From 1f8a366e816298efb231a629e8070f687c0ea593 Mon Sep 17 00:00:00 2001
From: Grant Forrest <forrest@progress.com>
Date: Thu, 3 Sep 2026 14:50:04 -0400
Subject: [PATCH] fix: preserve lastArgs of queued execution (#257) clear
 lastArgs upon execution, not after, to avoid resetting lastArgs of upcoming
 queued execution after current completes

---
 .changeset/little-planets-serve.md           |  5 +++
 packages/pacer/src/async-debouncer.ts        |  3 +-
 packages/pacer/tests/async-debouncer.test.ts | 33 ++++++++++++++++++++
 3 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/little-planets-serve.md

diff --git a/.changeset/little-planets-serve.md b/.changeset/little-planets-serve.md
new file mode 100644
index 000000000..b445640d2
--- /dev/null
+++ b/.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
diff --git a/packages/pacer/src/async-debouncer.ts b/packages/pacer/src/async-debouncer.ts
index 5a6a0b657..fcd1a0089 100644
--- a/packages/pacer/src/async-debouncer.ts
+++ b/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)
diff --git a/packages/pacer/tests/async-debouncer.test.ts b/packages/pacer/tests/async-debouncer.test.ts
index 393d6158c..9ee9415ce 100644
--- a/packages/pacer/tests/async-debouncer.test.ts
+++ b/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', () => {