title: All vue props and data give the error 'Property ' ' does not exist on type ' with typescript
body: <p>I recently added typescript to my vue project and every time I access a value in props or data I get this error </p>
  
  <pre><code>37:46 Property 'activity' does not exist on type '{ props: { activity: { type: ObjectConstructor; required: boolean; }; }; methods: { formatDistance: (distance: number, setting_unit: string) =&gt; string; }; mounted: () =&gt; void; }'.
      35 |         },
      36 |         mounted: function () {
    &gt; 37 |             var activitymap = L.map('map' + this.activity['activity']['id']).setView([51.505, -0.09], 13)
         |                                              ^
  </code></pre>
  
  <p>This is my props</p>
  
  <pre><code>    props: {
          activity: {
              type: Object,
              required: true
          }
      },
  </code></pre>
  
  <p>and this is the method it is being used in</p>
  
  <pre><code>    mounted: function () {
          var activitymap = L.map('map' + this.activity['activity']['id']).setView([51.505, -0.09], 13)
          L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
              attribution: '&lt;a href="https://www.openstreetmap.org/"&gt;OpenStreetMap&lt;/a&gt;',
              maxZoom: 18,
              id: 'mapbox.streets',
              accessToken: process.env.VUE_APP_MAPBOX_KEY
          }).addTo(activitymap)
  
          var gpxGroup = new L.FeatureGroup();
  
          activitymap.addLayer(gpxGroup);
  
          omnivore.gpx(this.activity['activity']['activity_log_url']).addTo(gpxGroup).on('ready',
              function () {
                  activitymap.fitBounds(gpxGroup.getBounds());
              }
          )
      }
  </code></pre>
  
  <p>What could be causing this issue?</p>