A trailing comma, also known as a dangling or terminal comma, is a comma symbol that is typed after the last item of a list of elements. Since the introduction of the JavaScript language, trailing commas have been legal in array literals. Later, object literals joined arrays.

[ "hello", "world" ]

# Out[1]: ['hello', 'world']

[ "hello", "world", ]

# Out[2]: ['hello', 'world']