[JSC] JSObject::makePropertiesImmutable(): an object's own properties and prototype stop changing, and its property attributes stay as they are

[dylan-conway]

What this does

Adds JSObject::makePropertiesImmutable(VM&) and JSObject::hasImmutableProperties(). Nothing in JavaScriptCore calls them and there is no JavaScript-visible API; the jsc shell gets $vm.makePropertiesImmutable(object) and $vm.hasImmutableProperties(value) for the tests.

It makes the object non-extensible and stops its own properties and its prototype from changing. Unlike Object.freeze(), it does not touch property attributes: a data property of such an object still reads as writable: true. So nothing changes for the objects that inherit from it. An assignment on an inheritor that finds the name on such a prototype creates an own property on the inheritor, as it always did; with a frozen prototype that assignment fails.

"Immutable" is used as it is for an immutable prototype exotic object, whose [[SetPrototypeOf]] succeeds for the prototype it already has and fails otherwise: this extends the same rule to the object's own properties.

On an object with immutable properties	Result
[[Set]] that starts at it, with it as the receiver (an assignment): data property, accessor or custom setter, own or inherited, named or indexed	fails (false; TypeError in strict mode), the value the property has included. A setter is not called.
[[Set]] that starts at it with another receiver: assignment on an inheritor, Reflect.set(object, key, value, receiver)	as before: the receiver decides
[[Set]] that starts on another object with it as the receiver: Reflect.set(other, key, value, object), super.key = value	the ordinary steps: its [[DefineOwnProperty]] decides, so it fails unless it changes nothing; a setter of the other object runs
[[DefineOwnProperty]]	succeeds if it changes nothing, fails otherwise
[[Delete]]	false for a property it has, true for one it does not have
[[SetPrototypeOf]]	succeeds for the prototype it already has, fails otherwise
[[PreventExtensions]] / [[IsExtensible]]	true / false: it already is non-extensible
Object.seal(), Object.freeze()	TypeError, unless there is nothing for them to change
Error.captureStackTrace()	TypeError: it defines a stack property
adding a private field or a private method	TypeError
writing a private field it already has	works: that value is the object's private state, as an internal slot is
reads, enumeration, calls	as before
{ ...object }, Object.assign({}, object)	the copy is an ordinary object

Internal slots are not properties and are not affected: such a Map still takes set(), such a Date still takes setTime().

