new String( "foo" ) VS. "foo"

[dhtmlkitchen]

Why does "foo" instanceof String return false?

It just seems counterintuitiv e to me.

I mean, sure, I can always check the constructor:

var fooVar = "foo";

var isString = fooVar.construc tor == String;

var isNotStringObje ct = fooVar instanceof String;

It seems counterintuitiv e to me that an object is not an instance of
it's constructor.

Garrett

[Richard Cornford]

<dh**********@g mail.comwrote in message
news:11******** **************@ k79g2000hse.goo glegroups.com.. .

Why does "foo" instanceof String return false?

String literals result in string primitive values and primitive values
are not objects.

It just seems counterintuitiv e to me.

Are you thinking of javascript as being in some way related to Java?

I mean, sure, I can always check the constructor:

var fooVar = "foo";

var isString = fooVar.construc tor == String;

And let the type-converting side effects of the dot operator fool you
into thinking that - fooVar - might be an object?

Dot operators need their left-had side arguments to be objects so they
call the internal - ToObject - function with the retrieved value of that
operand as an argument. The internal - ToObject - function returns a
String object when its argument is a string primitive value and so with,
for example, the expression - "foo".indexOf(' s') - the - "foo". - part
effectively evaluates to a string object equivalent to - new
String("foo") -, and it is this (internal and intermediate) object on
which the - indexOf - method is called.

var isNotStringObje ct = fooVar instanceof String;

Here the left operand for - instanceof - is the primitive value (no type
conversion is implied by - instanceof -) and as primitive values are not
objects the - String - constructor's - [[HasInstance]] - method will
return false in step 1 of its algorithm.

It seems counterintuitiv e to me that an object is not an
instance of it's constructor.

But makes perfect sense when you know that it is not an object at all.

Richard.