export class Client {
   months = []

   activate(id) {
      ...
      this.loadData()
   }

   loadData(){
      this.months.push("Item1")
    }

<section class="scrollable">
    ${months.length}
    <div repeat.for="month of months">
       Index = ${$index}
    </div>
</section>

1
Index = 0
Index = 1