How

• One bit, one transition. Structure gets a hasImmutableProperties flag, in the family of hasNonConfigurableProperties, set only by a new non-property transition, TransitionKind::MakePropertiesImmutable, which also prevents extensions. Every later transition inherits the flag. The operation always moves the object to a new structure, so every inline cache and every compiled store made for the old structure stops matching. Two things about the new structure are as they were for the old one: it stays watchable (the transition clears transitionWatchpointIsLikelyToBeFired, which a structure inherits from a watched predecessor, so the for-in enumerator cache and the DFG go on watching a built-in prototype), and it keeps didTransition(), the bit behind JSObject::hasCustomProperties(), since the transition adds and changes no property.
• The checks are on the operations a program can observe, and refuse by default, so their callers need no change. A direct define (a class field, the indexed direct define) reaches defineOwnProperty() for such an object, by the path a non-extensible object already takes, so it succeeds exactly when [[DefineOwnProperty]] would. A put that starts on another object and lands on such a receiver takes the generic receiver path (definePropertyOnReceiverSlow(), the specification's steps) and not the shortcuts that put, so [[DefineOwnProperty]] decides on every route, and a native setter of the receiver is not called. Error.captureStackTrace() throws. The primitives: putDirectInternal in every mode, putDirectWithoutTransition and its accessor variants, putDirectIndex, setPrototypeDirect, putInlineSlow, putByIndex, deleteProperty, validateAndApplyPropertyDescriptor, and the prototype, attribute, seal, freeze and prevent-extensions paths of JSObject. The transition factories behind those assert that JSObject refused first. Changes of representation are not refused: a dictionary conversion, an element-storage conversion, becoming a prototype, the realm starting to have a bad time. The one exception is ensureArrayStorageSlow(), which compiled code calls before it stores an element in place: such an object keeps the storage it has.
• The put fast path gains no instruction. The flag joins the bit test canPerformFastPutInlineExcludingProto() already makes for the receiver (both are bits of the same field), which sends such a put to putInlineSlow(), where it is refused. putDirectInternal() tests the flag once and refuses, unless the engine is materializing a property.
• Lazily materialized properties still appear. Static property tables, a function's name, length and prototype, an error's stack, line and column, and an arguments object's callee and Symbol.iterator are properties the object logically already has. Those writes run under an AllowLazyMaterializationOfImmutableProperties (a counted scope on the VM, which counts as a DisallowVMEntry too), which is never around anything that can run JavaScript. The operation therefore does not reify static properties first, and a built-in prototype keeps a cacheable structure.
• No tier keeps a write fast path into such an object. tryCachePutBy and tryCacheArrayPutByVal give up for a structure with the flag (a refused delete is never cacheable), and PutByStatus::computeFor reports the slow path. The store inside a for-in loop already tests a mask of structure bits before it writes by offset, in the interpreter, the baseline JIT, the DFG and the FTL: the flag joins that mask, so reads through for-in keep their fast path. The C++ paths that write slots directly check the flag (that store's slow path, the JSON reviver walk) or are already behind canPerformFastPutInlineExcludingProto() and an extensibility test (Object.assign() and its batching). A copy ({ ...object }, Object.assign({}, object)) takes the structure the object had before the transition, as it already does for a frozen object, so the copy is fast and ordinary.
• Elements. Compiled code decides an element store from the indexing type in the cell header, not from the structure, so the elements have to be in storage it does not store into in place. There are two such kinds.
  • A JSArray whose elements are Int32, Double or Contiguous without holes, and which has no named properties, keeps them: they are copied into a JSCellButterfly of the same shape and the array's indexing type gains CopyOnWrite (an array literal is there already and keeps its cell). Every tier reads copy-on-write storage in place, built-ins included, and none stores into it in place: the interpreter tests the bit, the DFG and FTL include it in the check for a write or ask tryMakeWritable*Slow() first, and C++ writers call ensureWritable() or convertFromCopyOnWrite(). JSObject::tryMakeWritable() is ensureWritable() that returns false for such an array instead of converting, and replaces it in the writers that are not behind one of the checks above: JSArray::fastFill, fastCopyWithin, appendMemcpy, fastShift, Array.prototype.reverse, JSArray::put (which converts before any put, whoever the receiver is) and switchToSlowPutArrayStorage (slow-put storage makes a store into a hole consult the prototype chain, and such an array takes no stores, so it is left as it is when the realm starts having a bad time). For an array that is not copy-on-write it is the same test ensureWritable() makes. The copy-on-write branch of the three tryMakeWritable*Slow() returns nothing, which compiled code treats as a failed conversion, and convertFromCopyOnWrite() release-asserts that the flag is absent. Options::useCopyOnWriteArraysForImmutableProperties (default true) turns this off; every array then takes the path below.
