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

, 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.

Step 2: One-Time Events

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()

.

Step 3: Repeated Method Calls

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.

Step 4: Model-Tied DisposableStores

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

.

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 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 Verification After fixing leaks, verify by: Checking listener counts before/after repeated operations Running ensureNoDisposablesAreLeakedInTestSuite() in tests Confirming object counts stabilize (don't grow linearly with usage)