Fix trailing commas in JSON

Why trailing commas fail

JSON objects and arrays cannot end with a comma before } or ]. Many JavaScript object literals allow the trailing comma; JSON.parse and most API gateways do not. A config copied from a JS module into an API body is a common source of “Unexpected token” errors on the final property.

{
  "id": "00127",
  "tags": ["mug", "blue",]
}

The comma after "blue" is illegal in JSON. The same rule applies after the last key in an object.

Steps in this browser

1. Paste the payload into the JSON Formatter.
2. Run validate or format. The first offending line and column are reported when the document is strict-invalid.
3. Remove the trailing comma (or convert the document in the source emitter).
4. Format again and confirm a clean tree. Minify only after the document validates.

Fix trailing commas in JSON Formatter →

Related mistakes

Single-quoted strings, unquoted keys and // comments are also rejected by strict JSON even though they appear in some config dialects. Large integers may validate here but lose precision in JavaScript Number — compare with your application’s parser. For CSV handoffs that become JSON, see CSV to JSON.