  • Any other object with elements (holes, named properties on the array, undecided or sparse storage, an array-like object, an arguments object) goes to dictionary indexing mode, so every indexed store, delete and length change reaches a C++ path that refuses. Reads of those elements cost what they cost for a frozen object. An object without element storage, which is every built-in prototype, is left as it is.
• Built-ins that ask whether an object is pristine get the answer they got before. Most ask by structure identity and then by hasCustomProperties(), which is unchanged (above). canUseDefaultArrayJoinForToString() and JSArray::isToPrimitiveFastAndNonObservable() ask by identity only: after that test fails they also accept the structure one MakePropertiesImmutable transition from an original array structure, which has the same (no) properties and the same prototype.
• A class with write hooks of its own can change an object before, or without, reaching JSObject's checks, so makePropertiesImmutable() supports only the classes whose hooks were read for this. ErrorInstance, StringObject, RegExpObject and ClonedArguments need no change: their hooks only materialize lazy properties, or refuse on their own account, before they call JSObject's implementation. JSFunction::put and JSFunction::defineOwnProperty (which reset the allocation profile and put a lazily made prototype directly) and JSArray::defineOwnProperty (which can flip the length-writable flag) hand such an object to JSObject's implementation first; JSArray::setLength, pop, push, and the shift and unshift fast paths, which write storage in place, refuse, each inside the cases such an array can be in (copy-on-write storage, no storage, array storage) and never on the path of a writable Int32, Double or Contiguous array. JSArray::put refuses an assignment to length before it converts the value, which can run user code; defining length converts first, as ArraySetLength does, and then succeeds only for the length the array has. JSGlobalObject is covered below. The decision is made from the method table: an object is supported if its write hooks are JSObject's or exactly those of one of these classes, so a subclass that brings a hook of its own is not. For any other class, makePropertiesImmutable() returns false and changes nothing: ProxyObject, typed arrays, DirectArguments and ScopedArguments (a mapped element is a view of a parameter variable, which the function can still assign), and classes outside JavaScriptCore that override a write hook, a C API global object among them.
• RegExpObject. Compiled code tests the object's own lastIndex-is-writable flag rather than its structure, so the operation sets that flag, and fires the realm's regExpLastIndexWritableWatchpointSet as RegExpObject::defineOwnProperty() does when it sets it, and lastIndex then reads as non-writable. This is the one attribute that changes. Matching that has to update lastIndex fails as it does for a frozen RegExp. An object that inherits from such a RegExp cannot assign lastIndex either, as with a frozen RegExp. The fast path of RegExp.prototype[@@search] skips the two writes of lastIndex the generic one makes, so it requires a writable lastIndex, as before, or, for a RegExp with immutable properties only, that nothing would be written: an expression that is neither global nor sticky, with a lastIndex of 0. A frozen RegExp takes the path it took.
• JSGlobalObject. Its properties go through JSObject's paths. Its top-level var and function declarations live in its symbol table, whose slots compiled code writes directly, so the operation marks each entry read-only and fires varReadOnlyWatchpointSet: the same two steps JSGlobalObject::defineOwnProperty takes when a script makes a global variable non-writable. Those bindings then read as non-writable, the second attribute that changes, and an object that inherits from the global object cannot assign those names, as with a frozen global object. A later script cannot declare a new top-level var or function, or replace an existing function. Top-level let, const and class live in the global lexical environment, a separate object, and are not affected. makePropertiesImmutable() and hasImmutableProperties() on a JSGlobalProxy act on its target.

