=head1 NAME

JE::Types - JavaScript types and objects

This is just documentation, not a module.

=head1 DESCRIPTION

The various JavaScript types and objects are represented by Perl
classes in JE (which are listed below). This document describes the basic 
interface implemented by
these classes. Information specific to each class can be found on its own
manual page.

=head1 UPGRADING VALUES

When a value is passed from Perl to JavaScript, it will be "upgraded" to a 
Perl object
representing a JavaScript value. This is done by the C<upgrade> method of
the global object.

If the value to be upgraded is a blessed reference, and the class into
which it is blessed has been bound using JE's C<bind_class> method, it is
wrapped up in a proxy object that provides the methods JS needs. A blessed
reference whose class has not been bound will be
left alone (we assume you know what you are doing). 
Otherwise the
conversion is as follows:

  From            To
  -------------------------
  undef           undefined
  array ref       Array
  hash ref        Object
  code ref        Function
  '0'             number
  other scalar    string

B<WARNING:> The 'upgrading' of simple scalars (strings/numbers) and regexps
is still 
subject to change.

B<To do:> Make &JE::upgrade detect whether a simple scalar is a string or 
number.

B<To do:> Convert Regexp objects to
JE::Object::RegExp objects.

=head1 WHICH CLASSES ARE WHICH

Each built-in JavaScript class or primitive type is a Perl class 
underneath. Here 
is the complete list of object classes:

  JavaScript   Perl
  -----------------
  Object          JE::Object
  Function        JE::Object::Function
  Array           JE::Object::Array
  String          JE::Object::String
  Boolean         JE::Object::Boolean
  Number          JE::Object::Number
  Date            JE::Object::Date
  RegExp          JE::Object::RegExp
  Error           JE::Object::Error
  RangeError      JE::Object::Error::RangeError
  ReferenceError  JE::Object::Error::ReferenceError
  SyntaxError     JE::Object::Error::SyntaxError
  TypeError       JE::Object::Error::TypeError
  URIError        JE::Object::Error::URIError
  
And here are the primitive types:

  string          JE::String
  number          JE::Number
  boolean         JE::Boolean
  null            JE::Null
  undefined       JE::Undefined

And I might also mention a few special cases:

  Global              JE
  Math                JE::Object::Math
  Arguments           JE::Object::Function::Arguments
  Function scope      JE::Object::Function::Call
  RegExp constructor  JE::Object::Function::RegExpConstructor

The last three are for internal use. 
  
=head1 PUBLIC API

=head2 Using JS Values as Scalars

Every JS data type can be used as a string, boolean or number. It works
exactly as it does in JavaScript. For example:

  $num = $je->eval('42');
  $num2 = $je->eval('NaN');
  print $num2; # prints NaN
  print 0+$num2; # prints nan or NaN, depending or your system
                 # (or something really weird on Windows).

  $zero_str = $je->eval("'0'");
  print "true" if $zero_str; # prints 'true'
  print "false" unless 0+$zero_str; # prints 'false'

  $false = $je->eval('false');
  print $false; # prints 'false'
  print "false" unless $false; # also prints 'false'

=head2 Property Access

To access the property of a JS object,
or of the
JS environment itself (i.e., a global variable), just use it as a hash ref:

  $je->{String};    # gives you the String constructor function
  $je->{undefined}; # the undefined value
  my $obj = $je->eval('var obj = new Object; return obj');
  $obj->{foo} = 'bar';

C<keys> will return a list of the object's enumerable properties, including
those inherited from its prototype. The following example prints
S<'baz foo '>:

  $obj = $je->eval('Object.prototype.foo="bar"; ({baz:43}) ');
  print "$_ " for keys %$obj;

C<exists> and C<delete> act upon properties of the object itself, ignoring
those of
its prototype, so C<< exists $obj->{foo} >> will return false.

=head2 Calling Methods

To call a method on an object or primitive data type, use the C<method>
method:

  my $number = $je->eval('42');
  $number->method('toString', 16); # returns the number in hexadecimal

=head2 Calling Functions

Just use a function as though it were a coderef:

  $je->{Array}->();

If you need to specify the invocant ('this' value), use the C<call_with>
method:

  $je->{Number}{prototype}{toString}->call_with($je->eval('42'), 16);

=head2 Just Getting a Simple Perl Scalar

To convert one of the fancy objects returned by JE into a simple Perl
value, use the C<value> method.

  $number->value; # simple Perl scalar
  $str->value;    # likewise
  $obj->value;    # hash ref
  $array->value;  # array ref

Currently the C<value> method of objects and arrays is not recursive, but
I plan to make it so later on. The only way to get consistent behaviour
between this and future versions is to pass C<< recursive => 0 >> as
arguments.

=head1 DATA TYPE API (in more detail)

If you are going to write you own custom data types, proxy objects, or
subclasses of JE's classes, you'll need to read
this. If not, you shouldn't need to, but you might like to anyway. :-)

Be warned that some of the methods described here can be hard
to use, and can easily result in code that's hard to debug, if misused.
This applies to those that expect their arguments already to be objects
compatible with the JE::Types interface. If you are not sure that a value you have is such, run it through the global object's C<upgrade> method (or
just use the L</PUBLIC API>, above).

