How to Fix Broken JSON (Trailing Commas, Single Quotes, Python Dicts & More)

You ran JSON.parse() and got SyntaxError: Unexpected token. Frustrating — but the cause is almost always one of a handful of common mistakes. Here is each one, why it breaks, and how to fix it (by hand or in one click).

First: find out what's actually wrong

Before fixing, see the error. A JSON validator shows the exact line and column of the first problem. Then come back here to fix it — or skip straight to the repair tool, which does both: diagnoses and fixes.

The common causes of broken JSON — and the fix

1. Trailing commas

{ "a": 1, "b": 2, }   // remove the last comma

Legal in JavaScript, illegal in JSON. Fix: delete the comma before the closing } or ].

2. Single quotes

{ 'name': 'Ada' }   ->   { "name": "Ada" }

JSON only allows double quotes. Fix: replace every ' with " (watch for apostrophes inside values, which must be escaped or left alone).

3. Unquoted keys

{ name: "Ada" }   ->   { "name": "Ada" }

JavaScript object syntax, not JSON. Fix: wrap every key in double quotes.

4. Comments

{
  "port": 8080  // not allowed in JSON
}

Standard JSON has no comments. Fix: remove // and /* */ blocks.

5. Python dicts (True / False / None)

{ 'active': True, 'role': None }
->
{ "active": true, "role": null }

Pasting a Python dict or a print() dump is one of the most common causes of "invalid JSON." Python uses True, False, None and single quotes. Fix: convert them to true, false, null and double quotes. The repair tool does this conversion automatically — it's effectively a Python-dict-to-JSON converter.

6. Truncated / cut-off JSON

{ "items": [1, 2, 3   // stream cut off — brackets unclosed

Common with log files, byte-limited API responses, or clipboard truncation. Fix: close the open brackets/braces in the right order. The repair tool detects unclosed structures and appends the minimum tokens to make it valid (then tells you it did).

7. Wrong value types

{ "count": 01, "note": undefined }   // invalid

No leading zeros; undefined and NaN don't exist in JSON. Fix: use plain numbers and null.

Fix it automatically in 3 steps

1. Open the JSON Repair tool and paste your broken JSON (or drag in a .json file).
2. It repairs the JSON instantly and lists every change — switch to the side-by-side diff to see exactly what changed.
3. Copy the result, Download it as a .json file, or click Apply fix to replace the input. Everything runs locally — nothing is uploaded.

When the data is truly incomplete (a truncated stream), no tool can invent the missing values — the repair tool will close the structure and flag that some data may still be missing so you can finish it by hand.

Then validate and format

• Repair to fix the syntax.
• Validate to confirm it's clean — see How to Validate JSON.
• Format to pretty-print it — see What Is JSON Formatting?

Frequently asked questions

Can a repair tool fix any broken JSON? It fixes syntax mistakes (commas, quotes, keys, comments, Python literals, unclosed brackets). It can't recover data that was never there — truncated content has to be completed manually.

Is it safe to paste sensitive JSON? Yes — repair runs entirely in your browser; nothing is sent to a server.

Does fixing change my data? No. It only corrects syntax so a parser will accept it; your values are preserved.

Wrapping up

Broken JSON is almost always one of these seven patterns — and a repair tool that shows you a diff of every change turns a tedious hunt into a single click.

Related free tools: JSON Validator · JSON Formatter · All JSON Tools