fix race condition and add test (#261)

Author: shuding

src/use-swr.ts

@@ -275,13 +275,13 @@ function useSWR<Data = any, Error = any>(
 
           CONCURRENT_PROMISES_TS[key] = startAt = Date.now()
 
+          newData = await CONCURRENT_PROMISES[key]
+
           setTimeout(() => {
             delete CONCURRENT_PROMISES[key]
             delete CONCURRENT_PROMISES_TS[key]
           }, config.dedupingInterval)
 
-          newData = await CONCURRENT_PROMISES[key]
-
           // trigger the success event,
           // only do this for the original request.
           config.onSuccess(newData, key, config)
@@ -470,7 +470,10 @@ function useSWR<Data = any, Error = any>(
 
     // set up reconnecting when the browser regains network connection
     let reconnect = null
-    if (typeof addEventListener !== 'undefined' && config.revalidateOnReconnect) {
+    if (
+      typeof addEventListener !== 'undefined' &&
+      config.revalidateOnReconnect
+    ) {
       reconnect = addEventListener('online', softRevalidate)
     }
 

test/use-swr.test.tsx

@@ -852,6 +852,28 @@ describe('useSWR - local mutation', () => {
     await act(() => new Promise(res => setTimeout(res, 110)))
     expect(container.textContent).toMatchInlineSnapshot(`"data: 999"`)
   })
+
+  it('should ignore in flight requests when mutating', async () => {
+    // set it to 1
+    mutate('mutate-2', 1)
+
+    function Section() {
+      const { data } = useSWR(
+        'mutate-2',
+        () => new Promise(res => setTimeout(() => res(2), 200))
+      )
+      return <div>{data}</div>
+    }
+
+    const { container } = render(<Section />)
+
+    expect(container.textContent).toMatchInlineSnapshot(`"1"`) // directly from cache
+    await act(() => new Promise(res => setTimeout(res, 150))) // still suspending
+    mutate('mutate-2', 3) // set it to 3. this will drop the ongoing request
+    expect(container.textContent).toMatchInlineSnapshot(`"3"`)
+    await act(() => new Promise(res => setTimeout(res, 100)))
+    expect(container.textContent).toMatchInlineSnapshot(`"3"`)
+  })
 })
 
 describe('useSWR - context configs', () => {