JSON Formatter and Validator

A JSON formatter turns a dense, one-line API response or config file into indented text you can actually read, and tells you exactly where the syntax breaks when it doesn’t parse. This one also minifies JSON for sending over the wire and can sort keys so two files are easier to compare. It runs on your browser’s own JSON parser, so nothing you paste is uploaded.

How to use the JSON formatter

1. Paste your JSON into the text box.
2. Choose the indent: 2 spaces, 4 spaces or Tab.
3. Tick Sort keys alphabetically if you want the keys of every object in A to Z order.
4. Select Format to pretty-print the JSON, or Minify to strip all the whitespace.
5. Read the status line: Valid JSON, or the parser’s error message with the line and column where it failed.
6. Use Copy to put the result on your clipboard, or Download .json to save it as a file.

How it works

The tool parses your text with JSON.parse, the parser built into every browser, and writes it back out with JSON.stringify:

• Format writes each value on its own line and indents every nesting level by the indent you chose.
• Minify writes the same data with no whitespace at all. JSON allows spaces, tabs and line breaks only around its structural characters ({ } [ ] : ,), so removing them never changes the data (RFC 8259, section 2).
• Sort keys reorders the keys of every object, at every level, before the output is written. Arrays keep their order, because in an array the order is part of the data.

Because the browser’s parser does the work, the rules are exactly the JSON standard, with two behaviors worth knowing:

• Big numbers lose precision. JavaScript stores every JSON number as a 64-bit float, which represents whole numbers exactly only between −(2^53 − 1) and 2^53 − 1, that is up to 9,007,199,254,740,991. RFC 8259 calls this the range where implementations agree. A longer integer is rounded to the nearest number a float can hold.
• Duplicate keys keep the last value. RFC 8259 says object names “SHOULD be unique” and that software behaves unpredictably when they aren’t. The ECMAScript specification settles it for JavaScript: earlier values for the same key are overwritten.

Note: The wording of the error message comes from your browser, so it differs between Chrome, Firefox and Safari. When a browser reports only a character position, the tool works out the line and column for you.

Worked examples

Format a minified API response (2 spaces). This input:

{"name":"Ada","languages":["en","fr"],"active":true,"id":7}

becomes:

{
  "name": "Ada",
  "languages": [
    "en",
    "fr"
  ],
  "active": true,
  "id": 7
}

With Sort keys alphabetically ticked, the same object comes out in the order active, id, languages, name, and the array still reads "en", "fr".

Find a trailing comma. This JSON fails because of the comma after 36:

{
  "name": "Ada",
  "age": 36,
}

The parser expects another property name after a comma and finds } instead, so it reports an error at line 4, column 1. Delete the comma and it validates.

A 64-bit ID that changes. {"id": 12345678901234567890} is valid JSON, but after parsing in JavaScript the number becomes 12345678901234567000, because it is far above 2^53 − 1. If an API sends IDs that large, it should send them as strings ("id": "12345678901234567890"), which the formatter keeps exactly.

Common JSON errors

Error	Invalid	Valid
Trailing comma	[1, 2, 3,]	[1, 2, 3]
Single quotes	{'name': 'Ada'}	{"name": "Ada"}
Unquoted key	{name: "Ada"}	{"name": "Ada"}
Comment	{"a": 1} // note	{"a": 1}
Undefined value	{"a": undefined}	{"a": null}

Trailing commas and single quotes are rejected by JSON.parse, as MDN’s reference notes. Comments and undefined are JavaScript, not JSON: the JSON grammar in RFC 8259 has no comment syntax, and its only literal names are true, false and null.

Related guides

• Working with XML instead? Use the XML formatter to pretty-print and check it.
• A JWT payload is JSON too: the JWT decoder decodes and formats it.
• Practice reading and writing JSON files with the to-do list and weather app in our Python projects for beginners.
• More developer guides and error fixes are in Programming.