Question

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?

Answer 1

This happens because the property accessors, (. and []) convert the value ToObject.

Something equivalent to this happens behind the scenes:

var a="abc";
new Object(a).b = 123;  
alert(a.b); // undefined

Basically an object is created on the fly, by the property accessor, see the Step 5:

The production MemberExpression : MemberExpression [ Expression ] (or MemberExpression . Identifier) is evaluated as follows:

1. Evaluate MemberExpression.

2. Call GetValue(Result(1)).

3. Evaluate Expression.

4. Call GetValue(Result(3)).

5. Call ToObject(Result(2)).

6. Call ToString(Result(4)).

7. Return a value of type Reference whose base object is Result(5) and whose property name is Result(6).