<div id="app">
  <demo key="key1"></demo>
</div>
</body>
<script>
    import Vue from 'vue';
    import demo from '../demo.vue';
    new Vue({
        el: '#app',
        components: {
           demo
        }
    })
</script>

<template>
     <div>test demo</div>
</template>
<script>
    export default{
        props: {
            key:String
        },
        created(){
            console.log(this.key);  //print undefined
        }
    }
</script>