Why doesn't concat concatenate JSON correctly?

Question

JavaScript

There are three JSON strings.
You need to fold them. I found the concat function in JS:
jsonArray1 = answers_string.concat(sub_answers);
As a result, not valid JSON is obtained. It's missing a comma between curly braces.
{"QID":3,"AID":2}{"SubAID":[]}
Question. Why and how to fix?

Answer 1

Why

because you are concatenating strings.

jsonArray1 = JSON.stringify(
    [answers_string, sub_answers].map((str) => JSON.parse(str))
);

or

jsonArray1 = '[' + [answers_string, sub_answers].join(', ') + ']';