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ES2015 Symbols in Duktape 2.x


Duktape 2.x adds ES2015 Symbol support. Duktape 1.x internal keys are unified with the Symbol concept, and are considered a custom "hidden symbol" type which is not normally visible to ECMAScript code. C code can access hidden symbols, however.

The internal implementation is similar to existing internal keys. Symbols are represented as duk_hstring heap objects, with the string data containing a byte prefix which is invalid (extended) UTF-8 so that it can never occur for normal ECMAScript strings, or even strings with non-BMP codepoints. Object coerced strings have a special object class and the underlying symbol is stored in _Value similarly to e.g. Number object.

Representation basics:

  • Symbols have an external type DUK_TYPE_STRING.
  • Symbols have internal type tag DUK_TAG_STRING.
  • Symbols can be distinguished internally from ordinary strings by looking up the DUK_HSTRING_FLAG_SYMBOL flag. Hidden symbols also have the DUK_HSTRING_FLAG_HIDDEN set.

Behavior basics:

  • Symbols are visible to ECMAScript code as required by ES2015 and later. Hidden symbols are not visible through e.g. Object.getOwnPropertySymbols(). They can only be accessed if a reference to the hidden symbol string is somehow available, e.g. via a C binding.
  • Symbols are visible to the public C API as strings: duk_is_string() is true, duk_get_string() returns a pointer to the symbol internal string representation, etc. C code can create symbols simply by pushing C strings with a specific format, see below.
  • While symbols are strings in the C API, coercion semantics are based on the ECMAScript behavior. For example, duk_to_string() applied to a symbol throws a TypeError.


Internal key formats

Initial bytes in the ranges [0x00,0x7F] and [0xC0,0xFE] are valid for Duktape's extended UTF-8 flavor. The byte 0xFF and the range [0x80,0xBF] are free to be used as symbol markers.

Internal string format Description
<ff> anyValue Hidden symbol (Duktape specific) used by application code. Prior to Duktape 2.2 Duktape internal hidden symbols also used the 0xFF prefix followed by a capital letter (A-Z). Starting from Duktape 2.2 all 0xFF prefixed strings are reserved for application code.
<80> symbolDescription Global symbol with description 'symbolDescription' created using Symbol.for().
<81> symbolDescription <ff> uniqueSuffix Local symbol with description 'symbolDescription'. Trailing unique string makes the symbol unique. The unique suffix is opaque and chosen arbitrarily by Duktape. It's unique within a Duktape heap (across all global environments).
<81> <ff> uniqueSuffix Local symbol with an empty description. Unique suffix makes each such symbol unique. The unique suffix is arbitrary but must not contain the 0xFF byte.
<81> <ff> uniqueSuffix <ff> Local symbol with undefined description. ES2015 differentiates internally between symbols with an empty string description vs. symbols with an undefined description.
<81> symbolDescription <ff> Well known symbol with description 'symbolDescription'. Well known symbols (like Symbol.iterator) are local symbols which are still shared across "code realms". Any fixed suffix never colliding with runtime generated unique local symbols works, currently an empty suffix is used.
<82> anyValue Hidden symbol (Duktape specific) used by Duktape internals. User code should never use this byte prefix or rely on any Duktape internal hidden Symbols.
<83 to be> Reserved for future use, behavior is undefined (Duktape 2.1 interprets as Symbols, Duktape 2.2 does not, don't rely on either behavior.
<bf> Initial byte marker for bytecode dump format since Duktape 2.2.
<00 to 7f> Valid ASCII initial byte.
<c0 to f7> Valid standard UTF-8 (or CESU-8) initial byte.
<f8 to fe> Valid extended UTF-8 initial byte.

There are public API macros (DUK_HIDDEN_SYMBOL() etc) to create symbol literals from C code.

Global symbols

Global symbols are the same across separate global environments and even across Duktape heaps. ES2015 Section

The GlobalSymbolRegistry is a List that is globally available. It is shared by all Code Realms.

and ES2015 Section 8.2:

Before it is evaluated, all ECMAScript code must be associated with a Realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

The current approach satisfies these simply by making a globally registered Symbol have a fixed format so that if a Symbol with the same description is created in another Duktape thread (or even Duktape heap), its internal representation will be identical. No explicit registry is maintained.

Well-known symbols

Well-known symbols (such as Symbol.iterator) are distinct from any local or global symbols. ES2015 Section

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all Code Realms (8.2).

The fixed representation described above ensures that well-known symbols are the same across all code realms (and even across Duktape heaps). The internal representation is essentially the same as for a unique local symbol, but the suffix that makes local symbols unique is missing. Thus, they behave like local symbols other than having a fixed representation.

Unifying with Duktape internal keys

Necessary changes to add symbol behavior:

  • Strings with initial byte 0x80, 0x81, 0x82 or 0xFF are flagged as symbols (DUK_HSTRING_FLAG_SYMBOL). If the initial byte is 0xFF or 0x82, also the hidden symbol flag (DUK_HSTRING_FLAG_HIDDEN) is set.
  • typeof(sym) should return "symbol" rather than string. This is done for Duktape hidden symbols too.
  • ToString(sym) must be rejected for a symbol, while String(sym) must specifically check for symbols. Coercion needs to strip possible "unique suffix" when coming up with the Symbol description.
  • Symbols should be safe from accidental enumeration, JSON serialization, etc. This is actually already the case because internal keys are already excluded in Duktape 1.x.
  • Object.getOwnPropertySymbols() should return a list of symbol properties for an object, but filter out Duktape hidden symbols.
  • Object(sym) should create an object with internal class "Symbol", with the plain symbol value stored behind _Value (hidden symbol property) as for Number objects, etc.
  • Non-strict comparison needs to handle symbols. ToPrimitive() coercion is maybe enough to ensure sym == Object(sym) is accepted.
  • Property code needs to accept plain Symbols as is (treated like any other strings), and Symbol objects should look up their internal string value (instead of being coerced to e.g. Symbol(symbolDescription). Current code just uses ToString() which causes a TypeError.
  • Dozens of similar semantics checks throughout the code base.

Some design questions

How should C code see Symbols?

Easiest approach:

  • Symbols are not enumerated by duk_enum() unless requested. Either fold in with internal keys, add a separate flags. Maybe rename existing internal keys flag.
  • Property operations work with symbols and internal keys without distinction.
  • API call to create a symbol from C code. Hides the construction of the internal string.

Best naming for Duktape internal keys

With Duktape internal properties would become unreachable from ECMAScript code, even if you construct the internal string using a buffer and then try to use it as an object key. This offers more protection for sandboxing than ES2015 Symbols which can be enumerated.

Current naming for Duktape 1.x internal keys is "hidden symbols". Some alternatives considered:

  • Internal symbol: easy to confuse with specification symbols for example. One benefit would be that as a term close to "internal property".
  • Hidden symbol: conveys semantics (assuming GH-797) pretty well.
  • Private symbol
  • Native symbol
  • Invisible symbol