fix: preserve lastArgs of queued execution (#257)

.changeset/little-planets-serve.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/pacer': patch
+---
+
+Fixed a bug in AsyncDebouncer where a scheduled execution is dropped if an in-flight execution completes before it is invoked

packages/pacer/src/async-debouncer.ts

@@ -366,7 +366,7 @@ export class AsyncDebouncer<TFn extends AnyAsyncFunction> {
     const currentMaybeExecuteCount = this.store.state.maybeExecuteCount + 1
 
     try {
-      this.#setState({ isExecuting: true })
+      this.#setState({ isExecuting: true, lastArgs: undefined })
       const currentAsyncRetryer = new AsyncRetryer(
         this.fn,
         this.options.asyncRetryerOptions,
@@ -391,7 +391,6 @@ export class AsyncDebouncer<TFn extends AnyAsyncFunction> {
       this.#setState({
         isExecuting: false,
         isPending: false,
-        lastArgs: undefined,
         settleCount: this.store.state.settleCount + 1,
       })
       this.options.onSettled?.(args, this)

packages/pacer/tests/async-debouncer.test.ts

@@ -237,6 +237,39 @@ describe('AsyncDebouncer', () => {
       expect(mockFn).toHaveBeenLastCalledWith('fifth')
       await promise5
     })
+
+    it('should execute a trailing call scheduled after a prior execution', async () => {
+      const mockFn = vi.fn(
+        (_: string) =>
+          new Promise((resolve) => setTimeout(() => resolve('result'), 500)),
+      )
+      const debouncer = new AsyncDebouncer(mockFn, {
+        wait: 1000,
+        leading: true,
+        trailing: true,
+      })
+
+      // First call - should execute immediately
+      const promise1 = debouncer.maybeExecute('first')
+      expect(mockFn).toBeCalledTimes(1)
+      expect(mockFn).toBeCalledWith('first')
+
+      // Second call enqueued during first execution
+      const promise2 = debouncer.maybeExecute('second')
+      expect(mockFn).toBeCalledTimes(1)
+
+      // Advance past first execution
+      vi.advanceTimersByTime(500)
+      await promise1
+
+      // Trigger second execution
+      vi.advanceTimersByTime(1000)
+      await promise2
+
+      // Verify trailing execution occurred
+      expect(mockFn).toBeCalledTimes(2)
+      expect(mockFn).toHaveBeenLastCalledWith('second')
+    })
   })
 
   describe('Promise Handling', () => {