instanceof Operator

P T Withington ptw at
Mon Oct 22 04:53:01 PDT 2007

On 2007-10-21, at 15:20 EDT, Brendan Eich wrote:

> On Oct 21, 2007, at 11:48 AM, Jeff Dyer wrote:
>>> The wrapper classes String, Number, Boolean are similar to the
>>> (primitive) values they wrap, but they're not really related to  
>>> those
>>> value types in a type sense, and in ES4 the wrappers are of even  
>>> less
>>> utility than in ES3, I would say.
>> Right, thanks for the correction. This is an obvious consequence of
>> the
>> recent return of primitive wrappers to ES4. 'string' is the new
>> 'String'!
> String has its uses, not just embedded in content that requires
> compatibility: string and String inter-convert, String is non-final,
> and the two share a prototype object.

I think Dylan would have made String an open subclass of the sealed  
class string:

> The need for string comes directly from ES1-3, because the primitive
> string type in JS1 was auto-wrapped when you used a string value as
> if it were an object, say to call a method on it. But since a new
> wrapper was created each time (or so it appeared; implementations
> could optimize), the object-ness of the primitive string was
> ephemeral: you couldn't set methods and get them later; any get on
> the wrapper for a property not found on String.prototype or above
> would return undefined.
> In ES4, string has catchalls to satisfy this ephemeral wrapper get/
> set compatibility, but method calls on string instances do not
> require String wrapper creation.

Are you saying for compatibility you have to maintain this 'ephemeral  
wrapper' behavior?

> Wrapper creation may or may not be observable in ES3 -- 9.9 says
> "create a new String" but does not say by evaluating |String| in
> order to construct -- but ES4 does not allow any type name to be re-
> bound, so the implementations that chose to construct the wrapper by
> reflecting on the current value of 'String' in the global object,
> thereby allowing someone to replace that binding and spy on wrapper
> construction, no longer need to reflect each time.
> The shared prototype object allows for replacement of prototype
> methods, which per ES1-3 and much compatibility-constraining content
> on the web, remain mutably bound in the shared prototype object. Thus
> in ES1-3, if you wrapped String.prototype.charAt with some AOP advice
> function, your code calling "hi".charAt(0) will still run your advice.
> Since ("hi instanceof String) was never true, it should not be true
> in the new world. This is one reason string and String are peer
> subtypes of Object, that is, string is not a subtype of String.

IWBNI there were a single test for string-ness.  In ES3, you have to  

   typeof x == 'string' || (typeof x == 'object && x instanceof String)

Am I right in thinking that in ES4 you will be able to say:

   x is string

to ask the same question?

> Tucker and others have hoped for a way to customize String, and since
> it is non-final and convertible to string, the use-cases that want to
> make custom objects that delegate to String.prototype should work
> too. But it would be good to see some examples.

My use case is for an annotated string, the result of a sprintf-like  
formatter that remembers the objects associated with each  
representation (so you can inspect a message for more detailed  

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