Trailing commas in JSON

Why JSON rejects trailing commas

In the JSON grammar, a comma separates two items. After a comma the parser expects another property or value, so when it finds } or ] instead, it stops with an error. The same comma is fine between items: only the one before the closing bracket has to go.

{
  "name": "Demo",
  "tags": ["api", "json",],
}

{
  "name": "Demo",
  "tags": ["api", "json"]
}

JSON’s syntax was defined in the early 2000s from a subset of JavaScript, before JavaScript allowed trailing commas in object literals. The grammar has never changed since, and the current standards, RFC 8259 and ECMA-404, still leave them out.

The error you’ll see

Few parsers mention the comma. They report the character after it, so look just before the reported position.

Other parse errors look similar, and the JSON errors reference covers the rest.

Where trailing commas come from

• Deleting the last item. Remove the final property by hand and the comma on the line above stays behind.
• Copying from code. JavaScript and TypeScript objects often end with a comma, especially with code formatters set to add them.
• Building JSON as text. A loop that appends item + "," leaves one extra comma at the end.
• Config files that allow them. tsconfig.json and VS Code’s settings.json accept trailing commas, so the habit carries over to package.json, which doesn’t.

Removing them in code

Use a parser rather than find-and-replace. A regular expression such as ,\s*([}\]]) works on simple files, but it also changes text inside strings, so a value like "a ,]" would be edited too.

In JavaScript, jsonrepair (the library behind the validator’s Fix My JSON button) returns valid JSON text. JSON5 parses trailing commas directly:

import { jsonrepair } from "jsonrepair";
const data = JSON.parse(jsonrepair(text));

// or
import JSON5 from "json5";
const data = JSON5.parse(text);

In Python, the json5 package reads them:

import json5  # pip install json5

data = json5.loads(text)

From the command line, jsonrepair and Prettier can both fix a file. Prettier removes trailing commas whenever it formats a .json file:

npx jsonrepair broken.json --output fixed.json
npx prettier --write settings.json

Avoiding them

Generate JSON with JSON.stringify in JavaScript or json.dumps in Python instead of writing it by hand. They never add trailing commas. If you do have to build text, collect the parts in a list and join them with commas, so the separator only goes between items. For hand-edited files, an editor that understands JSON underlines the stray comma, and Prettier removes it every time the file is saved.

Frequently asked questions