Audit code for memory leaks and disposable issues. Use when reviewing event listeners, DOM handlers, lifecycle callbacks, or fixing leak reports. Covers addDisposableListener, Event.once, MutableDisposable, DisposableStore, and onWillDispose patterns.
The #1 bug category in VS Code. This skill encodes the patterns that prevent and fix leaks.
Work through each check in order. A single missed pattern can cause thousands of leaked objects.
Rule: Never use raw .onload, .onclick, or addEventListener() directly. Always use addDisposableListener().
// BAD — leaks a listener every call
this.iconElement.onload = () => { ... };
// GOOD — tracked and disposable
this._register(addDisposableListener(this.iconElement, 'load', () => { ... }));
Validated by: PR #280566 — Extension icon widget leaked 185 listeners after 37 toggles.
Rule: Use Event.once() for events that should only fire once (lifecycle events, close events, first-change events).
// BAD — listener stays registered forever after first fire
model.onDidDispose(() => store.dispose());
// GOOD — auto-removes after first invocation
Event.once(model.onDidDispose)(() => store.dispose());
Validated by: PRs #285657, #285661 — Terminal lifecycle hacks replaced with Event.once().
Rule: Objects created in methods called multiple times must NOT be registered to the class this._register(). Use MutableDisposable or return IDisposable to the caller.
// BAD — every call adds another listener to the class store
startSearch() {
this._register(this.model.onResults(() => { ... }));
}
// GOOD — MutableDisposable ensures max 1 listener
private readonly _searchListener = this._register(new MutableDisposable());
startSearch() {
this._searchListener.value = this.model.onResults(() => { ... });
}
When the event should only fire once per method call, combine Event.once() with MutableDisposable — this auto-removes the listener after the first invocation while still guarding against repeated calls:
private readonly _searchListener = this._register(new MutableDisposable());
startSearch() {
this._searchListener.value = Event.once(this.model.onResults)(() => { ... });
}
Validated by: PR #283466 — Terminal find widget leaked 1 listener per search.
Rule: When creating a DisposableStore tied to a model's lifetime, register model.onWillDispose(() => store.dispose()) to the store itself.
const store = new DisposableStore();
store.add(model.onWillDispose(() => store.dispose()));
store.add(model.onDidChange(() => { ... }));
Validated by: Pattern used in chatEditingSession.ts, fileBasedRecommendations.ts, testingContentProvider.ts.
Rule: When using factory methods that create pooled objects (lists, trees), disposables must be registered to the individual item, not the pool class.
// BAD — registers to pool, never cleaned per item
createItem() {
const item = new Item();
this._register(item.onEvent(() => { ... }));
return item;
}
// GOOD — wrap with item-scoped disposal
createItem(): IDisposable & Item {
const store = new DisposableStore();
const item = new Item();
store.add(item.onEvent(() => { ... }));
return { ...item, dispose: () => store.dispose() };
}
Validated by: PR #290505 — Chat content parts CollapsibleListPool and TreePool leaked disposables.
Rule: Every test suite that creates disposable objects must call ensureNoDisposablesAreLeakedInTestSuite().
import { ensureNoDisposablesAreLeakedInTestSuite } from '../../../../base/test/common/utils.js';
suite('MyFeature', () => {
ensureNoDisposablesAreLeakedInTestSuite();
test('does something', () => {
// test disposables are tracked automatically
});
});
| Scenario | Pattern | Anti-Pattern |
|---|---|---|
| DOM events | addDisposableListener() | .onclick =, addEventListener() |
| One-time events | Event.once(event)(handler) | event(handler) for lifecycle |
| Repeated methods | MutableDisposable or return IDisposable | this._register() in non-constructor |
| Model lifecycle | store.add(model.onWillDispose(...)) | Forgetting cleanup |
| Pooled objects | Item-scoped DisposableStore | Pool-scoped this._register() |
| Tests | ensureNoDisposablesAreLeakedInTestSuite() | No leak checking |
After fixing leaks, verify by:
ensureNoDisposablesAreLeakedInTestSuite() in tests