title: Vue mutation change instance (MyArray to Array)
body: <p>I'm extending builtIn <strong>Array</strong> with my class <strong>MyArray</strong>
  When i try to store MyArr to Vuex, I loose instance, and get bultIn Array</p>
  
  <p>What's wrong?</p>
  
  <pre class="lang-js prettyprint-override"><code>class MyArray extends Array{/*some methods*/}
  
  const state = {
    arr: new ModuleOperationArray(),
  }
  
  const mutations = {
    setArr: (state) =&gt; {
     console.log(state.arr instanceof MyArray); //false -- WHY?
  
     let arr = new MyArray();
     console.log(arr instanceof MyArray); //true -- expectedly
  
     state.arr = arr;
     console.log(state.arr instanceof MyArray); //false -- WHY?
    }
  }
  </code></pre>