After cloning JS array using JSON.parse & JSON.stringify, why array's "undefined" value converts to "null"? [duplicate]

Question

var arr1 = [1, 2, 3, undefined, 4, 5];
var arr2 = JSON.parse(JSON.stringify(arr1));
console.log(arr2);

Output of the above code will be....

[1, 2, 3, null, 4, 5]

Why it converts it's 4th value from "undefined" to "null"?

ECMA-404 (no joke, the standard of JSON) does not know undefined. — Nina Scholz
– Nina Scholz, Commented Aug 17, 2020 at 13:24
stackoverflow.com/questions/13796751/json-undefined-value-type — marsnebulasoup
– marsnebulasoup, Commented Aug 17, 2020 at 13:24
Undefined is not valid json, so JSON.parse converts it to null. Then when converted back, since null is valid JS, it keeps the null — marsnebulasoup
– marsnebulasoup, Commented Aug 17, 2020 at 13:25

Community · Accepted Answer · 2021-10-07 08:58:39Z

2

Because undefined is a Javascript type. There is no undefined in JSON.

According to the spec:

JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).

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answered Aug 17, 2020 at 13:24

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