A program that never calls it takes the same paths as before. What it executes in addition is a test of a bit of a structure that is already loaded, on the slow paths listed above and where an array literal's copy-on-write storage is converted before its first write.

Tests

Ten new files in JSTests/stress, each written to the suite's rules (loop counts from testLoopCount, under 200 ms in the release configurations) and run through run-jsc-stress-tests in its seventeen modes, and with the default tiers, --useJIT=0, --useDFGJIT=0, --useFTLJIT=0, and low tier-up thresholds with --useConcurrentJIT=0, with --useCopyOnWriteArraysForImmutableProperties on and off, on a release build and on a build with ENABLE_ASSERTS=ON:

• immutable-properties-basic.js: every row of the table above for an ordinary object, in sloppy and strict mode and through Reflect; definitions that change nothing, with full and partial descriptors; attributes before and after; bulk writers; a JSON reviver that makes its holder's properties immutable; the classes that do not support it.
• immutable-properties-inheritors.js: Object.create() of such a prototype; such a class instantiated and extended; Error subclasses that assign name with Error.prototype's properties immutable, and a constructor that assigns toString with Object.prototype's; a setter on such a prototype with an ordinary receiver and with one whose own properties are immutable; Reflect.set() with a separate receiver in both directions, and with a receiver whose static properties are not reified yet.
• immutable-properties-classes.js: functions, errors, RegExp objects, String objects and unmapped arguments objects, including the properties they materialize lazily afterwards; built-in prototypes and constructors, then used; Error.captureStackTrace(), after which an error still has the stack it had yet to materialize; an inheritor of such a RegExp; a subclass with a write hook of its own; an empty array, a filled array, an array-like function and Array.prototype across the realm starting to have a bad time.
• immutable-properties-array-storage.js: which arrays become copy-on-write arrays (Int32, Double, Contiguous, a literal, a derived class, one used as a prototype, 20,000 elements), with the shape kept; forty-six mutators made hot on ordinary arrays first, plus Object.freeze() and Object.seal(), against each of them, comparing a full property snapshot, the prototype and the indexing mode. Run by the runner a second time with --useCopyOnWriteArraysForImmutableProperties=0, as the next file is.
• immutable-properties-array-other-storage.js: the same mutators against the nine kinds that get dictionary indexing instead (holes, a named property, no elements yet, sparse, array storage, an array-like, an arguments object); an assignment to length whose value has a valueOf, which is not called; and last, the realm starting to have a bad time, with copy-on-write arrays and arrays that have no element storage, which must get none that can be stored into in place.
• immutable-properties-array-reads.js: reads of such arrays against an ordinary array of the same contents; an inheritor, which reaches JSArray::put with another receiver; full and eden collections with the elements reachable only through the array. An allocation site learns from what becomes of its arrays, so the arrays that have to start without holes, in Int32, Double or Contiguous shape, come from sites of their own and are built before anything else runs, and each one is asserted to be in copy-on-write storage before it is attacked.
• immutable-properties-pristine-objects.js: a RegExp that is neither global nor sticky against an ordinary one, hot, through replace, split, match, search, test, exec, matchAll and Symbol.replace; a global, a sticky and a global sticky one against a frozen one; search with a lastIndex of 3, which still fails; arrays through join, toString, template literals, slice, concat, map and the rest against ordinary ones; a RegExp with its own exec and arrays with their own constructor or join, made immutable afterwards, which are still honoured; Array.prototype.join replaced; a Promise, a bound function, an arguments object, a Map iterator and the spread clone.
• immutable-properties-private-names.js: adding a private field, a private method and a public field through a constructor that returns its argument, hot and cold; writing a private field added before; a public field that changes nothing, which succeeds as Object.defineProperty() does, with the equal value arriving as a rope, another string cell, a computed number, another BigInt cell, NaN or a symbol, against fields that do change something (-0 over +0 among them), for named and index keys.
• immutable-properties-compiled-code.js: named, keyed and indexed stores, adds, deletes, the for-in store and the spread clone, made hot and megamorphic and run against each target before, then run again after: plain, dictionary-mode, used-as-prototype, function, array and object-with-indexed-properties targets.
• immutable-properties-global-object.js: the global object, after a hot compiled writer of a property and of a top-level var; properties, the implicit global of sloppy code, var and function declarations of later scripts, and an object that inherits from the global object.

Also run on this branch with the jsc shell:

• A differential fuzzer for copy-on-write arrays: an array of a random Int32, Double or Contiguous kind and size, without holes, (one in six also used as a prototype) is made immutable and required to be in copy-on-write storage, then given a random sequence of 47 kinds of write, each also kept hot on ordinary arrays through the same function, and 26 kinds of read compared with an ordinary array of the same contents, with full collections at random. After every write the property snapshot and the indexing mode must be unchanged. Five tier configurations: 6.2 million operations on the build with assertions and 29.6 million on the release build, and with the option off 2.2 million and 7.4 million. No difference, no assertion.
• The 1,657 files of JSTests/stress whose names relate to arrays, copy-on-write, freezing, sealing, spread, RegExp, slicing, sorting and arguments, with the option on and off: 1,596 pass, 51 are marked skip, and the other ten (timeouts, tests that need options from their runner annotations, and two that need more memory than the run allowed) end the same way on a build from before the last two commits.
• All 5,928 files of JSTests/stress with the properties of every built-in reachable from the global object made immutable first. Tests that write to built-ins fail, as they must; the point is the exit codes, which equal those of a build from before the last two commits, apart from seven files that end the same way on both when run alone.

Performance

Measured with the engine changes above on 35e8970 (the commit they were developed on) through a Bun release build that exposes the two functions; medians of nine interleaved rounds on pinned cores, each ratio beside the same measurement of the unchanged build against itself.

