How to Fix Invalid JSON: Every Common Error, Decoded

Reading the error message

Parsers stop at the first violation and report where they gave up — which is near, but not always exactly at, the real mistake. 'Unexpected token }' usually means the problem is just before the brace (a trailing comma, a missing value). 'Unexpected end of input' means the document stopped early — an unclosed brace or bracket, or a truncated copy-paste. 'Unexpected token ' (a quote) often signals single quotes used where doubles are required.

A validator with line-and-column reporting (and editor markers) collapses the search: paste, read the position, look one token left.

The five classics and their fixes

Trailing commas — [1, 2, 3,] → remove the final comma; JSON forbids it everywhere. Single quotes — 'name' → "name"; both keys and strings need double quotes. Unquoted keys — {name: 1} → {"name": 1}; that's JavaScript syntax, not JSON. Comments — delete them, or switch the file to JSON5 if comments are genuinely needed. JavaScript values — undefined, NaN, Infinity have no JSON form; use null or strings.

// ✗ All five mistakes in one (not valid JSON):
{
  name: 'Ada',          // unquoted key, single quotes, comment
  "score": NaN,         // NaN doesn't exist in JSON
  "tags": ["a", "b",],  // trailing comma
}

// ✓ Fixed:
{ "name": "Ada", "score": null, "tags": ["a", "b"] }

Sneakier breakages

Some invalid JSON looks perfect. Smart quotes — a document that traveled through Word or a chat app may have “curly” quotes, which are not JSON string delimiters. Invisible characters — a BOM at the file start or a non-breaking space breaks parsers that show you nothing wrong. Truncation — logs and clipboards cut long payloads mid-string, producing 'unexpected end of input' on a document that looks complete because the end scrolled away. Double encoding — a JSON string of JSON (\" everywhere) isn't broken, it's wrapped: unescape it first.

When to auto-repair vs. fix by hand

Mechanical mistakes — quotes, commas, comments, unquoted keys, smart quotes — are exactly what automated repair handles: our JSON Repair (the wrench icon on the formatter) fixes all of them in one click and re-validates. Structural problems — truncated payloads, genuinely ambiguous nesting — need a human, because the repairer would have to guess your intent. The workflow that always works: Validate → read the position → repair or hand-fix → validate again.