Unexpected token , in JSON at position X when parsing trailing commas

Question

Unexpected token , in JSON at position X when parsing trailing commas
When JSON.parse encounters a trailing comma, the ECMAScript specification mandates a SyntaxError with the message "Unexpected token , in JSON at position X". However, several JavaScript engines historically accept trailing commas as a non‑standard extension, creating inconsistent behavior across browsers and Node.js versions.
Goal
Determine the current state of trailing‑comma support across major JavaScript engines and decide on a strategy for handling such JSON data in a cross‑platform codebase.
Constraints & Uncertainty

Spec forbids trailing commas; no TC39 proposal currently exists to change this.
Engine behavior varies: Chrome, Firefox, Safari, and recent Node.js may throw, while older V8 or JavaScriptCore may silently accept.
Relying on this extension risks future compatibility if engines drop the non‑standard behavior.

Unresolved Questions

Will upcoming ECMAScript releases or engine updates standardize or remove trailing‑comma support in JSON?
Is there an ongoing TC39 proposal to extend the JSON grammar to allow trailing commas?
What is the most robust approach to ensure JSON data with trailing commas is parsed correctly across all target engines?

Tasadduq Burney · Answer

Short answer
JSON.parse follows the ECMAScript specification and RFC 8259, which forbid trailing commas. All current mainstream engines—V8 (Chrome, Node.js), SpiderMonkey (Firefox), and JavaScriptCore (Safari)—throw a SyntaxError such as "Unexpected token , in JSON at position X" when they encounter a trailing comma. No engine accepts it as a standard feature.
Confirmed facts

The JSON grammar does not allow a comma before the closing brace or bracket.
Engines that appear to accept trailing commas are either evaluating JavaScript literals, using eval() or new Function(), or employing a lenient third‑party parser such as JSON5.
No TC39 proposal exists to extend JSON with trailing commas; the format is intentionally frozen for interoperability.

Steps to handle trailing‑comma JSON in a cross‑platform codebase

If you control the data producer, fix the serializer so it never emits trailing commas.
If you do not control it, preprocess the incoming text at the boundary:

Strip trailing commas that are outside string literals (track quote and escape state).
After stripping, feed the cleaned string to JSON.parse.

Optionally, use a well‑documented tolerant parser like JSON5, but remember that its output is not guaranteed to match native JSON.parse; validate the result afterward.

Missing diagnostic detail that changes the recommendation
Is the JSON payload produced by a service you control, or by a third party you cannot modify? Knowing this determines whether you should fix the source or apply preprocessing.

Unexpected token , in JSON at position X when parsing trailing commas