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新出的 RFC 8259:The JavaScript Object Notation (JSON) Data Interchange Format

JSON 的規格書又被更新了 XD

在「The Last JSON Spec」這邊,Tim Bray 寫了這篇關於新的 RFC 8259 跟之前的差異,以及大家對於雙重標準的顧慮。

最大的差異在於,在 RFC 8259 規定了「如果 JSON 被用在非封閉的系統交換資料,必須使用 UTF-8」:

8259 con­tains one new sen­tence: “JSON text ex­changed be­tween sys­tems that are not part of a closed ecosys­tem MUST be en­cod­ed us­ing UTF-8 [RFC3629].” Giv­en that, by 2017, an at­tempt to ex­change JSON en­cod­ed in any­thing but UTF-8 would be ir­ra­tional, this hard­ly needs say­ing; but its ab­sence felt like an omis­sion.

而關於 ECMA-404 與 RFC 8259 都定義了 JSON 的問題他也說明了,因為很多人花了很多力氣在確保這兩份文件的正確性上,所以應該不會有問題 (i.e. 衝突):

The rea­son 8259 ex­ists is that the ECMAScript gang went and wrote their own ex­treme­ly min­i­mal spec, Stan­dard ECMA-404: The JSON Da­ta In­ter­change Syn­tax, and there was rea­son for con­cern over du­el­ing stan­dard­s. But, af­ter a cer­tain amount of standards-org elephant-gavotte, each of ECMA 404 and RFC 8259 nor­ma­tive­ly ref­er­ences the oth­er and con­tains a com­mit­ment to keep them con­sis­tent in case any er­rors turn up. Which is a good thing, but this text has been re-examined and re-polished so many times that I doubt ei­ther side will ev­er re­vis­it the ter­ri­to­ry, thank good­ness.

另外他也提到了對於不同情境下可以看不同的文件。像是要了解 JSON 的話,可以看當初發明 JSON 的 Doug Crockford 所設立的網站 (在「JSON」這邊);而在交換時應該參考 I-JSON (Internet JSON,RFC 7493):

Which spec should you use? · If you want to un­der­stand JSON syn­tax, you still can’t beat Doug Crockford’s orig­i­nal for­mu­la­tion at JSON.org. If you want to use an RFC as foun­da­tion for a REST API or some oth­er In­ter­net pro­to­col, I ac­tu­al­ly don’t rec­om­mend 8259, I rec­om­mend I-JSON, RFC 7493, which de­scribes ex­act­ly the same syn­tax as all the oth­er specs (by ref­er­enc­ing 7159), but ex­plic­it­ly rules out some legal-but-dumbass things you could do that might break your pro­to­col, for ex­am­ple us­ing any­thing but UTF-8 or hav­ing du­pli­cate mem­ber names in your ob­ject­s.

I-JSON 是 JSON 的子集合,比較重要的:

  • (MUST) 使用 UTF-8。
  • (SHOULD NOT) 浮點數的部份,不得超過 IEEE 754-2008 binary64 (double precision) 的範圍。
  • (SHOULD NOT) 整數的部份,不得超過 [-(2**53)+1, (2**53)-1]) 的範圍。
  • (RECOMMEND) 有超過的需求使用字串表示。
  • (MUST NOT) JSON object 內不得有重複的 name。
  • (SHOULD NOT) 最上層的型態不得使用字串,只能使用 object 或是 array。
  • (MUST NOT) 遇到先前沒有定義過的元素不得視為錯誤。(像是新版 API 內會在 object 裡增加元素)
  • (RECOMMEND) 時間使用 ISO 8601 表示 (在 RFC 3339 有提到),英文字的部份全部使用大寫,一定要標上時區,而秒數的 0 一定要加上去 (也就是 00 秒)。
  • (RECOMMEND) 時間長度也建議依照 RFC 3339 處理。
  • (RECOMMEND) Binary 資料用 base64url 傳 (RFC 4648)。