• A program that never calls it. 103 workloads, one per operation this touches (puts, defines, deletes, for-in, spread, Object.assign, array methods, Proxy, Reflect, JSON, classes, private names, RegExp): geometric mean 1.0013 of the unchanged build's time (the unchanged build against itself: 1.0012). Two workloads differ by more than their own noise when rerun alone: fill + copyWithin takes 8% longer while executing 0.7% fewer instructions, and a Proxy [[Set]] without a trap executes 1% more (about 14 of 1,430 per set: the flag tests on the C++ put and define paths), while a Proxy [[Get]] and Reflect.set() execute fewer.
• Ordinary code after every built-in is made immutable (the 964 prototypes, constructors, functions and namespace objects reachable from the global object), against the same walk without the operation: 0.9961 over the same 103 workloads.
• Reads of the object itself, as a multiple of the same object left ordinary, with a frozen one beside it:

	immutable	frozen
copy-on-write array: index loops (Int32, Double, Contiguous, 5,000 elements), for…of, destructuring, entries/keys/at, forEach/map/filter/reduce, spread, JSON.stringify, apply, join/toString, toReversed/with	0.91 – 1.05	2.6 – 19.8
copy-on-write array: find/some/every	0.79	22.8
copy-on-write array: indexOf/includes, Array.from, reads through an array of objects, slice + concat	1.09, 1.09, 1.08, 1.20	69.5, 6.6, 11.6, 27.7
array with holes, array-like object (index loop, Array.prototype.map.call), arguments object	14.8, 16.4 and 6.1, 8.1	15.1, 16.1 and 3.3, 8.1
RegExp, not global, varying input: split, exec, test, replace, match, search	1.0, 1.0, 1.2, 1.4, 1.4, 1.6	22.9, 1.0, 1.3, 7.0, 8.4, 3.9
Object.isFrozen() / isSealed() of a record	1.26	1.54

What is above 1.0 for a copy-on-write array or a RegExp is the optimizing compiler's inlined form of the call, which checks for the one original structure; the C++ fast path behind it is taken.

• An allocation site learns from what becomes of its arrays. Freezing arrays that came from map makes later map results in the program ArrayWithArrayStorage, and so does giving them dictionary indexing here. Copy-on-write storage does not: later map results stay ArrayWithInt32.

Also run through that build, with the option at its default:

• A write-route harness: 44 kinds of object, each given immutable properties and then attacked through about 95 ways of writing to an object (assignment forms, Reflect, Object.*, array and string methods, class fields and private names through return override, JSON.parse revivers, Error.captureStackTrace, legacy __define*__, structuredClone, and so on), comparing a full property snapshot before and after: 3,800 runs, no such object changed.
• A tier harness: every store form driven hot before and run again after, 308 scenarios, in default tiers, useJIT=0, useDFGJIT=0 and useFTLJIT=0: no such object changed. The same for built-ins that run user code part-way through an operation (a sort comparator, a valueOf, a getter) where that code makes the properties of the object being written immutable: 60 scenarios, no change afterwards.
• A differential fuzzer over objects of every supported class (random shaping, then the operation, then random writes; any observable change is a bug): 12 seeds in three tier configurations: no bugs.

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Walkthrough

JavaScriptCore adds APIs and structure state for immutable properties. It updates object, array, runtime, and optimized property-write paths to account for that state. New stress tests cover mutation rules, array storage, inherited writes, lazy properties, built-ins, and compiled stores.

