JSON Formatter, Validator & Viewer — Free & Online

What is JSON?

JSON — short for JavaScript Object Notation — is a lightweight, text-based data format used to exchange structured data between systems. It was originally derived from JavaScript object syntax, but today every major programming language has a JSON parser, which is why JSON has become the default wire format for REST APIs, configuration files, log records, and inter-service messaging.

A JSON document is just text. It represents one of six value types: an object (key/value pairs in curly braces), an array (ordered values in square brackets), a string (double-quoted), a number, a boolean (true or false), or null. That tiny grammar — codified in RFC 8259 and the ECMA-404 standard — is the entire spec. Its smallness is the point: any system can parse JSON without a schema, framework, or runtime dependency.

Why developers use JSON

JSON earned its place as the lingua franca of web development for a few practical reasons. It is language-independent: a Python service can emit JSON that a Go service or a browser can read with no glue code. It is human-readable: you can inspect a payload in any text editor and understand it immediately, which makes debugging dramatically easier than with binary formats. It is compact compared to XML — less ceremony around tags, attributes, and namespaces — which keeps API responses small and cacheable. And because every modern stdlib ships a JSON parser, there is no library lock-in: switching languages or frameworks never breaks your data interchange.

JSON syntax rules at a glance

Most JSON errors come from forgetting that JSON is not JavaScript. Here are the rules the spec enforces — and that our JSON validator checks for:

• Strings must use double quotes. 'hello' is invalid; use "hello".
• Object keys must be quoted strings. {name: "Ada"} is invalid; {"name": "Ada"} is valid.
• No trailing commas. [1, 2, 3,] and {"a": 1,} are both rejected.
• No comments. Strict JSON forbids // and /* */. (Our Auto-Fix mode strips them for you.)
• Numbers are decimal only. No hex (0xFF), no leading zeros (007), no NaN or Infinity.
• Use lowercase literals. true, false, null — not True or None.
• Strings cannot contain raw control characters. Newlines and tabs inside a string must be escaped (\n, \t).
• UTF-8 is the default encoding. Non-ASCII characters are allowed in strings as long as the file is valid UTF-8.
• The MIME type is application/json; the file extension is .json.

Common JSON errors and how to fix them

Native parsers tend to surface errors as terse messages like “Unexpected token in JSON at position 47.” That tells you something broke, but not what. Here are the failures we see most often — and how the Auto-Fix mode resolves them.

1. Trailing comma. Common when you copy from JavaScript or remove the last item from a list.

// ❌ Invalid
{
  "users": ["ada", "linus", "grace",],
}

// ✅ Fixed
{
  "users": ["ada", "linus", "grace"]
}

2. Single-quoted strings. JavaScript accepts both quote styles; JSON does not.

// ❌ Invalid
{ 'name': 'Ada Lovelace' }

// ✅ Fixed
{ "name": "Ada Lovelace" }

3. Unquoted object keys. Valid in JavaScript object literals, invalid in JSON.

// ❌ Invalid
{ name: "Ada", born: 1815 }

// ✅ Fixed
{ "name": "Ada", "born": 1815 }

4. Comments left in a config file. Frequently seen with tsconfig.json-style JSONC files passed to a strict parser.

// ❌ Invalid (strict JSON)
{
  // primary host
  "host": "api.example.com",
  /* TLS only */
  "tls": true
}

// ✅ Fixed
{
  "host": "api.example.com",
  "tls": true
}

5. Missing comma between values. Hard to spot in long objects.

// ❌ Invalid — error reported many lines below the actual problem
{ "host": "api.example.com" "port": 443 }

// ✅ Fixed
{ "host": "api.example.com", "port": 443 }

6. Unescaped control characters in strings. Pasting a multi-line value directly into a JSON string fails parsing.

// ❌ Invalid — raw newline inside the string
{ "message": "line 1
line 2" }

// ✅ Fixed
{ "message": "line 1\nline 2" }

7. Number precision loss. JSON numbers are 64-bit floats. IDs like 9007199254740993 silently lose their last digit if treated as a JavaScript Number. Our validator warns when a payload contains integers outside the safe range — the fix is to send those as strings.

How our JSON validator works

Behind the scenes the validator runs the native JSON.parse first — the fastest possible path for valid input. When parsing fails, we hand the source to a small custom tokenizer that walks the input character by character to locate the exact line and column of the failure, then maps the failure class to a plain-English explanation (“trailing comma before ]”, “unquoted key name”, etc.) and a suggested fix.

The Auto-Fix mode applies only safe, deterministic transforms: stripping comments, converting single quotes to double, quoting unquoted keys, and removing trailing commas. It will never invent values, guess at missing brackets, or reorder fields — the output is always a strict superset of what you typed, which means it’s safe to round-trip through the tool without losing data. Everything runs entirely in your browser: no upload, no server log, no telemetry on the payload itself. Open DevTools → Network and you’ll see zero requests fire when you paste JSON.