fix(javascript): emit no event operation for a missing listener argument

src/domain/javascript/javascriptSemanticAnalysis.rejected_event_listener.test.ts

@@ -27,6 +27,11 @@ const eventsFor = (call: string) =>
     eventName,
   }));
 
+const eventsForSource = (source: string) =>
+  analyzeJavaScriptSemantics(source).eventOperations.map(
+    ({ kind, method, eventName }) => ({ kind, method, eventName }),
+  );
+
 it.each([
   ["a numeric listener", `e.on("x", 1)`],
   ["a string listener", `e.on("x", "fn")`],
@@ -67,6 +72,118 @@ it.each([
   ]);
 });
 
+/**
+ * A listener method called without a listener argument passes `undefined` to
+ * Node's `checkListener`, which throws before anything is registered or
+ * removed. The argument position is empty, so no function can arrive there.
+ *
+ * Measured on Node 22.23.3:
+ *
+ *   emitter.on()                     -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.on("x")                  -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.once("x")                -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.addListener("x")         -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.prependListener("x")     -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.prependOnceListener("x") -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.removeListener("x")      -> THREW ERR_INVALID_ARG_TYPE
+ *   emitter.off("x") / emitter.off() -> THREW ERR_INVALID_ARG_TYPE
+ *
+ * A spread still reaches the position through the call, so it is untouched.
+ */
+it.each([
+  ["a missing listener", `e.on("x")`],
+  ["a missing once listener", `e.once("x")`],
+  ["a missing addListener listener", `e.addListener("x")`],
+  ["a missing prependListener listener", `e.prependListener("x")`],
+  ["a missing prependOnceListener listener", `e.prependOnceListener("x")`],
+  ["a missing removeListener listener", `e.removeListener("x")`],
+  ["a missing off listener", `e.off("x")`],
+  ["no arguments at all", `e.on()`],
+  ["no off arguments at all", `e.off()`],
+] as const)("emits no event operation for %s", (_name, call) => {
+  expect(eventsFor(call)).toEqual([]);
+});
+
+it("keeps the event operation for a spread call with no fixed listener", () => {
+  expect(eventsFor(`e.on(...pairs)`)).toEqual([
+    {
+      kind: expect.any(String) as unknown as string,
+      method: "on",
+      eventName: null,
+    },
+  ]);
+});
+
+it.each([
+  [
+    "a named import",
+    `import { EventEmitter } from "node:events";`,
+    "EventEmitter",
+  ],
+  ["a default import", `import EE from "node:events";`, "EE"],
+  [
+    "a destructured require",
+    `const { EventEmitter } = require("events");`,
+    "EventEmitter",
+  ],
+  [
+    "a member require",
+    `const events = require("node:events");`,
+    "events.EventEmitter",
+  ],
+] as const)(
+  "drops a missing listener on a receiver built from %s",
+  (_name, header, constructor) => {
+    expect(
+      eventsForSource(`${header}
+const e = new ${constructor}();
+e.on("x");`),
+    ).toEqual([]);
+  },
+);
+
+it("keeps a missing listener when the receiver was reassigned", () => {
+  expect(
+    eventsForSource(`import { EventEmitter } from "node:events";
+const e = new EventEmitter();
+e = other;
+e.on("x");`),
+  ).toContainEqual({ kind: "register", method: "on", eventName: "x" });
+});
+
+/**
+ * `checkListener` is Node's own contract. An EventEmitter-style receiver that
+ * this module does not construct from `events.EventEmitter` may define a
+ * one-argument form: Socket.IO's `socket.off("x")` removes every listener for
+ * the event, so the operation must survive as an unresolved candidate.
+ */
+it("keeps a one-argument removal on a receiver with no proven Node emitter", () => {
+  expect(
+    eventsForSource(`function g(socket) { socket.off("x"); }`),
+  ).toContainEqual({ kind: "remove", method: "off", eventName: "x" });
+});
+
+/**
+ * A subclass may default the listener position, which makes `bus.on("ready")`
+ * a real registration even though the module never proves Node's rejection.
+ * Measured on Node 22.23.3: this class stores `"ready"` and does not throw.
+ */
+it("keeps the registration of a subclass that defaults its listener", () => {
+  const events = eventsForSource(
+    `import { EventEmitter } from "node:events";
+class Bus extends EventEmitter {
+  on(name, fn = () => {}) { return super.on(name, fn); }
+}
+const bus = new Bus();
+bus.on("ready");`,
+  );
+  expect(events).toContainEqual({
+    kind: "register",
+    method: "on",
+    eventName: "ready",
+  });
+});
+
 /**
  * `emit` passes its remaining arguments to listeners that already exist, so it
  * does not validate them the way a registration does.

