I'll write what I know before I start asking.

The object type data is stored heap area because Object type data can grow countless times and cannot be measured in size.

The string type data is assigned an address of a new day when a value is assigned.

let a = 'test'; // a has 'test' data's address
a = 'hello'; // a's value change 'test' adress into 'hello' address.

then I have a question. why string is not stored like object?

The string can grow countless like object, too.

please...

I use english translator... sorry