add test

packages/runed/src/lib/utilities/watch/watch.test.svelte.ts

@@ -57,7 +57,7 @@ describe("watch", () => {
  }
  );
 
- testWithEffect("lazy watchers correctly pass the initial value as the previous value", () => {
+ testWithEffect("lazy watchers pass the initial value as the previous value", () => {
  return new Promise((resolve) => {
  let count = $state(0);
 
@@ -100,4 +100,24 @@ describe("watch", () => {
  await sleep(0);
  expect(runs).toBe(1);
  });
+
+ testWithEffect("once watchers pass the initial value as the previous value", () => {
+ return new Promise((resolve) => {
+ let count = $state(0);
+
+ watchOnce(
+ () => count,
+ (count, prevCount) => {
+ expect(count).toBe(1);
+ expect(prevCount).toBe(0);
+ resolve();
+ }
+ );
+
+ // Wait for the watcher's initial run to determine its dependencies.
+ sleep(0).then(() => {
+ count = 1;
+ });
+ });
+ });
 });