10 Common JSON Syntax Errors and How to Fix Them

Why JSON errors are so common#

JSON is strict. Unlike JavaScript object literals, it allows no shortcuts, so small habits from other languages break parsing. Below are the ten mistakes we see most, each with a fix.

1. Trailing comma#

{
  "a": 1,
  "b": 2,
}

Remove the comma after the last item:

{
  "a": 1,
  "b": 2
}

2. Single quotes#

{ 'name': 'Ada' }

JSON requires double quotes:

{ "name": "Ada" }

3. Unquoted keys#

{ name: "Ada" }

Wrap the key in double quotes: { "name": "Ada" }.

4. Comments#

{
  // user name
  "name": "Ada"
}

JSON has no comments. Remove them, or move the note into a field such as "_comment". If you need comments in config files, consider JSONC or JSON5, but parse them with a tool that supports those formats.

5. Missing comma between items#

{
  "a": 1
  "b": 2
}

Add the comma after 1.

6. Unescaped quotes inside strings#

{ "quote": "She said "hi"" }

Escape inner quotes with a backslash:

{ "quote": "She said \"hi\"" }

7. Raw newlines or tabs inside strings#

A literal line break inside a string is invalid. Use the escape sequence \n (or \t for tabs) instead.

8. Invalid values: undefined, NaN, Infinity#

These JavaScript values do not exist in JSON. Use null, or a string or number that your application understands.

9. Mismatched brackets#

{ "items": [1, 2, 3 }

Every [ needs a ] and every opening brace needs a closing one. An editor with bracket matching, like the one in NiceJSON, highlights the partner so you can spot the gap.

10. Leading zeros and bad numbers#

{ "zip": 01234, "price": .5 }

JSON numbers cannot have leading zeros or start with a decimal point. Use "zip": "01234" (as a string) and "price": 0.5.

Reading the error message#

Fastest way to find the problem#

Paste the text into the NiceJSON validator. It reports the line and column of the first error, so you can fix issues one at a time instead of guessing.