Changes

Immutable properties

Layer / File(s)	Summary
Structure state and object rules Source/JavaScriptCore/runtime/Structure*, Source/JavaScriptCore/runtime/JSObject*, Source/JavaScriptCore/runtime/VM.h, Source/JavaScriptCore/tools/JSDollarVM.cpp, JSTests/stress/immutable-properties-basic.js, JSTests/stress/immutable-properties-private-names.js, JSTests/stress/immutable-properties-inheritors.js	Structures gain an immutable-properties transition and flag. JSObject implements the operation, property rules, and private-field handling. $vm exposes the operation and state query. Stress tests cover object mutation, private state, and inherited writes.
Array storage and mutation paths Source/JavaScriptCore/runtime/OptionsList.h, Source/JavaScriptCore/runtime/JSArray*, Source/JavaScriptCore/runtime/ArrayPrototype.cpp, Source/JavaScriptCore/runtime/JSGlobalObject*, JSTests/stress/immutable-properties-array-*	Array mutation paths check whether storage can be made writable and reject writes to immutable arrays. Packed-array storage can use copy-on-write, controlled by a new option. Tests cover packed and non-packed arrays, storage modes, reads, and mutation attempts.
Runtime operations and lazy properties Source/JavaScriptCore/runtime/ClonedArguments.cpp, Source/JavaScriptCore/runtime/Error*, Source/JavaScriptCore/runtime/JSFunction.cpp, Source/JavaScriptCore/runtime/JSGlobalObject.cpp, Source/JavaScriptCore/runtime/JSONObject.cpp, Source/JavaScriptCore/runtime/Lookup.*, Source/JavaScriptCore/runtime/ObjectConstructor*, Source/JavaScriptCore/runtime/RegExp*, JSTests/stress/immutable-properties-classes.js, JSTests/stress/immutable-properties-global-object.js, JSTests/stress/immutable-properties-pristine-objects.js	Runtime paths account for immutable properties when materializing properties or performing operations on functions, errors, globals, and RegExp objects. Tests cover these paths and related built-in behavior.
Optimized property-write paths Source/JavaScriptCore/bytecode/*, Source/JavaScriptCore/dfg/DFGSpeculativeJIT64.cpp, Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp, Source/JavaScriptCore/jit/JITPropertyAccess.cpp, Source/JavaScriptCore/llint/LowLevelInterpreter64.asm, Source/JavaScriptCore/runtime/CommonSlowPathsInlines.h, JSTests/stress/immutable-properties-compiled-code.js	Write-status checks, inline caches, and enumerator store paths route immutable structures away from direct writes. The compiled-code stress test checks store attempts and an ordinary-object control.

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Title check	✅ Passed	The title clearly identifies the primary change: adding immutable-property behavior to JSObject while preserving property attributes. It is specific and related to the changeset.
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• JSTests/stress/immutable-properties-array-storage.js
• JSTests/stress/immutable-properties-basic.js
• JSTests/stress/immutable-properties-classes.js
• JSTests/stress/immutable-properties-compiled-code.js
• JSTests/stress/immutable-properties-global-object.js
• JSTests/stress/immutable-properties-inheritors.js
• JSTests/stress/immutable-properties-pristine-objects.js
• JSTests/stress/immutable-properties-private-names.js
• Source/JavaScriptCore/bytecode/PutByStatus.cpp
• Source/JavaScriptCore/bytecode/Repatch.cpp
• Source/JavaScriptCore/dfg/DFGSpeculativeJIT64.cpp
• Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp
• Source/JavaScriptCore/jit/JITPropertyAccess.cpp
• Source/JavaScriptCore/llint/LowLevelInterpreter64.asm
• Source/JavaScriptCore/runtime/ArrayPrototype.cpp
• Source/JavaScriptCore/runtime/ClonedArguments.cpp
• Source/JavaScriptCore/runtime/CommonSlowPaths.h
• Source/JavaScriptCore/runtime/CommonSlowPathsInlines.h
• Source/JavaScriptCore/runtime/ErrorInstance.cpp
• Source/JavaScriptCore/runtime/JSArray.cpp
• Source/JavaScriptCore/runtime/JSArrayInlines.h
• Source/JavaScriptCore/runtime/JSFunction.cpp
• Source/JavaScriptCore/runtime/JSGlobalObject.cpp
• Source/JavaScriptCore/runtime/JSGlobalObject.h
• Source/JavaScriptCore/runtime/JSGlobalObjectInlines.h
• Source/JavaScriptCore/runtime/JSONObject.cpp
• Source/JavaScriptCore/runtime/JSObject.cpp
• Source/JavaScriptCore/runtime/JSObject.h
• Source/JavaScriptCore/runtime/JSObjectInlines.h
• Source/JavaScriptCore/runtime/Lookup.cpp
• Source/JavaScriptCore/runtime/Lookup.h
• Source/JavaScriptCore/runtime/ObjectConstructor.cpp
• Source/JavaScriptCore/runtime/ObjectConstructorInlines.h
• Source/JavaScriptCore/runtime/OptionsList.h
• Source/JavaScriptCore/runtime/RegExpObject.h
• Source/JavaScriptCore/runtime/RegExpObjectInlines.h
• Source/JavaScriptCore/runtime/RegExpPrototype.cpp
• Source/JavaScriptCore/runtime/Structure.cpp
• Source/JavaScriptCore/runtime/Structure.h
• Source/JavaScriptCore/runtime/StructureTransitionTable.h
• Source/JavaScriptCore/runtime/VM.h
• Source/JavaScriptCore/tools/JSDollarVM.cpp

