JavaScript Overview
Author: Norris Boyd

# Rhino Overview

### Overview of Rhino

Most people who have used JavaScript before have done so by adding scripts to their HTML web pages. However, Rhino is an implementation of the core language only and doesn't contain objects or methods for manipulating HTML documents.

Rhino contains

- All the features of JavaScript 1.5
- Allows direct scripting of Java
- A JavaScript shell for executing JavaScript scripts
- A JavaScript compiler to transform JavaScript source files into Java class files

### Language

The JavaScript language itself is standardized by Standard ECMA-262 ECMAScript: A general purpose, cross-platform programming language. Rhino 1.5 implements JavaScript 1.5, which conforms to Edition 3 of the Standard. The Standard may be downloaded or obtained by mail from ECMA, 114 Rue du Rhône, CH1204 Geneva, Switzerland.

Rhino 1.6 also implements ECMA-357 ECMAScript for XML (E4X). See the specification for more information on the standard, and Rhino version 1.6R1 for details on the implementation in Rhino.

In addition, Rhino has implemented JavaAdapters, which allows JavaScript to implement any Java interface or extend any Java class with a JavaScript object. See the `enum.js` example for more information.

Numerous books and tutorials on JavaScript are available.

### Deprecated Language Features

Several language features introduced in JavaScript 1.2 are now deprecated. These features allow "computational reflection": that is, the ability for a script to determine and influence aspects of the way it is evaluated. These features are generally not broadly useful, yet they impose significant constraints on implementations that hamper or prevent optimization. The deprecated features are the `__proto__` and `__parent__` properties, and the constructors `With`, `Closure`, and `Call`. Attempts to invoke these constructors with the language version 1.4 will result in an error. For other versions, a warning will be generated.

### Internationalization

The messages reported by the JavaScript engine are by default retrieved from the property file `org/mozilla/javascript/resources/Messages.properties`. If other properties files with extensions corresponding to the current locale exist, they will be used instead.

### JavaScript Language Versions

Some behavior in the JavaScript engine is dependent on the language version. In browser embeddings, this language version is selected using the LANGUAGE attribute of the SCRIPT tag with values such as "JavaScript1.2".

Version 1.3 and greater are ECMA conformant.

Operators `==` and `!=`

Version 1.2 only uses strict equality for the == and != operators. In version 1.3 and greater, == and != have the same meanings as ECMA. The operators === and !== use strict equality in all versions.

ToBoolean

`Boolean(new Boolean(false))` is false for all versions before 1.3. It is true (and thus ECMA conformant) for version 1.3 and greater.

Array.prototype.toString and Object.prototype.toString

Version 1.2 only returns array or object literal notation ("[1,2,3]" or "{a:1, b:2}" for example). In version 1.3 and greater these functions are ECMA conformant.

Array constructor

`Array(i)` for a number argument `i` constructs an array with a single element equal to `i` for version 1.2 only. Otherwise the ECMA conformant version is used (an array is constructed with no elements but with length property equal to `i`).

String.prototype.substring

For version 1.2 only, the two arguments are not swapped if the first argument is less than the second one. All other versions are ECMA compliant.

String.prototype.split

For version 1.2 only, split performs the Perl4 special case when given a single space character as an argument (skips leading whitespace, and splits on whitespace). All other versions split on the space character proper as specified by ECMA.

### Security

The security features in Rhino provide the ability to track the origin of a piece of code (and any pieces of code that it may in turn generate). These features allow for the implementation of a traditional URL-based security policy for JavaScript as in Netscape Navigator. Embeddings that trust the JavaScript code they execute may ignore the security features.

Embeddings that run untrusted JavaScript code must do two things to enable the security features. First, every `Context` that is created must be supplied an instance of an object that implements the `SecuritySupport` interface. This will provide Rhino the support functionality it needs to perform security-related tasks.

Second, the value of the property `security.requireSecurityDomain` should be changed to `true` in the resource bundle `org.mozilla.javascript.resources.Security`. The value of this property can be determined at runtime by calling the `isSecurityDomainRequired` method of `Context`. Setting this property to true requires that any calls that compile or evaluate JavaScript must supply a security domain object of any object type that will be used to identify JavaScript code. In a typical client embedding, this object might be a string with the URL of the server that supplied the script, or an object that contains a representation of the signers of a piece of code for certificate-based security policies.

When JavaScript code attempts a restricted action, the security domain can be retrieved in the following manner. The class context should be obtained from the security manager (see `java.lang.SecurityManager.getClassContext()`). Then, the class of the code that called to request the restricted action can be obtained by looking an appropriate index into the class context array. If the caller is JavaScript the class obtained may be one of two types. First, it may be the class of the interpreter if interpretive mode is in effect. Second, it may be a generated class if classfile generation is supported. An embedding can distinguish the two cases by calling `isInterpreterClass()` in the `Context` class. If it is the interpreter class, call the getInterpreterSecurityDomain() method of Context to obtain the security domain of the currently executing interpreted script or function. Otherwise, it must be a generated class, and an embedding can call `getSecurityDomain()` in the class implementing `SecuritySupport`. When the class was defined and loaded, the appropriate security domain was associated with it, and can be retrieved by calling this method. Once the security domain has been determined, an embedding can perform whatever checks are appropriate to determine whether access should be allowed.

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