⊹ FAST.JSON.VIEWER no file loaded 
{
  "app"     :  "fast json viewer",
  "price"   :  "free",
  "privacy" :  "secure: no upload, no server parsing",
  "scale"   :  "huge json files — 20 GB tested",
  "workers" :  8,
  "engine"  :  "streaming parser + virtual scroll"
}
DRAG & DROP, click, or PASTE (Ctrl/⌘+V) a .json file
TAP the button below to open a .json file
or try a sample JSON
▾ scroll for about & faq

JSON Viewer & Formatter

guide · what is json? · share json with a url · json test suite · test suite explained · json document generator · url #hash length tester · scale mode write-up · wasm vs javascript write-up · fastest json viewer benchmark

Fast JSON Viewer is a free online JSON viewer and formatter built for large files. Drop in a .json file and it is parsed, formatted and validated instantly — directly in your browser.

Your file never leaves your computer. There is no upload and no server-side parsing, so even huge, sensitive JSON files stay completely private. A streaming parser and virtual scrolling keep things fast on big, large and multi-gigabyte JSON files — tested with 20GB JSON files.

What you can do

  • Open large JSON files — view huge and big JSON files that crash other tools and text editors.
  • Format & beautify JSON — pretty-print minified JSON with clean, readable indentation.
  • Validate JSON — catch syntax errors with byte-accurate position reporting.
  • Search JSON — find keys and values fast, with regex support.
  • Convert JSON to YAML — view any JSON file as YAML and download it as a .yaml file, even for multi-gigabyte inputs.
  • Export to CSV — view a JSON array of records as a table and download it as a .csv file (comma, semicolon or tab), even for multi-gigabyte inputs.
  • View JSON online for free — no install, no signup, works in any modern browser.