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[claude]

Beyond the inline findings, I also looked at the had-a-bad-time path for an immutable copy-on-write array: switchToSlowPutArrayStorage now leaves such an array packed instead of converting to slow-put storage, which breaks the usual "every array is slow-put after a bad time" invariant, but since no tier or C++ path stores into that array it is not reachable as a bug, so it was ruled out as intentional. Note the bug hunt stopped at its finding cap rather than running dry, so the inline comments are not an exhaustive pass over this change.

Extended reasoning...

The change adds a new Structure bit and transition kind plus refusal checks across all four tiers and roughly thirty runtime files, with eight confirmed inline findings on receiver-mismatch, error stack, global-object and array length/shift paths. The hunt exited at max_bugs, which with the size of the change rules out approval; the ruled-out note records the one invariant question examined and settled as intentional.

Findings marked 🟡 are optional suggestions and need no follow-up push.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🔴 Source/JavaScriptCore/runtime/JSObject.cpp — A [[Set]] whose receiver is an object with immutable properties can still run a native setter or report success when the name is a not-yet-reified static property. definePropertyOnReceiver at JSObject.cpp:1031-1032 sends such a receiver to putInlineFastReplacingStaticPropertyIfNeeded, which has no hasImmutableProperties check: a static CustomValue entry calls customSetter (JSObject.cpp:1054), and a static Function/Builtin/ConstantInteger entry calls putDirect, ignores its refusal and returns true (JSObject.cpp:1057-1058). Fix: refuse (false, TypeError in strict mode) for every receiver whose structure hasImmutableProperties before any static-table write, as definePropertyOnReceiverSlow already does at JSObject.cpp:988-989. [also at: Source/JavaScriptCore/runtime/JSObject.cpp:990 - Callers that write with an immutable built-in prototype as the receiver get true back, and a native setter can still run, instead of the refusal the PR promises. definePropertyOnReceiver (JSObject.cpp:1031) sends an immutable receiver that still has unreified static properties to…; Source/JavaScriptCore/runtime/JSObject.cpp:1058 - A [[Set]] whose receiver is an immutable object with a non-reified static property reports success and may run a native…]

  Why this was flagged

  Trigger: a put that starts on another object and lands on an immutable receiver, e.g. $vm.makePropertiesImmutable(JSON); Reflect.set({}, 'parse', 1, JSON) before JSON.parse has been reified. JSObject::put on the holder reaches definePropertyOnReceiver because the receiver differs; the receiver's defineOwnProperty is JSObject's and it has no reified accessor, so the slow-path checks at JSObject.cpp:1021-1029 are skipped and JSObject.cpp:1031-1032 calls putInlineFastReplacingStaticPropertyIfNeeded. There, for a CustomValue static entry the native setter runs (JSObject.cpp:1054); for a Function or other static entry putDirect is called, putDirectWhenPropertiesAreImmutable refuses it (JSObject.cpp:841-857), the bool is discarded and the function returns true (JSObject.cpp:1057-1058). Result after merge: Reflect.set reports true and strict-mode code throws nothing even though the object was not changed, and a custom setter runs on an object the PR says takes no setter calls; the PR's own slow-path guard at JSObject.cpp:988-989 does not cover this path. On the base branch the write simply succeeds, so the mismatch is new.

  Verification: putInlineFastReplacingStaticPropertyIfNeeded (JSObject.cpp:1036-1063) contains no hasImmutableProperties() test. For a CustomValue entry it calls customSetter(...) at :1052-1054. For a Function/Builtin entry it calls putDirect(...) at :1057, then ignores that result and return true at :1058. Result: Reflect.set reports true and strict-mode super.x = v does not throw.

