About File Viewer

fileviewer.dev is a file viewer for JSON, YAML, XML, CSV and Markdown that runs entirely in your browser, built because the common alternative — pasting a config file into a website that uploads it — is a habit worth replacing.

The problem

Everyone has pasted something into an online JSON formatter. Usually it is harmless. Occasionally it is a config file with a live credential in it, and it has just been posted to a server belonging to someone you have never heard of, whose retention policy you did not read. The tools are useful enough that people keep doing it.

Browsers have been able to read and parse files locally for over a decade. There is no technical reason a viewer needs a server. This one does not have one.

How it is built

• React 19 and TypeScript in strict mode, bundled with Vite
• Monaco — the editor from VS Code — bundled rather than loaded from a CDN, so the app works offline and no third-party script origin is needed
• Web Workers for parsing, conversion, diffing and graph layout, so a large file does not freeze the interface
• Zustand for state, with a rule that stores never import each other; cross-store behaviour lives in one coordination file
• Tailwind CSS v4, configured in CSS, with design tokens as custom properties so themes reach every surface
• A hand-written service worker generated at build time with the real asset manifest

Decisions worth explaining

No server, deliberately

It removes a class of privacy question entirely. It also removes features — there is no sharing, no sync, no history across devices — and that is the trade being made, not an oversight.

Diagnostics are editor markers

A marker gives the squiggle, the hover, the ruler tick and F8 navigation for free, and screen readers announce it. Reimplementing that with decorations would be more code and a worse result.

One analysis pipeline

The status bar, the problems panel and the editor squiggles all read the same parse result, so they cannot disagree about how many problems a file has.

Loss reporting on conversions

Every cross-format conversion loses something. Saying what, per file, before you rely on the output, is more useful than a success message.

Honest limits

The file-size ceiling is stated rather than advertised away. A tool that promises no limit and then refuses your file has spent your trust to win a comparison table.

Accessibility

Targeting WCAG 2.2 AA. Everything is reachable by keyboard, the split divider implements the ARIA separator pattern with arrow-key resize, validation status and progress are live regions, prefers-reduced-motion is honoured with an in-app override, and format is conveyed by icon shape as well as by colour. Every built-in theme is tested against its own contrast requirements — two shipped palettes were corrected when that test caught them failing.

Open the app