Is JSON OS free to use?

Yes. JSON OS is free, requires no account, and works entirely in your browser.

Is my JSON data uploaded to a server?

No. All parsing, validation, formatting, and storage happen locally in your browser. Documents persist in your browser's IndexedDB and never leave your device unless you explicitly export them.

What can the Repair feature fix?

Repair handles comments (// /* */ #), smart quotes, single-quoted strings, unquoted object keys, trailing commas, missing commas, hex/octal/binary numbers, Python literals (True, False, None), undefined and NaN, unclosed brackets, and stray top-level values. After running, a banner lists each fix that was applied.

Can I compare two JSON documents side by side?

Yes. Click the ⇄ Compare button or press Cmd+Shift+C. Pick a peer document, choose tree or text view, and the workspace switches to two linked columns with synced scroll and color-coded differences (added, removed, changed, moved). Ignore rules let you skip specific paths, treat null as missing, ignore string case, or trim whitespace.

Can I validate JSON against a schema?

Yes. Open the side panel, pick the Schema tab, and paste a JSON Schema. The active document is continuously validated using Ajv with formats. Errors appear inline with their instance paths.

Does it work offline?

After the first load, the app and your saved documents work without a network. JSON parsing runs in a Web Worker; everything is stored in IndexedDB on your device.

Can I run MongoDB-style queries on a JSON document?

Yes. Press Cmd+Shift+K or click the 🔎 Query button to open the Query panel. Set a target path (e.g. $ or $.users) and write a MongoDB-style filter object. Supported operators include $eq, $ne, $gt, $gte, $lt, $lte, $in, $nin, $exists, $type, $regex, $mod, $size, $all, $elemMatch, $and, $or, $nor, and $not. Field paths use dot notation and auto-branch over arrays. Results are listed with their JSON paths and clicking one highlights it in the tree.

Can I edit JSON in a tree view?

Yes. The tree view supports inline key rename, value editing, duplicate, cut/copy/paste, insert before/after, sort keys, convert object⇄array⇄value, and remove. Right-click any row for the full menu. Cmd+A selects all, Cmd+V pastes JSON from the clipboard at the focused row, and Cmd+C copies.

Can I export JSON arrays as CSV?

Yes. Switch to the table view on any column, sort and filter the columns, then export the visible data as CSV with the Export button. The CSV includes a UTF-8 BOM so it opens cleanly in Excel.

Is there a keyboard shortcut for everything?

Yes. Press ? or open the Help dialog for the full list. Highlights: Cmd+/ format, Cmd+Z undo, Cmd+S download, Cmd+T new tab, Cmd+\ toggle side panel, Cmd+Shift+K query, Cmd+Shift+C compare, Cmd+Shift+W toggle wrap, Cmd+1/2/3 focus column.

Does JSON OS include a Markdown editor?

Yes. JSON OS ships with Markdown Studio, a separate workbench for writing and previewing Markdown. Click the Tools switcher in the top bar and pick Markdown Studio, or open jsonos.online/?tool=md directly. It supports GitHub-Flavored Markdown, Mermaid diagrams, KaTeX math, syntax-highlighted code blocks and embeds for YouTube, Vimeo, CodePen, CodeSandbox, JSFiddle, GitHub Gist and Twitter/X.

How do I render a Mermaid diagram in Markdown?

Open a fenced code block with the language identifier ```mermaid and write Mermaid syntax inside. The Markdown Studio replaces the block with an inline SVG that follows your active light or dark theme. Supported diagram types include flowchart, sequence, gantt, class, ER, state, mindmap, journey and more. Mermaid is lazy-loaded on first use, so it doesn't bloat the JSON workbench.

Does the Markdown previewer support math?

Yes, via KaTeX. Wrap inline expressions in $…$ and block expressions in $$…$$. The renderer extracts math before the Markdown parser runs, so symbols like underscores and braces inside equations are never mangled by Markdown syntax.

Which embed providers does the Markdown previewer support?

