Detailed explanation of the use of Vue.js render function

<div id="div1">
    <child :level="1">Hello world!</child>
</div>

<script type="text/x-template" id="child-template">
  <h1 v-if="level === 1">
    <slot></slot>
  </h1>
  <h2 v-if="level === 2">
    <slot></slot>
  </h2>
  <h3 v-if="level === 3">
    <slot></slot>
  </h3>
  <h4 v-if="level === 4">
    <slot></slot>
  </h4>
  <h5 v-if="level === 5">
    <slot></slot>
  </h5>
  <h6 v-if="level === 6">
    <slot></slot>
  </h6>
</script>

<script type="text/javascript">
    /**
     * Globally register the child component. Note that if the template value starts with #, it is used as an option symbol and the innerHTML of the matching element will be used as the template. A common trick is to use <script type="x-template"> to include the template. The advantage of this is that the HTML will not render the content inside* 
     * Using template here is not the best choice:
     * 1. The code is lengthy* 2. Inserting content in different titles requires repeated use of slots 
     * 3. Since components must have a root element, the title and content are wrapped in a useless div, such as <div><h1>hello world</h1></div>
     */

    Vue.component('child', {
      template: '#child-template',
      props: {
        level:
          type: Number,
          required: true
        }
      },
      data: function() {
        return {
          a: 1
        }
      }
    })

    new Vue({
        el:"#div1"
    })
  </script>

<div id="div1">
    <child :level="1">
        Hello world!
    </child>
    <child :level="2">
        <!-- will not be displayed -->
        <span slot="footer">this is footer slot</span>
        <p slot="header">this is header slot</p>
    </child>
</div>


<script>
    Vue.component('child', {
        render: function (createElement) {
            console.log(this.$slots);
            return createElement(
                'h' + this.level, // tagName tag name {
                    // Set the class for each h tag
                    'class': {
                        foo: true,
                        bar: false
                    },
                    //Finally rendered as inline style style: {
                        color: 'red',
                        fontSize: '14px'
                    },
                    // Other HTML attributes attrs: {
                        id: 'foo',
                        'data-id': 'bar'
                    },
                    // DOM attributes domProps: {
                        // innerHTML: 'from domProps',
                    },
                    // Event listener based on "on"
                    // So no longer supports modifiers such as v-on:keyup.enter on: {
                        click: this.clickHandler
                    },
                    // ...
                },
                // You can get the static content in the VNodes list from this.$slots // $slots.default is used to access the unnamed slot of the component
                // When you may need a named slot, you need to specify the slot name, this.$slots.header
                // [this.$slots.default,this.$slots.footer,this.$slots.header] //Display level 2 slots [this.$slots.default] //Display only unnamed slots
            )
        },
        template: '<div v-if="level===1"><slot></slot></div>', // will be ignored props: {
            level:
                type: Number,
                required: true
            }
        },
        methods: {
            clickHandler: function () {
                console.log('clickHandler')
            }
        }
    })

    new Vue({
        el: "#div1"
    })
</script>