Frequently asked questions

Does Fast JSON Viewer work on Windows, macOS, Linux, iPhone and Android?
Yes. Fast JSON Viewer runs entirely inside your web browser, so it works on every operating system — Windows, macOS and Linux on the desktop, and iOS (iPhone and iPad) and Android on mobile. There is nothing to install: just open the page in any modern browser (Chrome, Edge, Safari or Firefox) and you can load huge JSON files on any device. Every file is processed locally on your own device, with no upload.
How do I open a large JSON file?
Drag and drop your .json file onto this page, or click LOAD FILE to browse for it. Fast JSON Viewer streams the file inside your browser and starts rendering the first page before parsing finishes, so even large JSON files open quickly.
What is the largest JSON file I can open?
There is no fixed limit. The viewer streams JSON in small blocks across multiple web workers and has been tested with 20GB JSON files. Practical limits depend on your device's available memory and browser.
Is my JSON file uploaded to a server?
No. Your file is opened and parsed entirely on your own device. There is no upload and no server-side processing, so private and sensitive JSON files never leave your browser.
How do I format or beautify JSON?
Open your file and Fast JSON Viewer automatically pretty-prints it with clean indentation and syntax highlighting. It works as a JSON formatter and beautifier for minified or messy JSON of any size.
Can I view JSON online for free?
Yes. Fast JSON Viewer is completely free, with no signup and no installation. It runs in any modern browser, so you can view JSON online from any computer.
Does Fast JSON Viewer validate JSON?
Yes. Every file is validated as it is parsed. If the JSON is invalid, the viewer reports the error and its location so you can find and fix it.
Can I convert JSON to YAML?
Yes. Open your JSON file and switch to the YAML tab (or press 5) to see it converted to YAML instantly. The conversion runs locally in your browser with streaming and virtual scrolling, so it works even on huge multi-gigabyte files, and nothing is uploaded. Click Download .yaml to save the converted file to disk.
Can I export JSON to CSV?
Yes. When your JSON is an array of records, switch to the CSV tab (or press 6) to see it as a table — nested objects are flattened into dotted columns (such as address.city) and arrays are kept as JSON in a cell. Click Download .csv to save it, and pick a comma, semicolon or tab delimiter. The export streams locally in your browser with no upload, so it works even on huge multi-gigabyte files.
Can I see my JSON as a diagram?
Yes. Switch to the Graphical tab (or press 7) to see your JSON as an interactive node graph — every object and array becomes a card, scalar fields are colour-coded rows, and nested structures are linked by edges. Pan by dragging, zoom with the scroll wheel, and click Download PNG to save the diagram as an image. It runs entirely in your browser in a background worker. For very large documents it shows a partial graph of the first several thousand nodes so the tab stays fast; the tab is only unavailable when the JSON is invalid.
Are there keyboard shortcuts?
Yes. Press 1, 2, 3, 4, 5, 6, 7 to jump between the Viewer, Raw, Hex, Stats, YAML, CSV, and Graphical tabs. Use j/k or arrow keys to scroll, gg/G to jump to top/bottom, / or Ctrl+F to search, and ? to open the full shortcuts list.
Can I share JSON via a URL?
Yes. The quickest way is the Linkify icon (the chain link in the top-right of the viewer): it encodes the open document into a shareable URL and copies it to your clipboard. The same icon on the Graphical tab makes a link that reopens straight into the node-graph view. You can also build the URL by hand: append #jsonString= followed by your URL-encoded JSON, e.g. #jsonString=%7B%22I%22%3A%22love%20json%22%7D, optionally with &encoding=base64 for base64url-encoded JSON and &view=graphical to land on a tab. The JSON travels in the URL fragment (after #), so it never reaches our server. Links are capped at 1 MB of decoded JSON; for larger files, open the file locally. To shrink a long link, run it through a shortener like Bitly. See the share-with-a-URL write-up and the guide for more.
Is there a JSON test suite I can run?
Yes. The test suite page is a complete browser-runnable JSON test suite with 45 valid (pass-*) and 74 intentionally invalid (fail-*) fixtures covering RFC 8259 edge cases — number formats, string escapes, surrogate pairs, multi-byte UTF-8, deep nesting, duplicate keys, trailing garbage, commas and comments. Each fixture is the exact input used by npm run test, encoded into the page URL so a click opens it in the viewer with no upload. Use it as a reference set for building or stress-testing your own JSON parser.
How do I create a huge 20GB JSON test file?
Use the JSON generator page to stream a JSON file of any size up to 20 GB straight to your disk via the File System Access API. The data never touches a server and there is no temporary copy in memory — the file is written incrementally as it is generated. Use it to benchmark JSON parsers or to stress-test Fast JSON Viewer on huge inputs.
Why is some deeply nested data shown as plain text, or Collapse all labelled “(partial)”, on very large files?
To make collapsing instant, Fast JSON Viewer builds an in-memory map of every object and array in your file. On very large documents that map can grow big enough to risk exhausting your browser's memory and crashing the tab, so the viewer keeps it within a safe budget (around 25 million items). It does this in one of two ways, depending on the shape of your file:
  • Deep nesting shown as plain text. The viewer indexes the outer structure first, so the whole file stays collapsible at its top levels. When the budget gets tight it stops indexing the deepest levels — anything nested past that point is shown expanded, as plain text. The top-level-items stat then notes “deep nesting past level N shown as text”. This is the usual case for big, deeply nested files.
  • Collapse all / Expand all “(partial)”. Only when a file has an enormous number of top-level items — beyond about 25 million — does even the outer structure exceed the budget. Then the viewer indexes just the beginning of the file, and Collapse all / Expand all are labelled “(partial)”: they fold the indexed top while the rest stays plain text.
In both cases the whole file is still validated, viewable and downloadable — only the collapse controls are limited.
I have a huge CSV file — how do I view it?
Use Huge CSV Viewer — the fastest CSV viewer in the world. Like Fast JSON Viewer, it runs entirely in your browser with no upload: it streams multi-gigabyte CSV, TSV and delimited files in 256 KB blocks across web workers and starts rendering before parsing finishes, and it has been tested on files up to 20 GB. You get instant sorting and filtering on any column (numeric-aware, substring or regex), draggable column resizing with one-click auto-fit, and virtual scrolling so even files with millions of rows stay smooth. Nothing is uploaded and there is no server-side parsing, so private data never leaves your machine.
drop a file to see statistics
total: — copy instrumentation
drop a file to visualize the instrumentation waterfall
Streams the whole file in your browser — works on multi-GB JSON. Supports $.key, ['key'], [n], [*], .* and recursive descent ... Click a result to jump to it.
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