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JavaScript types and objects

JE::Types - JavaScript types and objects


JE::Types - JavaScript types and objects

This is just documentation, not a module.


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.


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 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 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

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

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

To do: Convert Regexp objects to JE::Object::RegExp objects.


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 call   JE::Object::Function::Call

The last two are for internal use.


Using JS Values as Scalars

Every JS data type can be use as as 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'

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';

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

  $obj = $je->eval('"bar"; ({baz:43}) ');

  print "$_ " for keys %$obj;

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

Calling Methods

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

  my $number = $je->eval('42');

  $number->method('toString', 16); # returns the number in hexadecimal

Calling Functions

Just use a function as though it were a coderef:


Just Getting a Simple Perl Scalar

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

  $number->value; # simple Perl scalar

  $str->value;    # likewise

  $obj->value;    # hash ref

  $array->value;  # array ref

Currently the 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 a future versions is to pass recursive => 0 as arguments.

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.

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, 'some_string'() will attempt to call the call method of JE::String, thus resulting in a TypeError).

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 the JE::Object manpage, for the prop({ ... }) > usage.

The new value will be converted to a JS value automatically if it is not one already. (See UPGRADING VALUES.) This is going to change. For the sake of efficiency, prop is going to stop upgrading automatically.

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

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.

the JE::Object manpage 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.

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

Runs the code associated with the object if it is a function. The arguments are automatically upgraded but that is going to change.

apply($obj, @args)
Runs the code associated with the object if it is a function. $obj will be passed to the function as its invocant (its 'this' value). The arguments are automatically upgraded but, again, that is going to change.

This is just like calling a function in JS with the 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).

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

To do: Make hasOwnProperty use the exists method. Right now it uses prop with hashref syntax.

To do: Implement this method in JE's classes.

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 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 prop method.

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

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

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 defoult JavaScript toString method. If you create your own object class without subclassing JE::Object, you should still provide the class method, so that this JS code will still work:

  YourClass.prototype.toString = Object.prototype.toString;

  (new YourClass).toString();

This returns a unique id for the object or primitive, used by the JavaScript === operator. This id is unique as a 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.

Returns true or false.

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 $obj is specified, the prototype is set to that object first. The prop method uses this method, as does JE::Object->new.

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

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

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

Returns a reference to the global object.


JE and all the modules listed above under WHICH CLASSES ARE WHICH.