Optimistic update — immediate UI, rollback on failure

What does this output?

async function toggleLike(id, liked, updateUI, revertUI, saveToServer) {
  // Optimistic: update UI immediately
  updateUI(!liked);

  try {
    await saveToServer(id, !liked);
    // Success: UI already correct
  } catch {
    // Failure: revert to original state
    revertUI(liked);
  }
}

let uiState = false;
let serverSaveFails = true;

const log = [];

toggleLike(
  1,
  false,
  val => { uiState = val; log.push('ui→' + val); },
  val => { uiState = val; log.push('revert→' + val); },
  () => serverSaveFails ? Promise.reject() : Promise.resolve()
).then(() => console.log(log.join(' | ')));

Correct Output

ui→true | revert→false

Why this output?