JSON Formatter & Editor
A complete JSON workbench — format, validate, repair, explore, query, diff, and convert. Everything runs locally in your browser, so the data you paste is never uploaded.
Input
Output
Repair fixes loose JSON: trailing commas, single quotes, unquoted keys, // and /* */ comments, and Python-style True / False / None. Flatten collapses nesting into dot-paths; Unflatten reverses it.
About ALT JSON Lab
ALT JSON Lab is a fast, no-nonsense JSON editor and viewer that runs entirely in your browser. We built it because we kept hopping between half a dozen websites just to format a response, untangle a deeply nested object, or work out why two API payloads didn't line up — and most of those sites quietly send whatever you paste off to a server. This one never does. Whatever you drop in stays on your own machine.
Format, validate and fix JSON
Paste a raw API response and hit Format for clean, indented output, or Minify to squeeze it back down to one line. The validator points straight to the spot where parsing breaks, so you're not left hunting for a missing bracket. And when the JSON is genuinely broken — a trailing comma here, single quotes there, unquoted keys, the odd // comment — the Repair button cleans up the usual mistakes and hands you valid JSON back.
Read it as a tree, then pull out what you need
The tree view turns a wall of text into something you can actually read, with collapsible branches and click-to-copy paths on every key. When you only want a slice of the data, the query box understands simple JSONPath-style expressions like $.users[*].email, so grabbing one field out of a huge document takes a couple of seconds.
Compare versions and convert formats
Need to know what changed between two responses? Drop them side by side and the diff marks everything that was added, removed or edited, right down to the path — a real time-saver when you're chasing down why two payloads no longer line up. Need to turn JSON into another format? Our dedicated JSON Converter handles CSV, YAML, TypeScript and JSON Lines on its own page.
Your data never leaves the page
It is worth saying twice: the JSON you paste into ALT JSON Lab is processed entirely in your browser and is never uploaded. We use privacy-respecting, consent-based analytics to understand which tools people rely on, but the content you work with is never transmitted, so you can safely paste access tokens, internal payloads or production data. The tools themselves keep working even with your Wi-Fi switched off.
Frequently asked questions
What is a JSON Formatter?
A JSON formatter is a tool that takes minified or messy JSON data and formats it with proper indentation and line breaks, making it easy for humans to read and understand. Our online JSON formatter also acts as an editor and validator.
Is this JSON Formatter safe?
Yes, absolutely. Unlike many online tools, our JSON Formatter runs 100% locally in your browser. We never upload, see, or store the JSON data you paste. It is completely safe for sensitive payloads or API keys.
How to use JSON Formatter?
Simply paste your JSON string into the left editor pane. The tool will automatically validate your code. You can then use the buttons to format (pretty print), minify, sort keys, or fix common errors instantly.
How to validate a JSON file?
Paste the contents of your JSON file into our editor. If there are syntax errors (like missing commas or quotes), the validator will immediately flag the exact line with the error. You can also click "Repair" to automatically fix common JSON mistakes.
How to add JSON Formatter in Notepad++ or use a Chrome extension?
While you can install plugins for Notepad++ or download a Chrome extension, our web-based tool provides a powerful, zero-install alternative. Because it runs locally and supports offline mode, you get the security of a desktop app with the convenience of a web tool—no downloads required.