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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef js_Id_h
#define js_Id_h
// [SMDOC] PropertyKey / jsid
// A PropertyKey is an identifier for a property of an object which is either a
// 31-bit unsigned integer, interned string or symbol.
// Also, there is an additional PropertyKey value, PropertyKey::Void(), which
// does not occur in JS scripts but may be used to indicate the absence of a
// valid key. A void PropertyKey is not a valid key and only arises as an
// exceptional API return value. Embeddings must not pass a void PropertyKey
// into JSAPI entry points expecting a PropertyKey and do not need to handle
// void keys in hooks receiving a PropertyKey except when explicitly noted in
// the API contract.
// A PropertyKey is not implicitly convertible to or from a Value; JS_ValueToId
// or JS_IdToValue must be used instead.
// jsid is an alias for JS::PropertyKey. New code should use PropertyKey instead
// of jsid.
#include "mozilla/Maybe.h"
#include "jstypes.h"
#include "js/GCAnnotations.h"
#include "js/HeapAPI.h"
#include "js/RootingAPI.h"
#include "js/TraceKind.h"
#include "js/TracingAPI.h"
#include "js/TypeDecls.h"
namespace js {
class JS_PUBLIC_API GenericPrinter;
class JSONPrinter;
} // namespace js
namespace JS {
enum class SymbolCode : uint32_t;
class PropertyKey {
uintptr_t asBits_;
// All keys with the low bit set are integer keys. This means the other type
// tags must all be even. These constants are public only for the JITs.
static constexpr uintptr_t IntTagBit = 0x1;
// Use 0 for StringTypeTag to avoid a bitwise op for atom <-> id conversions.
static constexpr uintptr_t StringTypeTag = 0x0;
static constexpr uintptr_t VoidTypeTag = 0x2;
static constexpr uintptr_t SymbolTypeTag = 0x4;
// (0x6 is unused)
static constexpr uintptr_t TypeMask = 0x7;
static constexpr uint32_t IntMin = 0;
static constexpr uint32_t IntMax = INT32_MAX;
constexpr PropertyKey() : asBits_(VoidTypeTag) {}
static constexpr MOZ_ALWAYS_INLINE PropertyKey fromRawBits(uintptr_t bits) {
PropertyKey id;
id.asBits_ = bits;
return id;
bool operator==(const PropertyKey& rhs) const {
return asBits_ == rhs.asBits_;
bool operator!=(const PropertyKey& rhs) const {
return asBits_ != rhs.asBits_;
MOZ_ALWAYS_INLINE bool isVoid() const {
MOZ_ASSERT_IF((asBits_ & TypeMask) == VoidTypeTag, asBits_ == VoidTypeTag);
return asBits_ == VoidTypeTag;
MOZ_ALWAYS_INLINE bool isInt() const { return !!(asBits_ & IntTagBit); }
MOZ_ALWAYS_INLINE bool isString() const {
return (asBits_ & TypeMask) == StringTypeTag;
MOZ_ALWAYS_INLINE bool isSymbol() const {
return (asBits_ & TypeMask) == SymbolTypeTag;
MOZ_ALWAYS_INLINE bool isGCThing() const { return isString() || isSymbol(); }
constexpr uintptr_t asRawBits() const { return asBits_; }
MOZ_ALWAYS_INLINE int32_t toInt() const {
uint32_t bits = static_cast<uint32_t>(asBits_) >> 1;
return static_cast<int32_t>(bits);
MOZ_ALWAYS_INLINE JSString* toString() const {
// Use XOR instead of `& ~TypeMask` because small immediates can be
// encoded more efficiently on some platorms.
return reinterpret_cast<JSString*>(asBits_ ^ StringTypeTag);
MOZ_ALWAYS_INLINE JS::Symbol* toSymbol() const {
return reinterpret_cast<JS::Symbol*>(asBits_ ^ SymbolTypeTag);
js::gc::Cell* toGCThing() const {
return reinterpret_cast<js::gc::Cell*>(asBits_ & ~TypeMask);
GCCellPtr toGCCellPtr() const {
js::gc::Cell* thing = toGCThing();
if (isString()) {
return JS::GCCellPtr(thing, JS::TraceKind::String);
return JS::GCCellPtr(thing, JS::TraceKind::Symbol);
bool isPrivateName() const;
bool isWellKnownSymbol(JS::SymbolCode code) const;
// A void PropertyKey. This is equivalent to a PropertyKey created by the
// default constructor.
