I propose that trailing commas should be enabled by default. IE8 is a non issue for most, and this will provide better diffing.

const arr = [
  'multi',
  'line'
]

should be formatted as:

const arr = [
  'multi',
  'line',
]

I would also suggest not enabling it for function arguments, considering that this is not in the official spec yet and won't be fully supported without a transpiler like babel. e.g.:

foo(
  arg1,
  arg2,
)

should be formatted as:

foo(
  arg1,
  arg2
)