Vue computed property with an array not updating

Props are not Reactive. Use data:

Vue.component('test', {
 props:['myarray'],
  data: function() { 
     return {
        arr: this.myarray.slice()
     }
  },
  template:'#arraybug',
  methods:{
    add:function(){

      this.arr.push(1);
    }
  },
  computed:{
    mycomputed: function(){
        let newLen = this.arr.length;
        return newLen;
    }
  }
 });

var test = new Vue({
  el:"#test"});

I'm copied props array to data.