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You can package an app’s interface in one HTML file and use IndexedDB for structured data, but “offline” does not mean the data is backed up or guaranteed to survive. A localStorage fallback is limited to strings and is not dependable when the file is opened directly with a file: URL. A hand-written ZIP writer is a separate format-implementation task that needs independent validation.
Choose the launch model before choosing storage
A single HTML file can contain the interface code, but where and how it is opened affects browser storage. A file opened directly has a file: URL; a page served from a website has an origin defined by its scheme, host and port. Browser storage is scoped to an origin, so test the actual browsers and launch path your app is intended to support. In particular, MDN says localStorage behavior for file: URLs is undefined and can differ between browsers: MDN: localStorage.
This distinction matters even if the app never contacts a server. Offline capability describes what the app can do without a network connection; it does not guarantee a uniform storage environment or permanent data retention.
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| Property | IndexedDB | localStorage |
|---|---|---|
| Data shape | Structured records, including substantial client-side data and binary values. MDN: IndexedDB API | String key-value pairs. Objects must be serialized, typically to JSON, and converted back when read. MDN: localStorage |
| Capacity | Quota is browser-specific; there is no universal safe quota figure. Data stored with best-effort persistence may be evicted. | MDN’s Web Storage guidance describes a combined maximum of 10 MiB per origin: up to 5 MiB for localStorage and 5 MiB for sessionStorage. MDN: storage quotas and eviction |
| Operation style | Asynchronous database operations and transactions. MDN: IndexedDB API | Synchronous access to string values through the Web Storage API. |
| Directly opened file | Use the actual target browsers and launch path to establish whether your IndexedDB workflow works as intended. | Behavior for file: URLs is undefined and may vary by browser. Do not rely on it as a consistent fallback. |
For an app with records, settings, or binary data, IndexedDB is the more appropriate primary store. It is asynchronous and designed for structured client-side data, including applications that need to work without network connectivity. It is tied to the browser’s origin, however, and it does not synchronize data across devices or act as a backup.
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Use localStorage only for small, string-valued items where its limits and launch behavior are acceptable. Treat it as a deliberately chosen fallback, not a transparent replacement for IndexedDB: it cannot preserve the same data model without serialization, and its file:-URL behavior may not meet the app’s needs.
Make storage failures visible and recoverable
Browser-managed storage is not guaranteed durable storage. A user can clear it, a browser can deny access, and an IndexedDB operation can fail because of quota limits, database errors, or an aborted transaction. Handle these outcomes as normal states in the app rather than assuming every read or write will succeed. MDN explains that quotas and eviction policies vary by browser: MDN: storage quotas and eviction.
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- Report when saving fails and avoid telling the user a change was saved unless the transaction completes successfully.
- Provide a clear recovery route for errors, such as retrying, freeing space, or exporting data.
- Offer user-controlled export for information that matters. Offline persistence is not a backup.
- Test denied storage, quota failures, database errors, aborted transactions, and data removed through browser settings.
Do not promise a specific IndexedDB capacity: the available quota depends on browser policy and circumstances. The documented 10 MiB Web Storage figure applies to localStorage plus sessionStorage guidance, not to IndexedDB.
Offline data does not automatically mean offline startup
If the app is a single file opened locally, its interface is packaged in that file. If instead you host a web app and want it to start without a network connection, storing its data in IndexedDB is not enough: the HTML, scripts, styles, and other required assets must also be available offline. A service worker and the Cache API can cache app assets, while IndexedDB stores application data. Those are separate layers, as illustrated in MDN’s offline application example: MDN: Offline and background operation.
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Validate a hand-written ZIP writer independently
Creating a ZIP archive is not the same as compressing a stream. The browser’s Compression Streams API documents gzip and deflate formats; that support does not create ZIP archives or establish that a custom ZIP writer is valid. MDN: Compression Streams API.
A ZIP implementation must produce the structures and metadata expected by ZIP readers, not merely compressed bytes. Check the ZIP format requirements that apply to your features—including headers, the central directory, CRC values, filename encoding, compression flags, and any need for ZIP64—against the format specification. Then test generated files in the actual readers and browsers your users rely on. Do not claim standards conformance or interoperability until those checks pass.
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Keep ZIP handling separate from persistence: IndexedDB stores the app’s working data, while export code builds a portable archive. Test exports with empty data, non-ASCII filenames, large files if supported, and error cases; verify both that the archive opens and that extracted contents match the source data.
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