Memory Leak Audit

The #1 bug category in VS Code. This skill encodes the patterns that prevent and fix leaks.

When to Use

• Reviewing code that registers event listeners or DOM handlers
• Fixing reported memory leaks (listener counts growing over time)
• Creating objects in methods that are called repeatedly
• Working with model lifecycle events (onWillDispose, onDidClose)
• Adding event subscriptions in constructors or setup methods

Audit Checklist

Work through each check in order. A single missed pattern can cause thousands of leaked objects.

Step 1: DOM Event Listeners

Rule: Never use raw

.onload

.onclick

addEventListener()

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.

Step 2: One-Time Events

Rule: Use

Event.once()

// 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()

Step 3: Repeated Method Calls

Rule: Objects created in methods called multiple times must NOT be registered to the class

this._register()

MutableDisposable

IDisposable

// 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()

MutableDisposable

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.

Step 4: Model-Tied DisposableStores

Rule: When creating a

DisposableStore

model.onWillDispose(() => store.dispose())

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

Step 5: Resource Pool Patterns

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.

Step 6: Test Validation

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
    });
});

Quick Reference

addDisposableListener()

.onclick =

addEventListener()

Event.once(event)(handler)

event(handler)

MutableDisposable

IDisposable

this._register()

store.add(model.onWillDispose(...))

DisposableStore

this._register()

ensureNoDisposablesAreLeakedInTestSuite()

Verification

After fixing leaks, verify by:

1. Checking listener counts before/after repeated operations
2. Running

   ensureNoDisposablesAreLeakedInTestSuite()

   in tests
3. Confirming object counts stabilize (don't grow linearly with usage)