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Narcissus internals

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dherman edited this page Dec 3, 2010 · 3 revisions

Terminology

Since JavaScript is both the language of the implementation and the language being implemented, it's helpful to distinguish the two levels conceptually:

  • host ("host code," "host JS," etc): JS code in the implementation of Narcissus
  • user ("user code," "user JS," etc): JS code being interpreted by Narcissus

Narcissus global object

Loading the Narcissus source files creates a global variable Narcissus.

Narcissus.options is an object which can be used to set some global configuration options for Narcissus.

Property Options Meaning
version "185" (default), "harmony" JavaScript language version
ecma3OnlyMode false (default), true strict adherence to ES3 syntax
parenFreeMode false (default), true experimental "paren-free" syntactic extensions

Currently there is just one configuration option, version, which selects the user language version.

Narcissus modules

Narcissus is divided into four "modules" (using the module pattern):

  • Narcissus.definitions (jsdefs.js): basic definitions shared by the other modules
  • Narcissus.lexer (jslex.js): lexer
  • Narcissus.parser (jsparse.js): parser
  • Narcissus.interpreter (jsexec.js): interpreter
  • Narcissus.decompiler (jsdecomp.js): decompiler

The Narcissus.interpreter and Narcissus.decompiler modules are optional; that is, it's possible to load just the first three files to obtain a JavaScript parser written in portable JavaScript.

Host language requirements

These are the host language versions required by each module/source file:

  • jsdefs.js: ES3 + const + __proto__ = null + Object.defineProperty
  • jslex.js: ES3 + const + __proto__ = null + Object.defineProperty
  • jsparse.js: ES3 + const + __proto__ = null + Object.defineProperty
  • jsdecomp.js: not yet specified
  • jsssa.js: not yet specified
  • jsexec.js: SpiderMonkey JS 1.8.5:
    • const (Harmony)
    • catch guards (replaceable with catch + if)
    • let declarations (Harmony)
    • Proxy (Harmony)
    • Object.defineProperty (ES5)
    • Object.getOwnPropertyDescriptor (ES5)
    • Object.getPrototypeOf (ES5)
    • Object.getOwnPropertyNames (ES5)
    • __proto__ = null (replaceable with ES5 Object.create or Harmony maps)
    • __proto__ = obj (replaceable with ES5 Object.create)

The first four modules are fairly web-portable, except for IE. The jsssa.js and jsexec.js files depend on SpiderMonkey extensions.

User languages

The versions of JavaScript interpreted by Narcissus are under development. Currently, the interpreter works reasonably well on ES3 code, with support for a few SpiderMonkey extensions. We will soon be including support for various Harmony proposals as well.

Interpreter

Narcissus is a meta-circular JavaScript interpreter with a very direct representation of values: primitives are self-representing, objects are represented as objects (with their properties accessible via usual property access), and functions are represented as functions. The interpreter is designed this way to allow existing JavaScript functions and objects (such as the standard libraries) to interface directly with Narcissus code without following any special protocol or requiring wrapping and unwrapping.

Values

User-JS primitive values are represented in host-JS directly as themselves.

User-JS objects are represented directly as host-JS objects; each user-property foo is directly represented as a host-property foo.

User-JS functions are represented as proxy functions that wrap a FunctionObject, which encapsulates the Node and ExecutionContext for the closure.

Control flow

Code is always executed with a current ExecutionContext, which contains the current scope chain and this binding. Execution contexts also contain a result property, which is used for the completion value of the current statement or expression or the result of returning from a function or throwing an exception.

Calling a user-JS function is represented by calling the host-JS __call__ method. Narcissus patches the host-JS Function.prototype to add a default __call__ method. Similarly, calling a user-JS function as a constructor (i.e., from new) is represented by calling the host-JS __construct__ method. (This is leaky and would be better implemented with private names; see bug 586095.)

Returning from a function via return is represented by throwing the constant RETURN. The return value is stored in the result property of the current execution context.

Throwing a user-JS exception is represented by throwing the constant THROW. The exception value is stored in the result property of the current execution context.

Breaking or continuing a loop is represented by throwing the constant BREAK or CONTINUE, respectively.

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