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`createRoot`'s dispose function keeps `fn` alive, so `<For>`/`<Index>` rows retain removed list items

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@boombang is already working on this.

Since Oct 15, 2026.

  • #4096 by @boombang — open

Assessment

Difficulty
4/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, typescript
Domain
frontend, performance

Research direction

Start with createRoot and the updateFn line that builds the dispose closure, then trace how mapArray and indexArray store each row's disposer in disposers. Run the repro script with node --expose-gc --conditions=browser repro.mjs. Done means both lines print false, and the existing tests for createRoot and the array helpers still pass.

Written by the indexing model from the issue text.

Description

1.x
Describe the bug

createRoot creates the dispose function it passes to fn inside its own scope:

updateFn = unowned ? ... : () => fn(() => untrack(() => cleanNode(root)));

That scope also holds fn, because updateFn closes over it. So anything that keeps the dispose function also keeps fn, and everything fn closes over.

mapArray and indexArray keep every row's dispose function in disposers. Every row's fn is mapper, and mapper closes over newItems, the list that was current when the row was created. So a mounted row keeps that whole list reachable, including items that were removed from the list later.

We found this in production. A long-lived <For> row kept a list it was created from, and with it a replaced document of a few hundred MB.

Your Example Website or App

Standalone script below (Node 22, solid-js 1.9.17, client build).

Steps to Reproduce the Bug or Issue
// npm i [email protected]
// node --expose-gc --conditions=browser repro.mjs
import { createRoot, createSignal, createMemo, mapArray, indexArray } from "solid-js";

const collect = async () => {
  // WeakRef targets stay alive until the current job ends.
  await new Promise(r => setTimeout(r, 0));
  globalThis.gc();
};

// A row created from [kept, removed] stays mounted after the list becomes [kept].
async function check(name, map) {
  const kept = { name: "kept" };
  let removed, setList;
  const dispose = createRoot(dispose => {
    const [list, set] = createSignal(
      (() => {
        const item = { name: "removed" };
        removed = new WeakRef(item);
        return [kept, item];
      })()
    );
    setList = set;
    createMemo(map(list))();
    return dispose;
  });
  setList([kept]);
  await collect();
  console.log(`${name}: removed item still reachable:`, removed.deref() !== undefined);
  dispose();
}

await check("mapArray (<For>)", list => mapArray(list, item => item.name));
await check("indexArray (<Index>)", list => indexArray(list, item => item().name));

Output:

mapArray (<For>): removed item still reachable: true
indexArray (<Index>): removed item still reachable: true

The dev build (dist/dev.js) gives the same output, and so does Bun (JavaScriptCore) with Bun.gc(true).

Expected behavior

Once the row for removed is disposed, nothing should keep removed reachable, so both lines should print false. They do once the dispose function is created outside createRoot's scope, in both V8 and JavaScriptCore.

Platform
  • solid-js: 1.9.17 (main)
  • Node 22.23 (V8), Bun 1.4.2 (JavaScriptCore)
Additional context

I haven't checked next (2.0) yet. The fix for 1.x is in the PR linked below.

Dominant language
TypeScript
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Merged PRs (30d)
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