static constexpr PropertyKey Void() { return PropertyKey(); }
static constexpr bool fitsInInt(int32_t i) { return i >= 0; }
static constexpr PropertyKey Int(int32_t i) {
uint32_t bits = (static_cast<uint32_t>(i) << 1) | IntTagBit;
return PropertyKey::fromRawBits(bits);
static PropertyKey Symbol(JS::Symbol* sym) {
MOZ_ASSERT(sym != nullptr);
MOZ_ASSERT((uintptr_t(sym) & TypeMask) == 0);
return PropertyKey::fromRawBits(uintptr_t(sym) | SymbolTypeTag);
// Must not be used on atoms that are representable as integer PropertyKey.
// Prefer NameToId or AtomToId over this function:
// A PropertyName is an atom that does not contain an integer in the range
// [0, UINT32_MAX]. However, PropertyKey can only hold an integer in the range
// [0, IntMax] (where IntMax == 2^31-1). Thus, for the range of integers
// (IntMax, UINT32_MAX], to represent as a 'id', it must be
// the case id.isString() and id.toString()->isIndex(). In most
// cases when creating a PropertyKey, code does not have to care about
// this corner case because:
// - When given an arbitrary JSAtom*, AtomToId must be used, which checks for
// integer atoms representable as integer PropertyKey, and does this
// conversion.
// - When given a PropertyName*, NameToId can be used which does not need
// to do any dynamic checks.
// Thus, it is only the rare third case which needs this function, which
// handles any JSAtom* that is known not to be representable with an int
// PropertyKey.
static PropertyKey NonIntAtom(JSAtom* atom) {
MOZ_ASSERT((uintptr_t(atom) & TypeMask) == 0);
return PropertyKey::fromRawBits(uintptr_t(atom) | StringTypeTag);
// The JSAtom/JSString type exposed to embedders is opaque.
static PropertyKey NonIntAtom(JSString* str) {
MOZ_ASSERT((uintptr_t(str) & TypeMask) == 0);
return PropertyKey::fromRawBits(uintptr_t(str) | StringTypeTag);
// This API can be used by embedders to convert pinned (aka interned) strings,
// as created by JS_AtomizeAndPinString, into PropertyKeys. This means the
// string does not have to be explicitly rooted.
// Only use this API when absolutely necessary, otherwise use JS_StringToId.
static PropertyKey fromPinnedString(JSString* str);
// Internal API!
// All string PropertyKeys are actually atomized.
MOZ_ALWAYS_INLINE bool isAtom() const { return isString(); }
MOZ_ALWAYS_INLINE bool isAtom(JSAtom* atom) const {
return isAtom() && toAtom() == atom;
MOZ_ALWAYS_INLINE JSAtom* toAtom() const {
return reinterpret_cast<JSAtom*>(toString());
MOZ_ALWAYS_INLINE JSLinearString* toLinearString() const {
return reinterpret_cast<JSLinearString*>(toString());
#if defined(DEBUG) || defined(JS_JITSPEW)
void dump() const;
void dump(js::GenericPrinter& out) const;
void dump(js::JSONPrinter& json) const;
void dumpFields(js::JSONPrinter& json) const;
void dumpPropertyName(js::GenericPrinter& out) const;
void dumpStringContent(js::GenericPrinter& out) const;
static bool isNonIntAtom(JSAtom* atom);
static bool isNonIntAtom(JSString* atom);
} // namespace JS
using jsid = JS::PropertyKey;
namespace JS {
// Handle<PropertyKey> version of PropertyKey::Void().
extern JS_PUBLIC_DATA const JS::HandleId VoidHandlePropertyKey;
template <>
struct GCPolicy<jsid> {
static void trace(JSTracer* trc, jsid* idp, const char* name) {
// This should only be called as part of root marking since that's the only
// time we should trace unbarriered GC thing pointers. This will assert if
// called at other times.
