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

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

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