is object mutable in javascript? If yes, why the object of string is immutable then?

Question

In javascript only object and arrays are mutable and remaining are primitive type which are immutable . However if we create an object of string,then it can't be mutated. why is that?

var a = new String("abc")
a.toUpperCase();
console.log(a) /*wont print "ABC" and prints an object with String:abc. Concludes 
that string object is immutable.*/

Answer 1

In general, yes, objects are mutable.

The reason that "string objects" - created with new String - are not mutable is that string objects result in objects which have an internal StringData property :

Set S.[[StringData]] to value.

That StringData is what is retrieved when the string object is coerced back to a primitive.

If the interpreter provided a way to change this StringData internal slot - then these sorts of objects would effectively be mutable. (Similarly, Set.add changes a Set's internal slot, and the same sort of mechanism exists for many other objects in JS - an exposed method changes values held in internal slots.)

But

a.toUpperCase();

alone wouldn't do anything - toUpperCase returns a new string. There are no methods (in standard implementations) that exist that can alter a string object's StringData internal slot - all you can do is create a new string object with a different StringData .

You could also strongly consider just not using string objects at all.