How to Format and Validate JSON for Free

JSON is how nearly every web service sends data. It is also strict in a way that catches people out: one trailing comma or one single quote and the whole document is rejected.

A formatter does two things. It tells you whether the JSON parses at all, and if it does, it lays it out so you can read it.

The rules JSON actually enforces

Almost every JSON error people hit is one of five things. It is worth knowing them by sight.

• Double quotes only. 'name' is not valid. Neither is a key without quotes.
• No trailing comma. A comma after the last item in an object or array is an error, even though JavaScript allows it.
• No comments. // and /* */ are not part of JSON, however many configuration files pretend otherwise.
• Numbers are plain. No leading zeros, no +, no NaN and no Infinity.
• Only six types. String, number, boolean, null, object and array. A date is a string, and it is up to both ends to agree on its format.

How to find what is wrong

1. 1

   Paste the whole document

   Not a fragment. A parser cannot judge a piece of an object, and most "invalid JSON" reports turn out to be half a document.

2. 2

   Read the line and column

   The error gives you a position. The mistake is normally on that line or the one directly above it.

3. 3

   Look for the five rules above

   In practice it is a trailing comma or a single quote about nine times out of ten.

Formatted or minified

Formatting adds indentation and line breaks so a human can follow the structure. Minifying strips every space that is not inside a string, which makes the payload smaller to send.

Neither changes the data. You can go back and forth as often as you like and get the same document out.

Reading a nested structure

Once formatted, indentation does the explaining. A key at two spaces is inside the top-level object; a key at six spaces is inside an object inside an array inside that.

{
  "order": "A-1042",
  "paid": true,
  "customer": {
    "name": "Northgate Supplies",
    "country": "GB"
  },
  "items": [
    { "sku": "A-100", "qty": 2 },
    { "sku": "A-102", "qty": 1 }
  ]
}

Common mistakes

• Copying JSON out of a terminal that wrapped the lines, and pasting the wrapping with it.
• Pasting a fragment rather than the whole document and reporting the parser as broken.
• Adding comments to a .json configuration file. Some tools tolerate it; the standard does not.
• Escaping quotes by hand inside a string when the value already came escaped. Two rounds of escaping produces something that parses and is wrong.
• Pasting a payload containing real customer data into an online formatter without checking where the formatting happens.