)
        if (childOb) {
          childOb.dep.depend();
          // 如果是數(shù)組，監(jiān)聽(tīng)整個(gè)數(shù)組
          if (Array.isArray(value)) {
            dependArray(value);
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      var value = getter ? getter.call(obj) : val;
      /* eslint-disable no-self-compare */
      // 如果重新設(shè)置的值和之前的設(shè)置相同之碗，return
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if ("development" !== 'production' && customSetter) {
        customSetter();
      }
      //屬性是否有setter方法蝙眶，如果有setter調(diào)用，如果沒(méi)有設(shè)置val等于newVal
      if (setter) {
        setter.call(obj, newVal);
      } else {
        val = newVal;
      }
      // 重新監(jiān)聽(tīng)newVal
      childOb = !shallow && observe(newVal);
      //依賴
      dep.notify();
    }
  });
}

/**
 * Set a property on an object. Adds the new property and
 * triggers change notification if the property doesn't
 * already exist.
 */
// 設(shè)置對(duì)象的屬性
function set (target, key, val) {
  // 如果target是undefined,null,基礎(chǔ)類型的數(shù)據(jù)，就發(fā)出警告
  if ("development" !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(("Cannot set reactive property on undefined, null, or primitive value: " + ((target))));
  }
  // target如果是數(shù)組幽纷，
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    // 設(shè)置長(zhǎng)度
    target.length = Math.max(target.length, key);
    // 將在key對(duì)應(yīng)位置的數(shù)組元素替換為val
    target.splice(key, 1, val);
    return val
  }
  // key是target的自帶屬性式塌，將其屬性值替換為val
  if (key in target && !(key in Object.prototype)) {
    target[key] = val;
    return val
  }
  var ob = (target).__ob__;
  if (target._isVue || (ob && ob.vmCount)) {
    "development" !== 'production' && warn(
      'Avoid adding reactive properties to a Vue instance or its root $data ' +
      'at runtime - declare it upfront in the data option.'
    );
    return val
  }
  if (!ob) {
    target[key] = val;
    return val
  }
  // 監(jiān)聽(tīng)之前監(jiān)聽(tīng)對(duì)象的value
  defineReactive(ob.value, key, val);
  // 觀察者的依賴跟新
  ob.dep.notify();
  return val
}

/**
 * Delete a property and trigger change if necessary.
 */
// 刪除對(duì)象的屬性或者數(shù)組的元素，并跟新依賴
function del (target, key) {
  if ("development" !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(("Cannot delete reactive property on undefined, null, or primitive value: " + ((target))));
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.splice(key, 1);
    return
  }
  var ob = (target).__ob__;
  if (target._isVue || (ob && ob.vmCount)) {
    "development" !== 'production' && warn(
      'Avoid deleting properties on a Vue instance or its root $data ' +
      '- just set it to null.'
    );
    return
  }
  if (!hasOwn(target, key)) {
    return
  }
  delete target[key];
  if (!ob) {
    return
  }
  ob.dep.notify();
}

/**
 * Collect dependencies on array elements when the array is touched, since
 * we cannot intercept array element access like property getters.
 */
//依賴數(shù)組
function dependArray (value) {
  for (var e = (void 0), i = 0, l = value.length; i < l; i++) {
    e = value[i];
    e && e.__ob__ && e.__ob__.dep.depend();
    if (Array.isArray(e)) {
      dependArray(e);
    }
  }
}

/*  */

/**
 * Option overwriting strategies are functions that handle
 * how to merge a parent option value and a child option
 * value into the final value.
 */
// 自定義處理方法
var strats = config.optionMergeStrategies;

/**
 * Options with restrictions
 */
{
  strats.el = strats.propsData = function (parent, child, vm, key) {
    if (!vm) {
      warn(
        "option \"" + key + "\" can only be used during instance " +
        'creation with the `new` keyword.'
      );
    }
    return defaultStrat(parent, child)
  };
}

/**
 * Helper that recursively merges two data objects together.
 */
//合并兩個(gè)對(duì)象
function mergeData (to, from) {
  if (!from) { return to }
  var key, toVal, fromVal;
  var keys = Object.keys(from);
  for (var i = 0; i < keys.length; i++) {
    key = keys[i];
    toVal = to[key];
    fromVal = from[key];
    if (!hasOwn(to, key)) {
      set(to, key, fromVal);
    } else if (isPlainObject(toVal) && isPlainObject(fromVal)) {
      mergeData(toVal, fromVal);
    }
  }
  return to
}

