How to validate JSON (and fix common errors)

If a formatter or parser says your input is "invalid JSON", the data almost always contains something that is fine in JavaScript, Python or YAML but not allowed by the much stricter JSON specification (RFC 8259). This guide walks through how validation works, the errors you will see most often, and how to fix each one quickly.

What "valid JSON" actually means

Validation has two levels, and it helps to know which one is failing:

• Syntax validation checks that the text follows JSON grammar: matching braces and brackets, quoted keys, commas in the right places, and only the allowed value types. This is what a JSON validator or JSON.parse() does.
• Structure validation checks that valid JSON has the shape you expect, for example that email is a string and age is a number. That needs a schema, which is covered in JSON Schema basics.

Everything below is about syntax. To check syntax in the browser, paste your data into the JSON Formatter. If it parses, the editor formats it. If not, it highlights the line and column where parsing stopped.

Reading the error message

Parsers report the first position where the input stopped making sense, which is often just after the real mistake. A message like Unexpected token } in JSON at position 42 usually means the problem is on the line before: a trailing comma, a missing value, or an unclosed string. Jump to the reported line, then look one or two tokens back.

1) Keys and strings must use double quotes

JSON only accepts double quotes. Unquoted keys and single quotes come from JavaScript object literals and Python dictionaries.

Not valid JSON (unquoted key, single quotes):

text
{a: 'hello'}

Valid JSON:

json
{"a": "hello"}

If you copied the output of Python's print(dict), use json.dumps(data) instead, which produces real JSON.

2) Remove trailing commas

A comma after the last item in an object or array is allowed in modern JavaScript but not in JSON.

Not valid JSON (trailing comma):

text
{"a": 1,}

Valid JSON:

json
{"a": 1}

3) JSON doesn't allow comments

Neither // nor /* */ comments are part of JSON. Some tools accept "JSON with comments" (for example tsconfig.json in VS Code), but a standard parser will reject them.

Not valid JSON (contains a comment):

text
{
  // comment
  "a": 1
}

Valid JSON:

json
{
  "a": 1
}

If you need to document a field, add a separate key such as "_comment": "...", or use a format that supports comments, such as YAML (see JSON vs YAML).

4) Watch for special values: NaN, Infinity, undefined

JSON supports exactly six kinds of values: objects, arrays, strings, numbers, true/false and null. JavaScript values like undefined, NaN and Infinity have no JSON representation.

Not valid JSON:

text
{"count": NaN, "limit": Infinity, "value": undefined}

Valid JSON (use null, or leave the key out):

json
{"count": null, "limit": null}

Note that JSON.stringify() silently drops keys whose value is undefined and turns NaN into null, so the problem usually appears when JSON is written by hand or generated with string concatenation.

5) Other common mistakes

• Unescaped characters in strings. A raw line break or an unescaped " inside a string breaks parsing. Use \n and \" instead; see Escape JSON strings.
• Leading zeros and number formats. 007, +5, .5 and hexadecimal like 0x1F are not valid JSON numbers. Write 7, 5, 0.5 and 31.
• Multiple top-level values. A file with two objects one after the other is not a single JSON document. Wrap them in an array, or treat the file as NDJSON if each line is a separate record.
• A byte order mark or invisible characters. Text copied from documents or exported on Windows can start with a BOM or contain non-breaking spaces. If the error points to position 0 and the text looks fine, retype the first character.
• Smart quotes. Word processors and chat apps convert " into curly quotes (“ ”), which JSON does not accept.

6) Validate in code

In JavaScript, wrap JSON.parse in a try/catch and log the message:

javascript
try {
  const data = JSON.parse(text);
} catch (err) {
  console.error('Invalid JSON:', err.message);
}

In Python, json.loads(text) raises json.JSONDecodeError, whose lineno and colno attributes point to the problem.

7) Format before you compare

Different whitespace and indentation make diffs noisy. Once both documents are valid, format them the same way and then compare the two JSON files.