trailingComma

Trailing comma, sometimes written as trailingComma, is a comma placed after the final element in a list, such as an array or object literal, or in a parameter or argument list. The usage varies by language, but the intent is to simplify editing and diffs by making it easier to add, reorder, or remove items without changing the previous last item’s line.

In JSON, trailing commas are not allowed by the standard specification; parsers will typically reject them. Some JSON extensions and formats such as JSON5 relax this rule, but standard JSON does not permit trailing commas.

JavaScript and related languages commonly support trailing commas in several constructs. In modern JavaScript, trailing commas are allowed in array and object literals, and are often permitted in function parameter and call lists in many environments. Style guides frequently encourage trailing commas to reduce merge conflicts when items are added or reordered.

Other languages have explicit conventions. Go requires a trailing comma after the last element in multi-line composite literals, a rule that helps maintain cleaner diffs and easier edits. Python allows trailing commas in lists, tuples, and dictionaries, and often uses them in multi-line function calls and definitions for readability and maintainability. Rust, among others, also favors trailing commas in multi-line lists to improve diffs and reordering.

Pros of using trailing commas include improved diffs, easier reordering, and more robust code-generation. Cons include potential portability concerns across languages or older tooling, and potential confusion for readers unfamiliar with the practice. Many languages and tools provide options or lint rules to enforce a consistent trailing comma style.