fix(fromEvent): allow fromEvent to handle symbols as event names

Allow passing a value of type `symbol` to `eventName` if
a `NodeStyleEventEmitter` is passed to `source`, according to
[the documentation](https://nodejs.org/api/events.html#emitteraddlistenereventname-listener).

Closes #7338

spec-dtslint/observables/fromEvent-spec.ts

@@ -56,6 +56,13 @@ it('should support a node-style source', () => {
   const b = fromEvent<B>(nodeStyleSource, "exit"); // $ExpectType Observable<B>
 });
 
+it('should support a node-style source and symbol eventName', () => {
+  const SYMBOL_EVENT = Symbol();
+  const source: NodeStyleEventEmitter = nodeStyleSource;
+  const a = fromEvent(nodeStyleSource, SYMBOL_EVENT); // $ExpectType Observable<unknown>
+  const b = fromEvent<B>(nodeStyleSource, SYMBOL_EVENT); // $ExpectType Observable<B>
+});
+
 it('should deprecate explicit type parameters for a node-style source', () => {
   const source: NodeStyleEventEmitter = nodeStyleSource;
   const a = fromEvent(nodeStyleSource, "exit"); // $ExpectNoDeprecation

spec/observables/fromEvent-spec.ts

@@ -121,6 +121,39 @@ describe('fromEvent', () => {
     expect(offHandler).to.equal(onHandler);
   });
 
+  it('should pass symbol to "addListener" and "removeListener"', () => {
+    let onEventName;
+    let onHandler;
+    let offEventName;
+    let offHandler;
+
+    const SYMBOL_EVENT = Symbol();
+
+    const obj = {
+      addListener(a: string | symbol, b: (...args: any[]) => void) {
+        onEventName = a;
+        onHandler = b;
+        return this;
+      },
+      removeListener(a: string | symbol, b: (...args: any[]) => void) {
+        offEventName = a;
+        offHandler = b;
+        return this;
+      },
+    };
+
+    const subscription = fromEvent(obj, SYMBOL_EVENT).subscribe(() => {
+      //noop
+    });
+
+    subscription.unsubscribe();
+
+    expect(onEventName).to.equal(SYMBOL_EVENT);
+    expect(typeof onHandler).to.equal('function');
+    expect(offEventName).to.equal(onEventName);
+    expect(offHandler).to.equal(onHandler);
+  });
+
   it('should setup an event observable on objects with "addListener" and "removeListener" returning nothing', () => {
     let onEventName;
     let onHandler;

src/internal/observable/fromEvent.ts

@@ -77,12 +77,15 @@ export function fromEvent<T, R>(
   resultSelector: (event: T) => R
 ): Observable<R>;
 
-export function fromEvent(target: NodeStyleEventEmitter | ArrayLike<NodeStyleEventEmitter>, eventName: string): Observable<unknown>;
+export function fromEvent(
+  target: NodeStyleEventEmitter | ArrayLike<NodeStyleEventEmitter>,
+  eventName: string | symbol
+): Observable<unknown>;
 /** @deprecated Do not specify explicit type parameters. Signatures with type parameters that cannot be inferred will be removed in v8. */
-export function fromEvent<T>(target: NodeStyleEventEmitter | ArrayLike<NodeStyleEventEmitter>, eventName: string): Observable<T>;
+export function fromEvent<T>(target: NodeStyleEventEmitter | ArrayLike<NodeStyleEventEmitter>, eventName: string | symbol): Observable<T>;
 export function fromEvent<R>(
   target: NodeStyleEventEmitter | ArrayLike<NodeStyleEventEmitter>,
-  eventName: string,
+  eventName: string | symbol,
   resultSelector: (...args: any[]) => R
 ): Observable<R>;
 
@@ -238,7 +241,7 @@ export function fromEvent<T, R>(
  */
 export function fromEvent<T>(
   target: any,
-  eventName: string,
+  eventName: string | symbol,
   options?: EventListenerOptions | ((...args: any[]) => T),
   resultSelector?: (...args: any[]) => T
 ): Observable<T> {
@@ -248,7 +251,7 @@ export function fromEvent<T>(
   }
 
   if (resultSelector) {
-    return fromEvent<T>(target, eventName, options as EventListenerOptions).pipe(mapOneOrManyArgs(resultSelector));
+    return fromEvent<T>(target, eventName as string, options as EventListenerOptions).pipe(mapOneOrManyArgs(resultSelector));
   }
 
   const isValidTarget = isNodeStyleEventEmitter(target) || isJQueryStyleEventEmitter(target) || isEventTarget(target);
@@ -274,7 +277,13 @@ export function fromEvent<T>(
   });
 }
 
-function doSubscribe(handler: (...args: any[]) => void, subscriber: Subscriber<any>, subTarget: any, eventName: string, options: any) {
+function doSubscribe(
+  handler: (...args: any[]) => void,
+  subscriber: Subscriber<any>,
+  subTarget: any,
+  eventName: string | symbol,
+  options: any
+) {
   const [addMethod, removeMethod] = getRegistryMethodNames(subTarget);
   if (!addMethod || !removeMethod) {
     throw new TypeError('Invalid event target');