These are the methods that the JavaScript engine itself uses (as opposed
to those provided for convenient access from the Perl side). Each class 
provides whichever of the following methods are applicable. If
an object does not support a particular method, a TypeError will be thrown
when JavaScript code (indirectly) tries to call that method. (For instance,
C<'some_string'()> will attempt to call the C<call> method of JE::String,
thus resulting in a TypeError).

=over 4

=item prop($name)

=item prop($name, $new_value)

Gets or sets a property. Setting a property returns the new
value. The return value will be a Perl undef if the
property does not exist. See also L<JE::Object>, for the
S<< C<prop({ ... })> >> usage.

The new value is expected already to be an object compatible with the
JE::Types interface.

=item keys

Returns a list of the names of enumerable properties. This is a list of
Perl strings, not JE::Strings.

=item delete($name)

Deletes the property named $name, if it is deletable. If the property did 
not exist or it was deletable, then
true is returned. If the property exists and could not be deleted, false
is returned.

L<JE::Object> will also take a second argument, that allows one to
indicate whether an undeletable property should be deleted. This is
required by custom classes if the object in question is the global object.

The return value is a Perl scalar, not a JE::Boolean.

=item value

This returns a value that is supposed to be useful in Perl. The 
C<< value >> method of a JE::Object,
for instance, produces an array ref.

=item call(@args)

Runs the code associated with the object if it is a function.
The arguments are passed as-is, and are not upgraded.

=item apply($obj, @args)

Runs the code associated with the object if it is a function. C<$obj> will
be passed to the function as its invocant (its 'this' value).
The arguments are passed, as-is, and are not upgraded.

=item construct(@args)

This is just like calling a function
in JS with the C<new> keyword (which itself calls this method). It calls 
the constructor, if this 
function has one (functions written in JS
don't have this). Otherwise, an empty object will be created and passed to 
the 
function as its invocant. The return value of the function will be
returned if it is an object. Otherwise it will be discarded, and the object 
originally passed to the function will be returned instead (possibly 
modified).

=begin comment

NOTE: I need to copy parts of this to those classes that don't describe the
C<new> method, before I go and delete it.

=item I<Class>->new($global_obj, @args)

The C<@args> are in the same order that they are passed to the constructor
function in JavaScript (for objects, not primitives. For primitive classes, 
there should be only two
arguments, the global object and the value).

Some object classes also provide a hash ref syntax. See each object class's
respective man page.

User-defined classes do not need to accept arguments in the same order as
those that come with JE. They can do whatever they like.

=end comment

=item exists($property_name)

Returns a boolean indicating whether the property exists and is not
inherited from a prototype. Used by 
C<Object.prototype.hasOwnProperty>. (The C<in> operator checks to see 
whether
the return value of C<prop> is defined.)

B<To do:> Implement this method in subclasses of JE::Object.

=item is_readonly($property_name)

Not supported by the primitive JE classes. This returns a boolean 
indicating whether a given property is readonly. If
it doesn't exist, then the C<is_readonly> method of the object's prototype
is called with the same arguments. If there is no prototype, false is 
returned.
This is used internally by JE::Object's C<prop> method.

=item is_enum($property_name)

Not supported (yet) by the primitive JE classes. This returns a boolean 
indicating whether a given property is enumerable.
This is used by C<Object.prototype.propertyIsEnumerable>.

=item typeof

Returns a Perl string containing the type of the object. Used
by the JS C<typeof> operator.

=item class

This applies to object classes only (though is going to change, so that
primitives can pretend to be objects). It returns 
a 
Perl string containing
the type of object. This is only used by the default JavaScript C<toString>
method. If you create your own object class without subclassing JE::Object,
you should I<still> provide the C<class> method, so that this
JS code will still work:

  YourClass.prototype.toString = Object.prototype.toString;
  (new YourClass).toString();

=item id

This returns a unique id for the object or primitive, used by 
the JavaScript C<===>
operator. This id is unique as a I<string,> not as a number.

The JE primitive classes provide a unique string beginning with the data
type. The JE::Object and its subclasses return the memory address of the
object itself. If you 
subclass JE::Object, you should not have to
implement this method, unless you have multiple objects that you would
like JS to consider the same object.

Note that the id 'num:nan' is treated specially. It is never
considered equal to itself.

=item primitive

Returns true or false.

=item prototype

=item prototype ( $obj )

This applies to objects only, not to primitives. This method returns the 
prototype of the object, or undef if there is no prototype. If C<$obj> is 
specified,
the prototype is set to that object first. The C<prop> method uses this
method, as does C<< JE::Object->new >>.

=item to_primitive($preferred_type)

=item to_boolean

=item to_string

=item to_number

=item to_object

These each perform the appropriate type conversion. $preferred_type, which 
is 
optional, must be either 'string' or 'number'.

Calling C<to_string> or C<to_number> on a object is not exactly the same as 
calling
C<to_primitive('string')> or C<to_primitive('number')>, because the
argument to C<to_primitive> is merely a I<suggestion.>

The last four methods in this list should not be overridden by subclasses
of JE::Object.

=item global

Returns a reference to the global object.

=item taint($taint_brush)

This will only be called if it is implemented. Of JE's types, only 
primitive strings and numbers implement this.

C<$taint_brush> will always be a tainted empty string. If the
object's internal value is not tainted, this method should return a tainted 
clone 
of
the object. Otherwise, it should return the object itself.

=back

=head1 SEE ALSO

L<JE> and all the modules listed above under L</WHICH CLASSES ARE WHICH>.

=cut