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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"events: addAbortListener ignores disposal and passes undefined event when signal is already aborted","articleBody":"### Version\n\nTested on v24.7.0, reproduces on `main` (commit a48e33fb6f).\n\n### Platform\n\n```text\nAll platforms (pure JS logic in `lib/internal/events/abort_listener.js`).\n```\n\n### Subsystem\n\nevents\n\n### What steps will reproduce the bug?\n\n\n```javascript\n'use strict';\nconst { addAbortListener } = require('node:events');\n\n// --- Bug 1: Disposing the listener does not cancel it if the signal was already aborted ---\nconst ac = new AbortController();\nac.abort();\n\nlet listenerCalledAfterDispose = false;\n{\n  using _ = addAbortListener(ac.signal, (event) => {\n    listenerCalledAfterDispose = true;\n    console.log('Event argument received:', event); // Bug 2: logs 'undefined' instead of an Event object\n  });\n  // Scope exits here -> disposable[Symbol.dispose]() is invoked synchronously.\n}\n\nqueueMicrotask(() => {\n  console.log('Listener called after disposal:', listenerCalledAfterDispose);\n  // Prints: true (Expected: false)\n});\n```\n\n\n### How often does it reproduce? Is there a required condition?\n\n100% reproducible whenever `addAbortListener()` is called on an `AbortSignal` where `signal.aborted === true`.\n\n\n### What is the expected behavior? Why is that the expected behavior?\n\n1. Calling `disposable[Symbol.dispose]()` (or exiting a `using` scope) before the microtask executes should disarm/cancel the scheduled listener. The main motivation for `addAbortListener()` is TC39 explicit resource management, so disposal should always prevent the callback from firing.\n2. The listener callback should receive an `Event` object (`type === 'abort'`), consistent with when the listener is triggered by an active signal.\n\n\n### What do you see instead?\n\n1. The listener runs unconditionally in the microtask, ignoring `Symbol.dispose()`.\n2. The listener receives `undefined` instead of an `Event` instance. Code following the documented `(e) => { ... }` signature that accesses properties like `e.type` crashes with a `TypeError` inside the microtask.\n\n\n### Additional information\n\nIn `lib/internal/events/abort_listener.js`:\n\n```javascript\n  let removeEventListener;\n  if (signal.aborted) {\n    queueMicrotask ??= require('internal/process/task_queues').queueMicrotask;\n    queueMicrotask(() => listener()); // \u003c-- 1. Listener called with no argument\n  } else {\n    kResistStopPropagation ??= require('internal/event_target').kResistStopPropagation;\n    abortListenerOptions ??= ObjectFreeze({ __proto__: null, once: true, [kResistStopPropagation]: true });\n    signal.addEventListener('abort', listener, abortListenerOptions);\n    removeEventListener = () => {\n      signal.removeEventListener('abort', listener);\n    };\n  }\n  return {\n    __proto__: null,\n    [SymbolDispose]() {\n      removeEventListener?.(); // \u003c-- 2. removeEventListener is undefined on the aborted branch\n    },\n  };\n```\n\nWhen `signal.aborted` is true:\n- `removeEventListener` is left `undefined`, making `SymbolDispose` a no-op.\n- `queueMicrotask(() => listener())` invokes `listener` without passing an `Event` object.\n\nA possible fix is tracking a `disposed` flag for the microtask and creating an `Event('abort')` instance:\n\n```javascript\n  let removeEventListener;\n  if (signal.aborted) {\n    queueMicrotask ??= require('internal/process/task_queues').queueMicrotask;\n    let disposed = false;\n    queueMicrotask(() => {\n      if (!disposed) {\n        const { Event } = require('internal/event_target');\n        listener(new Event('abort', { cancelable: false, bubbles: false }));\n      }\n    });\n    removeEventListener = () => {\n      disposed = true;\n    };\n  } else {\n    // ...\n```\n","author":{"url":"https://github.com/Abhirup0","@type":"Person","name":"Abhirup0"},"datePublished":"2026-08-29T07:23:50Z","url":"https://github.com/nodejs/node/issues/65637"}