fix: don't reassign closed-over parameter in fromFetch (ReactiveX#5234)

* test: add failing test for ReactiveX#5233

* fix: don't reassign closed-over parameter in fromFetch

Closes ReactiveX#5233

* chore: add type annotation for variable

spec/observables/dom/fetch-spec.ts

@@ -176,6 +176,25 @@ describe('fromFetch', () => {
     expect(mockFetch.calls[0].init.signal.aborted).to.be.true;
   });
 
+  it('should not immediately abort repeat subscribers', () => {
+    const fetch$ = fromFetch('/foo');
+    expect(mockFetch.calls.length).to.equal(0);
+    expect(MockAbortController.created).to.equal(0);
+    let subscription = fetch$.subscribe();
+    expect(MockAbortController.created).to.equal(1);
+    expect(mockFetch.calls[0].init.signal.aborted).to.be.false;
+
+    subscription.unsubscribe();
+    expect(mockFetch.calls[0].init.signal.aborted).to.be.true;
+
+    subscription = fetch$.subscribe();
+    expect(MockAbortController.created).to.equal(2);
+    expect(mockFetch.calls[1].init.signal.aborted).to.be.false;
+
+    subscription.unsubscribe();
+    expect(mockFetch.calls[1].init.signal.aborted).to.be.true;
+  });
+
   it('should allow passing of init object', done => {
     const fetch$ = fromFetch('/foo', {method: 'HEAD'});
     fetch$.subscribe({

src/internal/observable/dom/fetch.ts

@@ -58,6 +58,7 @@ export function fromFetch(input: string | Request, init?: RequestInit): Observab
     let abortable = true;
     let unsubscribed = false;
 
+    let perSubscriberInit: RequestInit;
     if (init) {
       // If a signal is provided, just have it teardown. It's a cancellation token, basically.
       if (init.signal) {
@@ -72,12 +73,14 @@ export function fromFetch(input: string | Request, init?: RequestInit): Observab
           init.signal.addEventListener('abort', outerSignalHandler);
         }
       }
-      init = { ...init, signal };
+      // init cannot be mutated or reassigned as it's closed over by the
+      // subscriber callback and is shared between subscribers.
+      perSubscriberInit = { ...init, signal };
     } else {
-      init = { signal };
+      perSubscriberInit = { signal };
     }
 
-    fetch(input, init).then(response => {
+    fetch(input, perSubscriberInit).then(response => {
       abortable = false;
       subscriber.next(response);
       subscriber.complete();