# A browser save file that never leaves you with nothing

> Lantern Valley's save uses a verified temp write, a backup of the previous good save, pure migrations and export/import, so a crash cannot erase progress.

Canonical page: https://andersonmancini.dev/blog/lantern-valley-save-that-never-loses-progress

Published: 2026-09-29 · 1 min read · Tags: game-dev, javascript, localstorage, lantern-valley, reliability

Losing someone's progress in silence is the one thing the save file of [Lantern Valley](https://andersonmancini.dev/blog/lantern-valley.md) exists to prevent. It is a small module with a few firm rules.

## An envelope, not a bag

The v2 save is `{ v, app, savedAt, payload }`, and the payload is split in one slice per system: clock, player, flags, quests, inventory, entities, scheduler, the journal collections, and one slice for each system such as the farm, the animals, the boat or the herd. Each system owns its slice.

## Writes that cannot half-succeed

A save does not overwrite the real key straight away:

1. serialize and re-parse the JSON, to catch anything that does not round-trip,
2. write to a temporary key and **read it back**, comparing it to what was written,
3. copy the previous good save to a `.bak` key,
4. only then write the real key, and remove the temporary one.

A full disk or a crash in the middle never leaves nothing.

## Loading goes down a ladder

The current save first. If it is unreadable, the previous good save. If that is missing, the old v1 key, which is only ever *read*. A save from a newer version of the game is not touched: the game says so, instead of "migrating" it backwards.

## Migrations are pure functions

Every migration takes one payload to the next, and the tests keep real saves of every version as fixtures. One detail I like: crates used to be numbered in creation order, so inserting one would have shuffled everyone's saves. They have stable ids now.

## Storage can be refused

Private windows and blocked site data make `localStorage` throw. A stand-in in memory keeps the game running, only without persistence.

## "New game" is not delete

Clearing keeps a copy as a backup, and there is an export and import of the file as text, so a player can move computers.

The project is on its [project page](https://andersonmancini.dev/projects/lantern-valley.md).

## Related projects

- [Lantern Valley](https://andersonmancini.dev/projects/lantern-valley.md) — Cozy adventure game for the browser