Paste a URL on its own line and these providers turn into a player or card: YouTube, Vimeo, CodePen, CodeSandbox, JSFiddle, GitHub Gist (rendered via the official gist.js script) and Twitter/X (styled card link). Iframe sources are restricted to the providers above — DOMPurify strips iframes that point anywhere else.

Can I export Markdown as HTML?

Yes. The Markdown Studio toolbar offers three export paths: Save downloads the source as .md, ⎘ HTML copies the rendered HTML to your clipboard, and ↧ HTML downloads a standalone .html file with light-theme styles inlined so it opens cleanly in any browser.

Is the Markdown editor safe? Does it sanitise HTML?

Yes. The output of the Markdown parser is run through DOMPurify, which strips on-handlers, javascript: URLs and any unknown tags. The only exception is the GitHub Gist embed script, which is allowed only for src URLs matching gist.github.com. Iframes are restricted to a known allowlist of embed providers.

JSON OS — Free JSON Editor & Markdown Previewer Online

JSON OS is a free, browser-based workbench with two tools: a JSON editor (format, validate, repair, compare, query) and a Markdown previewer (live preview with Mermaid diagrams, KaTeX math, code highlighting and embeds). Everything runs locally — no signup, no uploads, no server processing.

JSON Workbench

View & edit JSON in tree, text, or table — switch any column independently.
Format / beautify / minify / sort keys with one click.
Validate JSON against a JSON Schema (Ajv with format checks).
Repair broken JSON: smart quotes, trailing commas, unquoted keys, comments, hex/octal/binary numbers, Python literals, missing commas, unclosed brackets.
Compare two documents side by side with sync-scroll, move detection, and ignore rules.
Query JSON with MongoDB-style filters: $eq $gt $in $regex $exists $elemMatch $and $or and more — perfect for filtering arrays or validating a single document.
Find & replace with regex, case, and whole-word toggles.
Export JSON arrays to CSV (UTF-8 BOM, Excel-friendly).
Multi-column workspace (up to 3 panes), drag-resize, dark / light / system themes.
Local-first: parsing in a Web Worker, persistence in IndexedDB; nothing leaves your device.

Markdown Studio — open at jsonos.online/?tool=md

A second workbench inside JSON OS for writing and previewing Markdown. GitHub-Flavored Markdown, live preview, Mermaid diagrams, KaTeX math, syntax-highlighted code, and provider embeds — all rendered locally in your browser.

Live Markdown preview: GitHub-Flavored Markdown with tables, task lists, footnotes, autolinks and strikethrough, re-rendered as you type.
Mermaid diagrams: ```mermaid code blocks render inline as SVG. Supports flowchart, sequence, gantt, class, ER, state and mindmap diagrams.
KaTeX math: inline $E = mc^2$ and block $$…$$. Math is extracted before parsing so braces and underscores stay intact.
Code highlighting: fenced code blocks (TypeScript, JavaScript, HTML, CSS, JSON, and many more) with hover Copy buttons.
Embeds: paste a URL on its own line — YouTube, Vimeo, CodePen, CodeSandbox, JSFiddle, GitHub Gist and Twitter/X turn into players or cards. Iframe sources are restricted to a safe allowlist.
Three layouts: edit-only, split editor + preview, or preview-only. Toggle with Cmd+Shift+P.
Editor shortcuts: Cmd+B bold, Cmd+I italic, Cmd+E inline code, Cmd+K link, plus toolbar buttons for headings, lists, quotes, tasks, code blocks and Mermaid blocks.
Multi-tab workspace with drag-and-drop .md import, inline rename, undo / redo, and auto-save to IndexedDB.
Export as .md source, copy rendered HTML, or download a standalone .html file with light-theme styles inlined.
Sanitised output: rendered HTML is run through DOMPurify; scripts and untrusted iframes are stripped.

Get started

Paste JSON or Markdown (Cmd+V), open a file, or drop a .json / .md file anywhere on this page. Use the Tools switcher in the top bar to move between the JSON Workbench and Markdown Studio. Press ? in either tool for the shortcut cheat sheet.

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