How to Fix Invalid JSON

Find the first syntax error, then re-parse

A formatter can only pretty-print JSON that already parses. If you see Unexpected token, Expected property name, or JSON.parse throwing, fix the text at the reported line — usually a trailing comma, a single-quoted string, an unquoted key, or a comment.

Paste the payload into the JSON Formatter & Minifier to validate it in your browser. The tool reports line and column when the parser provides them. After it parses, use Format if you need indentation. For the pretty-print workflow, see How to Format JSON Online.

JSON is not a JavaScript object literal

Most “invalid JSON” is valid JavaScript. JSON (RFC 8259) is stricter: double-quoted keys and strings, no comments, no trailing commas, and no undefined, NaN, or Infinity.

Trailing commas

A comma after the last property or array item is the most common copy-paste failure — especially from TypeScript objects or Python dicts printed with extra commas.

{
  "id": 18,
  "role": "editor",
}

Fixed trailing comma

Remove the last comma. Do not add a dummy property just to “use” the comma.

{
  "id": 18,
  "role": "editor"
}

Single quotes and unquoted keys

JSON strings and object keys must use double quotes. Single quotes and bare identifiers are JavaScript only.

{
  name: 'Ada',
  active: true
}

Fixed quotes and keys

Quote every key. Switch string values to double quotes. Escape any double quote that belongs inside a string with \".

{
  "name": "Ada",
  "active": true
}

Comments are not allowed

// and /* */ are fine in many config dialects (JSONC) and in JavaScript. Standard JSON has no comment syntax. Strip comments, or keep a documented JSONC file and compile it to JSON in your build — do not ship comments to JSON.parse.

{
  "timeoutMs": 5000,
  // retry on 429
  "retries": 3
}

undefined, NaN, and trailing commas in nested objects

JSON’s empty value is null. undefined is not a JSON token. NaN and Infinity are not JSON numbers. A trailing comma buried in a nested object still fails the whole document — parsers stop at the first error, which is why nested payloads feel “randomly broken.”

{
  "user": {
    "id": 7,
    "label": undefined,
  },
  "score": NaN
}

A valid nested example

Replace undefined with null or omit the key. Use a number or null instead of NaN.

{
  "user": {
    "id": 7,
    "label": null
  },
  "score": null
}

How to work through a parser error

Do not re-indent until the document parses. Extra whitespace will not fix a trailing comma.

• Read the line and column from the error. Jump there first.
• Fix one issue, then parse again. Later errors often disappear.
• If the file is huge, copy the object around the reported line into a smaller fixture.
• If two versions look the same, compare the text to catch a smart quote or a hidden comma.

Validate, then format

Use the JSON Formatter & Minifier to parse and, once valid, pretty-print or minify. If the next step is a spreadsheet, convert a JSON array with the JSON to CSV Converter after the JSON is valid.