title: data variable undefined when I use setInterval in vue methods
body: <p>when I get a data variables from vue data data is undefined how can I get a variable for realtime exchange in vue methods </p>
  
  <pre><code>export default {
        name: 'Chart',
        components: {
       },
       data(){
           return {
               firstUpdateTime:0,
               price:[],
               timeLable:[],
               maxPrice: 20000,
           }
       },
       methods:{
  
           calculateChart : function(){
               var a = 300;
               setInterval(() =&gt; 
  
                 this.firstUpdateTime ++;
                 a--;
  
              );}, 30000);
  
  
           }
       },
  
  }
  </code></pre>