在HTML中 slot 元素 ，作为 Web Components 技术套件的一部分
是Web组件内的一个占位符该占位符可以在后期使用自己的标记语言填充

<template id="element-details-template">
  <slot name="element-name">Slot template</slot>
</template>
<element-details>
  <span slot="element-name">1</span>
</element-details>
<element-details>
  <span slot="element-name">2</span>
</element-details>

customElements.define('element-details',
  class extends HTMLElement {
    constructor() {
      super();
      const template = document
        .getElementById('element-details-template')
        .content;
      const shadowRoot = this.attachShadow({mode: 'open'})
        .appendChild(template.cloneNode(true));
  }
})

<template>
    <slot>
      <p>插槽的内容</p>
    </slot>
</template>

<Child>
  <div>默认插槽</div>  
</Child>

<template>
    <slot>插槽的内容</slot>
  <slot name="content">插槽的内容</slot>
</template>

<child>
    <template v-slot:default>具名插槽</template>
    <!-- 具名插槽是⽤插槽名做参数 -->
    <template v-slot:content>存放内容区域</template>
</child>

<template> 
  <slot name="footer" testProps="子组件的值">
          <h3>没有传递footer插槽</h3>
    </slot>
</template>

<child> 
    <!-- 把v-slot的值指定为作⽤域上下⽂对象 -->
    <template v-slot:default="slotProps">
      来自子组件数据：{{slotProps.testProps}}
    </template>
    <template #default="slotProps">
      来自子组件数据：{{slotProps.testProps}}
    </template>
</child>

Vue.component('button-counter', {
  template: '<div> <slot>我是默认的内容</slot></div>'
})

new Vue({
    el: '#app',
    template: '<button-counter><span>我是slot传入内容</span></button-counter>',
    components:{buttonCounter}
})

(function anonymous(
) {
with(this){return _c('div',[_t("default",[_v("我是默认内容")])],2)}
})

function renderSlot (
  name,
  fallback,
  props,
  bindObject
) {
  // 得到渲染插槽内容的函数    
  var scopedSlotFn = this.$scopedSlots[name];
  var nodes;
  // 如果存在插槽渲染函数，则执行插槽渲染函数，生成nodes节点返回
  // 否则使用默认值
  nodes = scopedSlotFn(props) || fallback;
  return nodes;
}

function initRender (vm) {
  ...
  vm.$slots = resolveSlots(options._renderChildren, renderContext);
  ...
}

function resolveSlots (
    children,
    context
  ) {
    if (!children || !children.length) {
      return {}
    }
    var slots = {};
    for (var i = 0, l = children.length; i < l; i++) {
      var child = children[i];
      var data = child.data;
      // remove slot attribute if the node is resolved as a Vue slot node
      if (data && data.attrs && data.attrs.slot) {
        delete data.attrs.slot;
      }
      // named slots should only be respected if the vnode was rendered in the
      // same context.
      if ((child.context === context || child.fnContext === context) &&
        data && data.slot != null
      ) {
        // 如果slot存在(slot="header") 则拿对应的值作为key
        var name = data.slot;
        var slot = (slots[name] || (slots[name] = []));
        // 如果是tempalte元素 则把template的children添加进数组中，这也就是为什么你写的template标签并不会渲染成另一个标签到页面
        if (child.tag === 'template') {
          slot.push.apply(slot, child.children || []);
        } else {
          slot.push(child);
        }
      } else {
        // 如果没有就默认是default
        (slots.default || (slots.default = [])).push(child);
      }
    }
    // ignore slots that contains only whitespace
    for (var name$1 in slots) {
      if (slots[name$1].every(isWhitespace)) {
        delete slots[name$1];
      }
    }
    return slots
}

vm.$scopedSlots = normalizeScopedSlots(
  _parentVnode.data.scopedSlots,
  vm.$slots,
  vm.$scopedSlots
);