How to Format and Validate JSON Online

Minified JSON is unreadable. A single-line blob from an API response or config file is impossible to debug without formatting it first. Here's how to format JSON instantly, validate it, and fix the common errors that make parsers choke.

What JSON formatting does

Formatting (also called pretty-printing) takes this:

{"name":"John Doe","age":30,"address":{"city":"New York","zip":"10001"},"tags":["developer","writer"]}

And produces this:

{
  "name": "John Doe",
  "age": 30,
  "address": {
    "city": "New York",
    "zip": "10001"
  },
  "tags": [
    "developer",
    "writer"
  ]
}

Same data, different whitespace — the JSON is still valid in both forms.

Formatting in the browser

Paste minified JSON into the formatter, click format, and get indented output. Most formatters also validate as you type, highlighting the exact line where a syntax error occurs. This is the fastest path for one-off debugging.

Formatting in JavaScript

const minified = '{"name":"John Doe","age":30}';
const formatted = JSON.stringify(JSON.parse(minified), null, 2);
console.log(formatted);

JSON.stringify(value, replacer, space) — the third argument is the indent. 2 gives 2-space indentation; '\t' gives tabs.

To format a JSON file in Node.js:

const fs = require('fs');
const data = JSON.parse(fs.readFileSync('data.json', 'utf8'));
fs.writeFileSync('data.json', JSON.stringify(data, null, 2));

Formatting in Python

import json

minified = '{"name": "John Doe", "age": 30}'
formatted = json.dumps(json.loads(minified), indent=2)
print(formatted)

Format a file in place:

import json

with open('data.json', 'r') as f:
    data = json.load(f)

with open('data.json', 'w') as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

ensure_ascii=False preserves Unicode characters (é, ñ, 你好) instead of escaping them to \uXXXX.

Command line: jq

jq is a command-line JSON processor. Format a file:

# Install
brew install jq        # macOS
sudo apt install jq    # Ubuntu

# Pretty-print
jq . data.json

# Minify
jq -c . data.json

# Format a piped API response
curl -s https://api.example.com/data | jq .

Python one-liner (no install needed):

python3 -m json.tool data.json

Common JSON errors and how to fix them

Trailing comma:

{
  "name": "John",
  "age": 30,    ← error
}

JSON does not allow trailing commas. Remove the last comma before } or ].

Single quotes instead of double quotes:

{'name': 'John'}   ← error

JSON requires double quotes for all strings and keys. Replace ' with ".

Unquoted keys:

{name: "John"}   ← error

JavaScript object literal syntax is not valid JSON. Keys must be quoted: {"name": "John"}.

Comments:

{
  // This is the user object
  "name": "John"
}

JSON does not support comments. Remove them, or switch to JSONC (JSON with Comments) format if your tool supports it.

Undefined or NaN values:

{"value": undefined}   ← error
{"value": NaN}         ← error

undefined, NaN, and Infinity are JavaScript values, not JSON values. Replace with null, 0, or remove the field.

Unescaped special characters in strings:

{"message": "He said "hello""}   ← error

Double quotes inside strings must be escaped: {"message": "He said \"hello\""}.

Newlines in strings also need escaping: \n not a literal line break.

Validating programmatically

function isValidJson(str) {
    try {
        JSON.parse(str);
        return true;
    } catch {
        return false;
    }
}

import json

def is_valid_json(s: str) -> bool:
    try:
        json.loads(s)
        return True
    except json.JSONDecodeError:
        return False

For production use with detailed error messages, both JSON.parse (JS) and json.loads (Python) throw exceptions with the position of the error.

Sorting keys

Alphabetically sorted keys make diff comparisons much cleaner in version control:

const sorted = JSON.stringify(data, Object.keys(data).sort(), 2);

json.dumps(data, sort_keys=True, indent=2)