不思議な JSON.stringify の第 2 引数。

replacer.coffee

# 参考: ネイティブ JSON を使う
#       (https://developer.mozilla.org/ja/docs/Using_native_JSON#replacer_.E3.83.91.E3.83.A9.E3.83.A1.E3.83.BC.E3.82.BF)
obj =
  a: 12
  b: 'hoge'
  c: true
  d: undefined
  e: 'fuga'
  f: [12, 'piyo', true]

f = (k, v) ->
  if k is 'e'
    return undefined
  switch typeof v
    when 'number' then v * 2
    when 'string' then v.repeat 2
    when 'boolean' then !v
    when 'undefined' then null
    else v

JSON.stringify obj, f, '  '
###
{
  "a": 24,
  "b": "hogehoge",
  "c": false,
  "d": null,
  "f": [
    24,
    "piyopiyo",
    false
  ]
}
###