---
title: "Keep a backup when overwriting"
description: "Turn on preserve so the bytes you are about to overwrite are kept in a sibling file."
source: "https://jostraca.org/how-to/keep-a-backup-when-overwriting/"
---

# Keep a backup when overwriting

Turn on preserve so the bytes you are about to overwrite are kept in a sibling file.

Rendered from [`docs/how-to/keep-a-backup-when-overwriting.md`](https://github.com/jostraca/jostraca/blob/master/docs/how-to/keep-a-backup-when-overwriting.md) in the generator repository — where a correction belongs, and where the test suite runs every example on this page or states why it does not.

`preserve` copies the current file to a `.old` sibling before writing the new version. It is the cheapest safety net: no merge to reason about, and the user’s work is one `mv` away.

```js
import { appendFileSync } from 'node:fs'
import { Jostraca, Project, File, Content } from 'jostraca'

const jostraca = Jostraca({
  existing: { txt: { write: true, preserve: true } },
})

const run = (body) => jostraca.generate({ folder: './out' }, () => {
  Project({}, () => File({ name: 'config.sh' }, () => Content(body)))
})

await run('PORT=8080\n')
appendFileSync('./out/config.sh', 'DEBUG=1\n')
await run('PORT=9090\n')
```

```text
config.old.sh
config.sh
```

`config.old.sh` holds what was on disk, edit included:

```text
PORT=8080
DEBUG=1
```

The suffix goes before the extension, so `config.sh` becomes `config.old.sh` and `bundle.min.js` becomes `bundle.min.old.js`. A file with no extension appends instead: `Makefile` becomes `Makefile.old`, and a dotfile does the same, so `.env` becomes `.env.old`.

No backup is written when the content has not changed, so a run that generates the same bytes leaves no `.old` files behind.

Add `write: false` to get a snapshot rather than a backup: the copy is taken and the original is left alone.

Backups accumulate, and each run overwrites the previous one. Add `*.old.*` to the generated project’s `.gitignore` if the output is checked in.

## See also

-   [Merge your changes with the user’s](https://jostraca.org/how-to/merge-generator-and-user-edits) when a backup is not enough.
-   [Options reference](https://jostraca.org/docs/reference-options#existing) for every mode.