Common JSON Errors (and How to Fix Them)

Trailing comma

Unlike JavaScript, JSON does not allow a comma after the last item in an object or array.

Invalid:

{
  "name": "Ada",
  "role": "engineer",
}

Fixed — remove the comma after the last value:

{
  "name": "Ada",
  "role": "engineer"
}

Single quotes instead of double quotes

JSON strings and keys must use double quotes. Single quotes are valid in JavaScript object literals, which is the usual source of the mix-up, but JSON has no concept of them.

{'name': 'Ada'}     // invalid
{"name": "Ada"}     // valid

Unquoted object keys

Every key must be a double-quoted string — not a bare identifier.

{name: "Ada"}       // invalid
{"name": "Ada"}     // valid

Comments aren't allowed

JSON has no comment syntax — not //, not /* */, not #. If you need comments, you're working with a JSON superset like JSON5 or JSONC, not JSON itself, and it needs to be stripped before it's valid JSON.

{
  // invalid: comments break standard JSON parsers
  "name": "Ada"
}

Mismatched or missing brackets

Every { needs a matching }, and every [ needs a matching ]. This is the error that benefits most from a real parser instead of eyeballing it — a validator reports exactly where the structure breaks down instead of you counting braces.

Using JavaScript-only values

undefined, functions, and NaN/Infinity are all valid in a JavaScript object but have no JSON equivalent. The closest JSON has is null.

{"middleName": undefined}   // invalid
{"middleName": null}        // valid