## Summary
- Change `js_repl` failed-cell persistence so later cells keep prior
bindings plus only the current-cell bindings whose initialization
definitely completed before the throw.
- Preserve initialized lexical bindings across failed cells via
module-namespace readability, including top-level destructuring that
partially succeeds before a later throw.
- Preserve hoisted `var` and `function` bindings only when execution
clearly reached their declaration site, and preserve direct top-level
pre-declaration `var` writes and updates through explicit write-site
markers.
- Preserve top-level `for...in` / `for...of` `var` bindings when the
loop body executes at least once, using a first-iteration guard to avoid
per-iteration bookkeeping overhead.
- Keep prior module state intact across link-time failures and
evaluation failures before the prelude runs, while still allowing failed
cells that already recreated prior bindings to persist updates to those
existing bindings.
- Hide internal commit hooks from user `js_repl` code after the prelude
aliases them, so snippets cannot spoof committed bindings by calling the
raw `import.meta` hooks directly.
- Add focused regression coverage for the supported failed-cell
behaviors and the intentionally unsupported boundaries.
- Update `js_repl` docs and generated instructions to describe the new,
narrower failed-cell persistence model.
## Motivation
We saw `js_repl` drop bindings that had already been initialized
successfully when a later statement in the same cell threw, for example:
const { context: liveContext, session } =
await initializeGoogleSheetsLiveForTab(tab);
// later statement throws
That was surprising in practice because successful earlier work
disappeared from the next cell.
This change makes failed-cell persistence more useful without trying to
model every possible partially executed JavaScript edge case. The
resulting behavior is narrower and easier to reason about:
- prior bindings are always preserved
- lexical bindings persist when their initialization completed before
the throw
- hoisted `var` / `function` bindings persist only when execution
clearly reached their declaration or a supported top-level `var` write
site
- failed cells that already recreated prior bindings can persist writes
to those existing bindings even if they introduce no new bindings
The detailed edge-case matrix stays in `docs/js_repl.md`. The
model-facing `project_doc` guidance is intentionally shorter and focused
on generation-relevant behavior.
## Supported Failed-Cell Behavior
- Prior bindings remain available after a failed cell.
- Initialized lexical bindings remain available after a failed cell.
- Top-level destructuring like `const { a, b } = ...` preserves names
whose initialization completed before a later throw.
- Hoisted `function` bindings persist when execution reached the
declaration statement before the throw.
- Direct top-level pre-declaration `var` writes and updates persist, for
example:
- `x = 1`
- `x += 1`
- `x++`
- short-circuiting logical assignments only persist when the write
branch actually runs
- Non-empty top-level `for...in` / `for...of` `var` loops persist their
loop bindings.
- Failed cells can persist updates to existing carried bindings after
the prelude has run, even when the cell commits no new bindings.
- Link failures and eval failures before the prelude do not poison
`@prev`.
## Intentionally Unsupported Failed-Cell Cases
- Hoisted function reads before the declaration, such as `foo(); ...;
function foo() {}`
- Aliasing or inference-based recovery from reads before declaration
- Nested writes inside already-instrumented assignment RHS expressions
- Destructuring-assignment recovery for hoisted `var`
- Partial `var` destructuring recovery
- Pre-declaration `undefined` reads for hoisted `var`
- Empty top-level `for...in` / `for...of` loop vars
- Nested or scope-sensitive pre-declaration `var` writes outside direct
top-level expression statements
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