$: files = files === folder1 ? folder2 : folder1;

<script>
  import { fade } from 'svelte/transition';
  let folders = [
    {
      id: 1,
      name: 'folder1',
      content: [1,2,3,4,5]
    },
    {
      id: 2,
      name: 'folder2',
      content: [6,7,8,9,10]
    }
  ];
  let folderId = 1;
  function changeFolderId() {
    folderId = folderId < 2 ? folderId + 1 : 1;
  }
</script>

<button on:click={changeFolderId}>
  ChangeFolderId: {folderId}
</button>

{#each [folders.find(f => f.id === folderId)] as folder (folder)}
  <div class="folder" in:fade>
    {#each folder.content as file}
      <div>{file}</div>
    {/each}
  </div>
{/each}

{#each folders.filter(f => f.id === folderId) as folder (folder)}