fix(async-queuer): keep pendingTick true during wait period

.changeset/fix-async-queuer-pending-tick.md

@@ -0,0 +1,9 @@
+---
+'@tanstack/pacer': patch
+---
+
+fix(async-queuer): keep pendingTick true during wait period
+
+When `addItem()` is called on a running queue during the wait period, it checks `isRunning && !pendingTick` to decide whether to trigger `#tick()`. Previously, `pendingTick` was set to `false` synchronously at the end of `#tick()`, even when async work was still pending. This caused `addItem()` to trigger immediate processing that bypassed the configured `wait` delay.
+
+This fix tracks whether async work was scheduled and only clears `pendingTick` when no async work is pending.

packages/pacer/src/async-queuer.ts

@@ -431,6 +431,7 @@ export class AsyncQueuer<TValue> {
     this.#checkExpiredItems()
 
     // Process items concurrently up to the concurrency limit
+    let scheduledAsyncWork = false
     const activeItems = this.store.state.activeItems
     while (
       activeItems.length < this.#getConcurrency() &&
@@ -444,6 +445,7 @@ export class AsyncQueuer<TValue> {
       this.#setState({
         activeItems,
       })
+      scheduledAsyncWork = true
       ;(async () => {
         await this.execute()
 
@@ -458,7 +460,9 @@ export class AsyncQueuer<TValue> {
       })()
     }
 
-    this.#setState({ pendingTick: false })
+    if (!scheduledAsyncWork) {
+      this.#setState({ pendingTick: false })
+    }
   }
 
   /**

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

@@ -836,6 +836,40 @@ describe('AsyncQueuer', () => {
       expect(results).toHaveLength(3)
       expect(results[2]).toBe('third')
     })
+
+    it('should respect wait period when addItem is called during processing', async () => {
+      const results: Array<string> = []
+      const asyncQueuer = new AsyncQueuer<string>(
+        async (item) => {
+          results.push(item)
+          return item
+        },
+        {
+          wait: 100,
+          concurrency: 1,
+          started: false,
+        },
+      )
+
+      asyncQueuer.addItem('first')
+      asyncQueuer.start()
+
+      // 'first' processes immediately
+      await vi.advanceTimersByTimeAsync(0)
+      expect(results).toEqual(['first'])
+
+      // During the 100ms wait period, add a new item
+      await vi.advanceTimersByTimeAsync(50)
+      asyncQueuer.addItem('second')
+
+      // 'second' should NOT have processed yet — still in the wait period
+      await vi.advanceTimersByTimeAsync(0)
+      expect(results).toEqual(['first'])
+
+      // After the remaining wait time, 'second' should process
+      await vi.advanceTimersByTimeAsync(50)
+      expect(results).toEqual(['first', 'second'])
+    })
   })
 
   describe('error handling', () => {