TraceRoot(trc, idp, name);
static bool isValid(jsid id) {
return !id.isGCThing() ||
static bool isTenured(jsid id) {
return true;
#ifdef DEBUG
MOZ_ALWAYS_INLINE void AssertIdIsNotGray(jsid id) {
if (id.isGCThing()) {
* Get one of the well-known symbols defined by ES6 as PropertyKey. This is
* equivalent to calling JS::GetWellKnownSymbol and then creating a PropertyKey.
* `which` must be in the range [0, WellKnownSymbolLimit).
extern JS_PUBLIC_API PropertyKey GetWellKnownSymbolKey(JSContext* cx,
SymbolCode which);
* Generate getter/setter id for given id, by adding "get " or "set " prefix.
extern JS_PUBLIC_API bool ToGetterId(
JSContext* cx, JS::Handle<JS::PropertyKey> id,
JS::MutableHandle<JS::PropertyKey> getterId);
extern JS_PUBLIC_API bool ToSetterId(
JSContext* cx, JS::Handle<JS::PropertyKey> id,
JS::MutableHandle<JS::PropertyKey> setterId);
} // namespace JS
namespace js {
template <>
struct BarrierMethods<jsid> {
static gc::Cell* asGCThingOrNull(jsid id) {
if (id.isGCThing()) {
return id.toGCThing();
return nullptr;
static void writeBarriers(jsid* idp, jsid prev, jsid next) {
if (prev.isString()) {
if (prev.isSymbol()) {
postWriteBarrier(idp, prev, next);
static void postWriteBarrier(jsid* idp, jsid prev, jsid next) {
MOZ_ASSERT_IF(next.isString(), !gc::IsInsideNursery(next.toString()));
static void exposeToJS(jsid id) {
if (id.isGCThing()) {
static void readBarrier(jsid id) {
if (id.isGCThing()) {
// If the jsid is a GC pointer type, convert to that type and call |f| with the
// pointer and return the result wrapped in a Maybe, otherwise return None().
template <typename F>
auto MapGCThingTyped(const jsid& id, F&& f) {
if (id.isString()) {
return mozilla::Some(f(id.toString()));
if (id.isSymbol()) {
return mozilla::Some(f(id.toSymbol()));
using ReturnType = decltype(f(static_cast<JSString*>(nullptr)));
return mozilla::Maybe<ReturnType>();
// If the jsid is a GC pointer type, convert to that type and call |f| with the
// pointer. Return whether this happened.
template <typename F>
bool ApplyGCThingTyped(const jsid& id, F&& f) {
return MapGCThingTyped(id,
[&f](auto t) {
return true;
template <typename Wrapper>
class WrappedPtrOperations<JS::PropertyKey, Wrapper> {
const JS::PropertyKey& id() const {
return static_cast<const Wrapper*>(this)->get();
bool isVoid() const { return id().isVoid(); }
bool isInt() const { return id().isInt(); }
bool isString() const { return id().isString(); }
bool isSymbol() const { return id().isSymbol(); }
bool isGCThing() const { return id().isGCThing(); }
int32_t toInt() const { return id().toInt(); }
JSString* toString() const { return id().toString(); }
JS::Symbol* toSymbol() const { return id().toSymbol(); }
bool isPrivateName() const { return id().isPrivateName(); }
bool isWellKnownSymbol(JS::SymbolCode code) const {
return id().isWellKnownSymbol(code);
uintptr_t asRawBits() const { return id().asRawBits(); }
// Internal API
bool isAtom() const { return id().isAtom(); }
bool isAtom(JSAtom* atom) const { return id().isAtom(atom); }
JSAtom* toAtom() const { return id().toAtom(); }
JSLinearString* toLinearString() const { return id().toLinearString(); }
} // namespace js
#endif /* js_Id_h */