code.ts

figma.showUI(__html__);

async function main() {
  for (let i = 0; i < 5; i++) {
    figma.ui.postMessage({ i });
    await new Promise((resolve) => setTimeout(resolve, 1000));
  }
}

main()
  .then(() => {
    figma.closePlugin();
  })
  .catch((e) => {
    console.error(e);
    figma.closePlugin();
  });

ui.html

<script>
  window.onmessage = (ev) => console.log(ev.data.pluginMessage);
</script>