⊹ 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
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JSON Viewer & Formatter

guide · json test suite · json document generator

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

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.
Are there keyboard shortcuts?
Yes. Press 1, 2, 3, 4, 5, 6 to jump between the Viewer, Raw, Hex, Stats, YAML, and CSV 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. Append #jsonString= followed by your URL-encoded JSON to share a viewer link, e.g. #jsonString=%7B%22I%22%3A%22love%20json%22%7D. Use &encoding=base64 for base64url-encoded JSON. The URL is capped at 1 MB of decoded JSON; for larger files, open the file locally. See 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.
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