JSON formatter & validator

What this JSON formatter does

Paste any JSON document and either pretty-print it with consistent indentation or minify it to a single line. The tool is a thin UI over the browser’s native JSON.parse and JSON.stringify, which means two things: it is as strict as the JSON spec (RFC 8259), and it is instant even for multi-megabyte API responses. If the input does not parse, you see the exact error the engine produced instead of a vague “invalid JSON” message. Nothing leaves your browser.

How to use it

1. Paste your JSON into the JSON box (the sample shows the expected shape).
2. Pick an Indent width — 2 spaces (the default used by most formatters and Prettier) or 4 spaces.
3. Click Format for readable output or Minify for the compact form.
4. If parsing fails, a red error line replaces the result. Fix the input and click again — the tool does not re-run on every keystroke, so you can edit freely.
5. Use Copy on the result to grab the output.

Reading the error message

Because the parser is the browser’s own, the wording differs slightly between engines, but all of them tell you where it stopped:

Browser	Typical message
Chrome / Edge (V8)	Unexpected token } in JSON at position 42
Firefox	JSON.parse: unexpected character at line 3 column 7 of the JSON data
Safari	JSON Parse error: Unexpected identifier "foo"

“Position 42” counts characters from the start of the input, including newlines. The most common failures, roughly in order of frequency:

Input	Why it fails	Fix
{"a": 1,}	Trailing comma	Remove the last comma
{'a': 1}	Single quotes	Use double quotes for keys and strings
{a: 1}	Unquoted key	Quote the key: {"a": 1}
// comment	Comments are not JSON	Strip them (JSONC is not JSON)
{"x": NaN}	NaN, Infinity, undefined are not JSON values	Use null or a string
{"p": "C:\Users"}	Backslash must be escaped	Write "C:\\Users"
[1, 2, 3	Unbalanced brackets	Close the array

A sneaky one: pasting from a chat app or a PDF often brings in “smart” quotes (“ ”) or a non-breaking space, both of which look fine but are not valid tokens. Re-type the quote and the error disappears.

Things JSON.stringify changes silently

Formatting is lossless for ordinary data, but a few JavaScript rules apply because the tool round-trips through a real object:

• Big integers lose precision. {"id": 12345678901234567890} becomes 12345678901234567000. Anything above Number.MAX_SAFE_INTEGER (9007199254740991) is at risk. Twitter/X snowflake IDs and some database keys hit this — send them as strings.
• Duplicate keys collapse to the last one: {"a":1,"a":2} formats as {"a": 2}. Validators in other languages may reject the input instead.
• Numeric-looking keys are reordered ahead of other keys because of how JavaScript orders object properties. {"b":1,"2":2,"a":3} comes back as {"2": 2, "b": 1, "a": 3}.
• Number formatting normalizes: 1.0 becomes 1, 1e3 becomes 1000, -0 becomes 0.
• Escaped Unicode is decoded: "\u00e9" is printed as "é". The meaning is identical; the bytes are not.

None of these are bugs, but they matter when you diff formatted output against the original — use the text diff tool on two formatted versions rather than on raw vs. formatted text.

Format vs. minify in practice

Minifying removes only whitespace outside strings, so savings depend on how “pretty” the input was. A typical 4-space-indented API response shrinks by 20–40%; once gzip or Brotli is applied on the wire the difference drops to a few percent, which is why most APIs do not bother minifying responses but do avoid pretty-printing them. Where minified JSON really helps is inside HTML attributes (data-config='{"a":1}'), inline <script type="application/json"> blocks, and localStorage, where there is no compression at all.

Related tools and snippets

• CSV ⇄ JSON converter — turn a spreadsheet export into an array of objects, or flatten JSON back to CSV.
• JWT decoder — the header and payload of a token are JSON; this decodes and pretty-prints them.
• Base64 encoder / decoder — JSON payloads are often shipped Base64-encoded inside other formats.
• Text diff — compare two formatted JSON documents line by line.
• Text and data tools overview — the full list of text utilities on WebTricks and how they fit together.