# `Session.undo`

> Contract 2.0.0 · package sha256 `a857eca95b35921109faf997094bb4192f12527d9d0fcd4be7e97f06425bad6e` · generated from `configurator-runtime.d.ts` by `services/onsite-scripter/scripts/build-contract-docs.mjs`; edit the source, not this file.

Method of [`Session`](Session.md) · inherited by [`testing.FakeSession`](testing.FakeSession.md) · stable · since 2.0.0 · called as `session.undo`

```ts
undo(options?: WriteOptions): Promise<Report>;
```

Undoes the last committed step (recorded). In the write queue: it runs after the writes called
before it. An undo that needs no load commits before the next frame. It ends an open
[`history.group()`](History.group.md) at its call, so at its turn it undoes the group's whole step.
Outcome: 'committed', or 'refused' with reason 'empty'. It always moves the history pointer:
a step whose change is look-only commits look-only; a step whose result equals the current
state (its owner was disposed) resolves 'committed' with `changed: false`, lists
'owner-disposed' in [`problems`](ReportFields.problems.md) and fires only
'history:changed' (spec section 6.1, O9). A reload that fails is not equal: the undo commits
with the node back in the model, unloaded, and 'asset-failed' in [`problems`](ReportFields.problems.md) (spec section 6.4).
The one exception: when applying it fails inside the runtime, it resolves 'rolled-back' with
reason 'internal-error' and the pointer and the step stay where they were (spec section 6.4).
Errors: never rejects for an empty history (that is 'refused' / 'empty'); rejects AbortError
when `signal` aborts first.

## Options

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `actor` | `Actor` | the `expectGroup`'s actor when one is given, else 'app' | Who made the write. |
| `dryRun` | `boolean` | false | Runs and reports, then rolls back by design ('dry-run'). |
| `expectGroup` | `HistoryGroup` | undefined | A guard, checked at the call: this write belongs to the undo group you are running. |
| `expectRevision` | `number` | undefined (no check) | If-match on [`session.revision`](Session.revision.md). |
| `origin` | `string` | null | Your tag, echoed unchanged in [`Report.origin`](ReportFields.origin.md) and `cause.origin`. |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |
| `supersede` | `boolean` | false (the write is queued behind the transaction and applies after it closes) | While a transaction is open: roll it back (it resolves 'superseded'); the writes queued behind it apply in call order, then this one, which supersedes an… |
| `undo` | `'rebase' \| (string & {})` | a recorded-state change gets one step labelled 'Change'; a look-only write gets none; the session's first model (cause 'load') gets none, whatever the label: it is the floor of history | An undo label makes this write one undo step. |

## Abort (`signal`)

This call is a write (`SignalOption.signal`, [the kinds of call](../read-this-first.md#conventions)).

> A write: before the commit point it rolls back and rejects AbortError; after it, no effect, except that a running camera move stops where the last frame drew it (`CameraMoveOptions`).
>
> Through a `tx` the commit point is the transaction's: aborting a write still loading rejects AbortError and stages nothing (catch it: unobserved, it rolls the transaction back, spec section 6.1, O6); aborting one already staged unstages it, its paths keeping the latest other write to them as after a failed savepoint (O7), and its Promise stays resolved 'staged'.

## Example

Imports: values (`has`, `validate`, the `Holo*Error` classes, the pure helpers) from `@holostep-configurators/runtime-client`, types from `@holostep-configurators/runtime-client/contract`, `/math` and `/testing` names from those entries; `boot` from the runtime module the root's `loadRuntimeAtFloor` resolves to. A type your page defines is your app's: declare it there, never import it from the package.

Compiled under the contract's strict settings (`examples/declaration/05-Session.undo.ts`):

```ts
import type { Session } from '@holostep-configurators/runtime-client/contract';
declare const session: Session;
declare const signal: AbortSignal;
declare const undoButton: HTMLButtonElement;
undoButton.addEventListener('click', () => {
  // 'refused' / 'empty' when there is nothing to undo
  void session.undo({ actor: 'shopper', origin: 'undo-button' });
}, { signal });
session.on('history:changed', ({ canUndo, undoLabel }) => {
  undoButton.disabled = !canUndo;
  undoButton.title = undoLabel ? `Undo ${undoLabel}` : '';
}, { signal });
```

## Generic-API test (spec 3.1)

| Rule | Result |
| --- | --- |
| 1. Neutral name | pass: `undo` names no product |
| 2. A precedent | pass: [ProseMirror history (addToHistory)](https://prosemirror.net/docs/ref/#history) (row `K62`); [Unity Undo](https://docs.unity3d.com/ScriptReference/Undo.html) (row `K62`) |
| 3. Two apps, or labs | pass: 20 apps (ledger: app-04c2774145, app-4762caacdc, app-f9a32142f1; fleet cross-check: app-04c2774145, app-37356bf09f, app-4762caacdc, app-4fd23bc869, app-822bb9d6aa, app-8675b3225c, app-b362a81c85, app-bcb7ef21da and 12 more) |
| 4. The cheaper fix first | decided per need in Appendix B: rows `K62` at tier stable (a member only where no rule, data, asset or app code serves the need) |