There's very little practical use for String objects as created by new String(foo). The only advantage a String object has over a primitive string value is that as an object it can store properties:

var str = foo;

str.prop = bar;

alert(str.prop); // undefined



var str = new String(foo);

str.prop = bar;

alert(str.prop); // bar

If you're unsure of what values can be passed to your code then I would suggest you have larger problems in your project. No native JavaScript object, major library or DOM method that returns a string will return a String object rather than a string value. However, if you want to be absolutely sure you have a string value rather than a String object, you can convert it as follows:

var str = new String(foo);

str =  + str;

If the value you're checking could be any object, your options are as follows:

1. Don't worry about String objects and just use typeof. This would be my recommendation.

   
   
   

   typeof str == string.




2. Use instanceof as well as typeof. This usually works but has the disadvantage of returning a false negative for a String object created in another window.

   
   
   

   typeof str == string || str instanceof String




3. Use duck typing. Check for the existence of one or more String-specific methods, such as substring() or toLowerCase(). This is clearly imprecise, since it will return a false positive for an object that happens to have a method with the name you're checking, but it will be good enough in most cases.

   
   
   

   typeof str == string || typeof str.substring == function