src/domain/javascript/javascriptSemanticAsyncEffects.ts

@@ -15,6 +15,7 @@ import {
 import {
   resolveSemanticBindingState,
   semanticResolutionBlocked,
+  semanticStableInitializer,
   type JavaScriptSemanticAnalysisState,
   type JavaScriptSemanticBindingState,
 } from "./javascriptSemanticState.js";
@@ -23,6 +24,7 @@ import { traverseJavaScriptValueAst } from "./javascriptSemanticTraversal.js";
 import { isUnmodifiedGlobalReference } from "./javascriptSemanticAmbientWrites.js";
 import { range } from "./javascriptStaticAnalysisHelpers.js";
 import {
+  builtinDataEffectCallablePath,
   builtinDataEffectMethod,
   dataEffectArgumentBindingId,
   dataEffectLiteralString,
@@ -161,6 +163,57 @@ const rejectsEventListener = (listener: t.Node): boolean => {
   );
 };
 
+/**
+ * Node's `events` module, whose `EventEmitter` validates every listener
+ * argument before it registers or removes anything.
+ */
+const NODE_EVENTS_SPECIFIERS = ["events", "node:events"] as const;
+
+/**
+ * Report whether a constructor reference resolves to Node's `EventEmitter`.
+ * Named and default imports, destructured requires, and member requires all
+ * denote that class.
+ */
+const isNodeEventEmitterConstructor = (
+  node: t.Node,
+  state: JavaScriptSemanticAnalysisState,
+): boolean => {
+  const value = unwrapJavaScriptExpression(node).node;
+  if (!t.isNewExpression(value)) return false;
+  const selected = builtinDataEffectCallablePath(
+    value.callee,
+    state,
+    NODE_EVENTS_SPECIFIERS,
+  );
+  // An empty selected path is the module's default export, which `events`
+  // defines as `EventEmitter`.
+  return (
+    selected !== null &&
+    (selected.length === 0 || selected.at(-1) === "EventEmitter")
+  );
+};
+
+/**
+ * Report whether the receiver is a value this module constructs from Node's
+ * `EventEmitter`. Only that class is bound by `checkListener`; other
+ * EventEmitter-style APIs define one-argument forms, such as Socket.IO's
+ * `socket.off("x")`, which removes every listener for the event. A receiver
+ * that is not proven this way keeps its operation as an unresolved candidate.
+ */
+const nodeEventEmitterReceiver = (
+  node: t.Expression | t.Super,
+  state: JavaScriptSemanticAnalysisState,
+): boolean => {
+  if (!t.isIdentifier(node)) return false;
+  const binding = resolveSemanticBindingState(state, node, node.name);
+  const initializer = semanticStableInitializer(binding, state);
+  return (
+    initializer !== undefined &&
+    initializer.projection.length === 0 &&
+    isNodeEventEmitterConstructor(initializer.node, state)
+  );
+};
+
 const collectEvent = (
   node: t.CallExpression | t.OptionalCallExpression,
   ownerCallableId: string | null,
@@ -177,9 +230,19 @@ const collectEvent = (
   if (method === undefined) return;
   const kind = eventKind(method);
   const eventArgument = node.arguments[0];
-  const listenerArgument = EVENT_LISTENER_METHODS.includes(method)
-    ? node.arguments[1]
-    : undefined;
+  const listenerMethod = EVENT_LISTENER_METHODS.includes(method);
+  const listenerArgument = listenerMethod ? node.arguments[1] : undefined;
+  // A listener method called without a listener argument passes `undefined` to
+  // Node's `checkListener`, which throws before it registers or removes
+  // anything. Only a spread argument can still reach that position, and only a
+  // receiver proven to be Node's `EventEmitter` is held to that contract.
+  if (
+    listenerMethod &&
+    listenerArgument === undefined &&
+    !node.arguments.some((argument) => t.isSpreadElement(argument)) &&
+    nodeEventEmitterReceiver(member.object, state)
+  )
+    return;
   // A leading spread shifts the runtime argument positions, so argument 1 is not
   // provably the listener: `e.on(...["x", fn], 1)` registers `fn` and ignores the
   // trailing `1`.