为vue中的data赋值computed计算属性后,出现undefined原因及解决

<template>
  <div class="container">
    <div>num1:{{num1}}</div>
    <div>num2:{{num2}}</div>
    <div>data中的数据{{sum}}</div>
  </div>
</template>
<script>
export default {
  data() {
    return {
      num1:2,
      num2:10,
      sum:this.result
    };
  },
  computed:{
    result(){
      return this.num1+this.num2
    }
  }
};
</script>

function initMixin (Vue) {
    Vue.prototype._init = function (options) {	//在这里定义了_init方法
      var vm = this;
      // a uid
      vm._uid = uid$3++;
      var startTag, endTag;
      /* istanbul ignore if */
      if (config.performance && mark) {
        startTag = "vue-perf-start:" + (vm._uid);
        endTag = "vue-perf-end:" + (vm._uid);
        mark(startTag);
      }
      // a flag to avoid this being observed
      vm._isVue = true;
      // merge options
      if (options && options._isComponent) {
        // optimize internal component instantiation
        // since dynamic options merging is pretty slow, and none of the
        // internal component options needs special treatment.
        initInternalComponent(vm, options);
      } else {
        vm.$options = mergeOptions(
          resolveConstructorOptions(vm.constructor),
          options || {},
          vm
        );
      }
      /* istanbul ignore else */
      {
        initProxy(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 (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);
      }
    };
  }

function initState (vm) {
    vm._watchers = [];
    var opts = vm.$options;
    if (opts.props) { initProps(vm, opts.props); }//初始化props
    if (opts.methods) { initMethods(vm, opts.methods); }//初始化methods
    if (opts.data) {
      initData(vm);
    } else {
      observe(vm._data = {}, true /* asRootData */);
    }	//初始化data
    if (opts.computed) { initComputed(vm, opts.computed); }//初始化computed
    if (opts.watch && opts.watch !== nativeWatch) {
      initWatch(vm, opts.watch);//初始化watch
    }
  }

<template>
  <div class="container">
    <div>num1:{{num1}}</div>
    <div>num2:{{num2}}</div>
    <div>data中的数据{{sum}}</div>
  </div>
</template>
<script>
export default {
  data() {
    return {
      num1:2,
      num2:10,
      sum:this.result
    };
  },
  computed:{
    result(){
      return this.num1+this.num2
    }
  }
};
</script>