I tried the code below:
console.log(JSON.stringify({ test: "\u30FCabc" }));
It gives the output below:
'{"test":"ーabc"}'
We know that mainly JSON.stringify() method converts a JavaScript object or value to a JSON string. And as per JSON std (https://www.json.org/json-en.html) we should be able to store \u30FC. Then why "\u30FC" is converted to "ー"?