JSON formatter & validator

How to use the JSON formatter

Paste JSON into the Input box — or drag a file onto it — and it is checked as you type. Press Format (or Ctrl+Enter) to pretty-print it with two spaces, four spaces or tabs, or Minify to strip every unnecessary space for the smallest payload. The output has line numbers and syntax colours that stay readable in light and dark mode, and can be copied, downloaded as a .json file or moved back into the input with Use output as input.

Switch the output to Tree to browse a large document: objects and arrays expand on click, show how many keys or items they contain, and long arrays load in pages of 300 so a 50,000-item array doesn’t lock up the tab. Clicking any key copies its path, such as $.orders[1].customer, ready to paste into code.

The search box accepts a key name (every key containing that text, anywhere in the document) or a path with dots, brackets and wildcards: address.city, orders[0], orders[-1] for the last item, or $.orders[*].total to pull one field out of every element.

Reading validation errors

Browsers’ own error messages are terse (“Unexpected token } in JSON at position 1843”). This validator is written for humans: it gives the line and column, shows the offending line with a caret under the exact character, and explains the likely cause. The mistakes it recognises include:

• Trailing commas — [1, 2, ] is fine in JavaScript but not in JSON.
• Single quotes and unquoted property names — JSON requires double quotes around every string and key.
• Comments — // and /* */ are not part of JSON, even though some config files allow them.
• Python literals — True, False and None from a Python print() instead of json.dumps().
• Missing commas, mismatched brackets, line breaks inside strings, leading zeros, NaN and undefined, and several top-level values pasted one after another.

Repair fixes the mechanical ones automatically — comments, trailing commas, single quotes, unquoted keys, Python literals — and tells you what it changed. Anything that needs judgement, such as a missing comma, is left for you so nothing is silently guessed.

Big numbers, duplicate keys and why formatting is exact

Most online formatters run JSON.parse then JSON.stringify. That quietly changes your data: JavaScript numbers are 64-bit floats, so an ID like 9007199254740993 becomes 9007199254740992, and when a key appears twice only the last value survives. For database IDs, Twitter/X snowflake IDs or payment references that is a real bug.

Format and Minify here work on the text itself — a streaming scanner re-indents the tokens exactly as written — so numbers keep every digit, duplicate keys stay, and key order never changes. The status line warns you when it sees integers larger than 2⁵³ or duplicate keys, because the next program to read the file probably won’t be as careful. Sort keys, the tree, search and CSV necessarily parse the data, so those features follow JavaScript’s rules; the warning tells you when that matters.

JSON to CSV, escaping and large files

JSON → CSV takes an array of objects (or the first array property of an object, as many APIs wrap results in {"data": [...]}) and builds one column per key across all rows, flattening nested objects into dot-named columns such as address.city. Values starting with =, +, - or @ are prefixed with an apostrophe so Excel and Google Sheets can’t execute them as formulas.

Escape to string turns the input into a JSON string literal, handy for embedding a JSON document inside another or in a test fixture; Unescape string reverses it, which is the fix for log lines full of \". To pass JSON through a URL or an HTTP header, encode it with Base64; for dummy text values in a mock payload, use the lorem ipsum generator.

Everything runs in your browser. Work is split into chunks that yield to the page, so a file of tens of megabytes formats in a few seconds without the tab freezing; above 1.5 MB the output drops syntax colouring to stay fast. The only limit is your device’s memory.

Frequently asked questions