Common JSON Errors and How to Fix Them

JSON is defined in RFC 8259. The rules that cause nearly all problems are these:

• Strings and object keys must use double quotes. Single quotes are not allowed.
• There are no comments of any kind.
• There is no trailing comma after the last item of an object or array.
• The only values are objects, arrays, strings, numbers, true, false and null. There is no undefined, NaN, Infinity, date type or function.
• Numbers cannot have a leading plus sign, leading zeros (other than a single 0), or a bare decimal point.
• A document is one value. Two objects side by side are not valid JSON.

The most common error by far, usually left behind after deleting or reordering the last item.

{
  "name": "Asha",
  "roles": ["admin", "beta",],
}

{
  "name": "Asha",
  "roles": ["admin", "beta"]
}

Python dictionaries and JavaScript objects accept single quotes, but JSON does not. This often happens when someone prints a Python dictionary with print(data) and pastes the output.

{'name': 'Asha', 'active': True}

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

Standard JSON has no comment syntax. Neither // nor /* */ is allowed. Some tools accept them as an extension (VS Code's settings files use a variant called JSONC, and TypeScript's tsconfig.json allows comments), but a strict parser will reject them.

{
  // number of retries
  "retries": 3
}

If you need to annotate data, add a field such as "_comment": "number of retries", or keep the explanation in documentation. If a file is meant to contain comments, name it for what it is (JSONC or JSON5) and parse it with a parser that supports it.

undefined, NaN, Infinity and -Infinity are JavaScript values with no JSON equivalent. JSON.stringify quietly drops properties whose value is undefined, and turns NaN and Infinity into null, so the failure often only shows up later.

JSON.stringify({ a: undefined, b: NaN, c: Infinity, d: new Date(0) });
// {"b":null,"c":null,"d":"1970-01-01T00:00:00.000Z"}

Decide how missing and unknown values should be represented, usually null or an omitted key, and use strings for dates (ISO 8601 is the convention).

Values like postal codes, phone numbers and account numbers with leading zeros should be strings, because a number cannot preserve them.

Inside a string, a double quote, a backslash and any control character (including a real line break or tab) must be escaped.

{"message": "She said "hello""}
{"path": "C:\Users\asha"}
{"text": "line one
line two"}

{"message": "She said \"hello\""}
{"path": "C:\\Users\\asha"}
{"text": "line one\nline two"}

The valid escapes are \", \\, \/, \b, \f, \n, \r, \t and \uXXXX. Anything else after a backslash, including \', is an error.

{
  "id": 1
  "name": "Asha"
}

Parsers usually report the position of the token after the missing comma, so look at the line before the one in the error message.

Every { needs a } and every [ needs a ], in the right order. Truncated downloads and copy-and-paste mistakes often cut off the closing brackets. A related error appears when there is more than one value in the text, for example two objects one after another, or a log file with one JSON object per line (often called JSON Lines or NDJSON). A normal parser reads the first value and then complains about the rest. Parse such files line by line.

• Smart quotes. Text copied from a word processor, a chat window or a web page can contain curly quotes, which look right but are different characters.
• Byte order mark (BOM). Some Windows editors add an invisible marker at the start of a UTF-8 file. RFC 8259 says generators must not add one, and some parsers fail on it.
• Non-breaking spaces. These look like spaces but are not valid whitespace in JSON.

If the text looks perfect but still fails, retype the suspicious punctuation, or save the file as UTF-8 without BOM.

Parsers describe where they gave up, which is usually right after the real mistake. Typical messages:

A validator that highlights the exact line and column is faster than counting characters by hand. Paste the text, find the marker, then check the line above it as well.