JSON.stringify(NaN)为什么等于null

Question

JSON.stringify(NaN)"null"JSON.stringify([NaN])"[null]"JSON.stringify(undefined)undefinedJSON.stringify(null)"null"JSON.stringify([undefined])"[null]"求解释原理

Answer 1

从ecma-262 规范文档 的角度解释标题的问题就非常简单。

直接看为什么 JSON.stringify(NaN) => "null"

NOTE 4
Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String null.

NaN 会被 当成字符串 null

为什么 
JSON.stringify(undefined) => undefined
JSON.stringify([undefined]) => "[null]"

NOTE 5
Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String null

Answer 2

undefined 直接返回 undefined ; 在数组中，就被当成 字符串null;