memory-leak-audit

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(() => { ... });
}