As far as I understand, every string is an object in Javascript. Still, it "does not work" as I expect it to be:

var a="abc";  //here we get a new string object
a.b = 123;    //I seem to declare a property "b" of that object
alert(a.b);   //alerts "undefined"

However, if I try to define a string in the "wrong way", everything works as expected

var a=new String("abc"); //
a.b = 123;
alert(a.b);    //alerts "123"

Why is that so?