/**
 * Data
 */
// 合并data或者函數(shù)
function mergeDataOrFn (
  parentVal,
  childVal,
  vm
) {
  if (!vm) {
    // in a Vue.extend merge, both should be functions
    if (!childVal) {
      return parentVal
    }
    if (!parentVal) {
      return childVal
    }
    // when parentVal & childVal are both present,
    // we need to return a function that returns the
    // merged result of both functions... no need to
    // check if parentVal is a function here because
    // it has to be a function to pass previous merges.
    return function mergedDataFn () {
      return mergeData(
        typeof childVal === 'function' ? childVal.call(this, this) : childVal,
        typeof parentVal === 'function' ? parentVal.call(this, this) : parentVal
      )
    }
  } else {
    return function mergedInstanceDataFn () {
      // instance merge
      var instanceData = typeof childVal === 'function'
        ? childVal.call(vm, vm)
        : childVal;
      var defaultData = typeof parentVal === 'function'
        ? parentVal.call(vm, vm)
        : parentVal;
      if (instanceData) {
        return mergeData(instanceData, defaultData)
      } else {
        return defaultData
      }
    }
  }
}

strats.data = function (
  parentVal,
  childVal,
  vm
) {
  if (!vm) {
    if (childVal && typeof childVal !== 'function') {
      "development" !== 'production' && warn(
        'The "data" option should be a function ' +
        'that returns a per-instance value in component ' +
        'definitions.',
        vm
      );

      return parentVal
    }
    return mergeDataOrFn(parentVal, childVal)
  }

  return mergeDataOrFn(parentVal, childVal, vm)
};

/**
 * Hooks and props are merged as arrays.
 */
// 合并生命周期友浸，最后是以數(shù)組的形式
function mergeHook (
  parentVal,
  childVal
) {
  return childVal
    ? parentVal
      ? parentVal.concat(childVal)
      : Array.isArray(childVal)
        ? childVal
        : [childVal]
    : parentVal
}

LIFECYCLE_HOOKS.forEach(function (hook) {
  strats[hook] = mergeHook;
});

/**
 * Assets
 *
 * When a vm is present (instance creation), we need to do
 * a three-way merge between constructor options, instance
 * options and parent options.
 */
function mergeAssets (
  parentVal,
  childVal,
  vm,
  key
) {
  var res = Object.create(parentVal || null);
  if (childVal) {
    "development" !== 'production' && assertObjectType(key, childVal, vm);
    return extend(res, childVal)
  } else {
    return res
  }
}

ASSET_TYPES.forEach(function (type) {
  strats[type + 's'] = mergeAssets;
});

/**
 * Watchers.
 *
 * Watchers hashes should not overwrite one
 * another, so we merge them as arrays.
 */
strats.watch = function (
  parentVal,
  childVal,
  vm,
  key
) {
  // work around Firefox's Object.prototype.watch...
  if (parentVal === nativeWatch) { parentVal = undefined; }
  if (childVal === nativeWatch) { childVal = undefined; }
  /* istanbul ignore if */
  if (!childVal) { return Object.create(parentVal || null) }
  {
    assertObjectType(key, childVal, vm);
  }
  if (!parentVal) { return childVal }
  var ret = {};
  extend(ret, parentVal);
  for (var key$1 in childVal) {
    var parent = ret[key$1];
    var child = childVal[key$1];
    if (parent && !Array.isArray(parent)) {
      parent = [parent];
    }
    ret[key$1] = parent
      ? parent.concat(child)
      : Array.isArray(child) ? child : [child];
  }
  return ret
};

/**
 * Other object hashes.
 */
strats.props =
strats.methods =
strats.inject =
strats.computed = function (
  parentVal,
  childVal,
  vm,
  key
) {
  if (childVal && "development" !== 'production') {
    assertObjectType(key, childVal, vm);
  }
  if (!parentVal) { return childVal }
  var ret = Object.create(null);
  extend(ret, parentVal);
  if (childVal) { extend(ret, childVal); }
  return ret
};
strats.provide = mergeDataOrFn;

/**
 * Default strategy.
 */
var defaultStrat = function (parentVal, childVal) {
  return childVal === undefined
    ? parentVal
    : childVal
};

/**
 * Validate component names
 */
// 組件的合法性檢查
function checkComponents (options) {
  for (var key in options.components) {
    validateComponentName(key);
  }
}
// 組件名的合法性檢查
function validateComponentName (name) {
  if (!/^[a-zA-Z][\w-]*$/.test(name)) {
    warn(
      'Invalid component name: "' + name + '". Component names ' +
      'can only contain alphanumeric characters and the hyphen, ' +
      'and must start with a letter.'
    );
  }
  if (isBuiltInTag(name) || config.isReservedTag(name)) {
    warn(
      'Do not use built-in or reserved HTML elements as component ' +
      'id: ' + name
    );
  }
}

/**
 * Ensure all props option syntax are normalized into the
 * Object-based format.
 */
// props規(guī)則化
function normalizeProps (options, vm) {
  var props = options.props;
  // 如果有props為null,undefined等,返回
  if (!props) { return }
  var res = {};
  var i, val, name;
  // 如果props為數(shù)組
  if (Array.isArray(props)) {
    i = props.length;
    while (i--) {
      val = props[i];
      // 如果是字符串類型的峰尝，type設(shè)置為null,首字母大寫(xiě)
      if (typeof val === 'string') {
        name = camelize(val);
        res[name] = { type: null };
      } else {
        warn('props must be strings when using array syntax.');
      }
    }
    // 若果是對(duì)象
  } else if (isPlainObject(props)) {
    for (var key in props) {
      val = props[key];
      name = camelize(key);
      res[name] = isPlainObject(val)
        ? val
        : { type: val };
    }
  } else {
    warn(
      "Invalid value for option \"props\": expected an Array or an Object, " +
      "but got " + (toRawType(props)) + ".",
      vm
    );
  }
  options.props = res;
}

/**
 * Normalize all injections into Object-based format
 */
// 注入的規(guī)則，并用一個(gè)變量保存
function normalizeInject (options, vm) {
  var inject = options.inject;
  if (!inject) { return }
  var normalized = options.inject = {};
  if (Array.isArray(inject)) {
    for (var i = 0; i < inject.length; i++) {
      normalized[inject[i]] = { from: inject[i] };
    }
  } else if (isPlainObject(inject)) {
    for (var key in inject) {
      var val = inject[key];
      normalized[key] = isPlainObject(val)
        ? extend({ from: key }, val)
        : { from: val };
    }
  } else {
    warn(
      "Invalid value for option \"inject\": expected an Array or an Object, " +
      "but got " + (toRawType(inject)) + ".",
      vm
    );
  }
}

/**
 * Normalize raw function directives into object format.
 */
// 指令的規(guī)則化收恢，如果指令是函數(shù)將指令的每個(gè)都綁定對(duì)應(yīng)的方法
function normalizeDirectives (options) {
  var dirs = options.directives;
  if (dirs) {
    for (var key in dirs) {
      var def = dirs[key];
      if (typeof def === 'function') {
        dirs[key] = { bind: def, update: def };
      }
    }
  }
}

// 對(duì)象如果不是通過(guò)字面量或者new Object()形式生成的對(duì)象武学，發(fā)出警告
function assertObjectType (name, value, vm) {
  if (!isPlainObject(value)) {
    warn(
      "Invalid value for option \"" + name + "\": expected an Object, " +
      "but got " + (toRawType(value)) + ".",
      vm
    );
  }
}

/**
 * Merge two option objects into a new one.
 * Core utility used in both instantiation and inheritance.
 */
//將兩個(gè)option合并
function mergeOptions (
  parent,
  child,
  vm
) {
  {
    checkComponents(child);
  }

  if (typeof child === 'function') {
    child = child.options;
  }

  normalizeProps(child, vm);
  normalizeInject(child, vm);
  normalizeDirectives(child);
  var extendsFrom = child.extends;
  if (extendsFrom) {
    parent = mergeOptions(parent, extendsFrom, vm);
  }
  if (child.mixins) {
    for (var i = 0, l = child.mixins.length; i < l; i++) {
      parent = mergeOptions(parent, child.mixins[i], vm);
    }
  }
  var options = {};
  var key;
  for (key in parent) {
    mergeField(key);
  }
  for (key in child) {
    if (!hasOwn(parent, key)) {
      mergeField(key);
    }
  }
  function mergeField (key) {
    var strat = strats[key] || defaultStrat;
    options[key] = strat(parent[key], child[key], vm, key);
  }
  return options
}

/**
 * Resolve an asset.
 * This function is used because child instances need access
 * to assets defined in its ancestor chain.
 */
//為子組件新建另外一條鏈
function resolveAsset (
  options,
  type,
  id,
  warnMissing
) {
  /* istanbul ignore if */
  if (typeof id !== 'string') {
    return
  }
  var assets = options[type];
  // check local registration variations first
  if (hasOwn(assets, id)) { return assets[id] }
  var camelizedId = camelize(id);
  if (hasOwn(assets, camelizedId)) { return assets[camelizedId] }
  var PascalCaseId = capitalize(camelizedId);
  if (hasOwn(assets, PascalCaseId)) { return assets[PascalCaseId] }
  // fallback to prototype chain
  var res = assets[id] || assets[camelizedId] || assets[PascalCaseId];
  if ("development" !== 'production' && warnMissing && !res) {
    warn(
      'Failed to resolve ' + type.slice(0, -1) + ': ' + id,
      options
    );
  }
  return res
}

/*  */
// 檢查是否是合法的prop參數(shù)
function validateProp (
  key,
  propOptions,
  propsData,
  vm
) {
  var prop = propOptions[key];
  var absent = !hasOwn(propsData, key);
  var value = propsData[key];
  // boolean casting
  var booleanIndex = getTypeIndex(Boolean, prop.type);
  if (booleanIndex > -1) {
    if (absent && !hasOwn(prop, 'default')) {
      value = false;
    } else if (value === '' || value === hyphenate(key)) {
      // only cast empty string / same name to boolean if
      // boolean has higher priority
      var stringIndex = getTypeIndex(String, prop.type);
      if (stringIndex < 0 || booleanIndex < stringIndex) {
        value = true;
      }
    }
  }
  // check default value
  if (value === undefined) {
    value = getPropDefaultValue(vm, prop, key);
    // since the default value is a fresh copy,
    // make sure to observe it.
    var prevShouldObserve = shouldObserve;
    toggleObserving(true);
    observe(value);
    toggleObserving(prevShouldObserve);
  }
  {
    assertProp(prop, key, value, vm, absent);
  }
  return value
}