[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

Still open from earlier reviews (7):

• 🔴 Source/JavaScriptCore/runtime/ErrorInstance.cpp:503 — Calling Error.captureStackTrace on an error whose properties were made immutable silently discards the error's lazily m…
• Also unresolved: 6 minor or pre-existing.

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🔴 Source/JavaScriptCore/runtime/JSArray.cpp — Scripts can crash the process: Array.prototype.concat whose Symbol.species constructor returns a packed array made immutable hits the RELEASE_ASSERT in convertFromCopyOnWrite. The JSArray* overload of JSArray::appendMemcpy at JSArray.cpp:1184 never calls tryEnsureWritable, so its ensureLength call at JSArray.cpp:1222 tries to convert the copy-on-write storage. Fix: every in-place element writer must refuse such an array; make this overload return false when tryEnsureWritable fails, as the span overload does at JSArray.cpp:1119, so concat falls back to moveArrayElements and the define is refused with a TypeError. On the base branch the call converts the storage and appends.

  Why this was flagged

  Trigger: let imm = $vm.makePropertiesImmutable([1, 2, 3]); let x = [0]; x.constructor = { [Symbol.species]: function () { return imm; } }; x.concat([4]). makePropertiesImmutable moves imm's Int32 elements to copy-on-write storage (JSObject.cpp:3130-3151). x has an own constructor, so arraySpeciesWatchpointIsValid fails and arrayProtoFuncConcat takes the generic path; speciesConstructArray (ArrayPrototypeInlines.h:138) calls construct, which returns imm, and ArrayPrototype.cpp:1844-1845 makes imm the result.

  Verification: normal — triggered when Array.prototype.concat's generic path obtains, through Symbol.species, a result array that was made immutable while packed (copy-on-write storage), and then spreads a JSArray into it.

  Mechanism verified in the code:

  • /home/claude/webkit/Source/JavaScriptCore/runtime/JSArray.cpp:1184-1248: the JSArray* otherArray overload of appendMemcpy was not changed by the diff and has no tryEnsureWritable check (the diff only added it to the span overload at JSArray.cpp:1119-1120).

• 🟡 Source/JavaScriptCore/runtime/JSObject.cpp — nit: Embedder C++ that calls putDirectAccessor or putDirectCustomAccessor on an object it made immutable gets false back, yet the shared immutable Structure is still marked as having read-only/accessor properties, and debug builds assert. putDirectNonIndexAccessor (JSObject.cpp:2386-2392) and putDirectCustomAccessor (JSObject.cpp:2353-2360) set the flags after putDirectInternal, which could not fail in PutModeDefineOwnProperty mode before this change and now returns ReadonlyPropertyChangeError. Fix: at both sites return early when putDirectInternal reports an error, before setContainsReadOnlyProperties and setHasAnyKindOfGetterSetterPropertiesWithProtoCheck run (and before the ASSERT at JSObject.cpp:2355).

  Why this was flagged

  Trigger: host C++ calls object->putDirectAccessor(...) or object->putDirectCustomAccessor(...) on an object after JSObject::makePropertiesImmutable(vm), outside an AllowLazyPropertyMaterialization scope. putDirectInternal at JSObjectInlines.h:508-511 calls putDirectWhenPropertiesAreImmutable (JSObject.cpp:865-877), which returns ReadonlyPropertyChangeError for a property the object does not have, so nothing is stored. putDirectNonIndexAccessor (JSObject.cpp:2386-2392) and putDirectCustomAccessor (JSObject.cpp:2353-2360) ignore that and still call structure->setContainsReadOnlyProperties() and structure->setHasAnyKindOfGetterSetterPropertiesWithProtoCheck() on the immutable structure, which the transition table shares with every object made immutable from the same predecessor; putDirectCustomAccessor also hits ASSERT(slot.type() == PutPropertySlot::NewProperty) at JSObject.cpp:2355. On the base branch PutModeDefineOwnProperty never fails, so the flags are only ever set after a property was really added.

  Verification: On the base, putDirectInternal can never return a non-null error. The diff adds at JSObjectInlines.h:508-511 a call to putDirectWhenPropertiesAreImmutable(), which at JSObject.cpp:864-876 returns ReadonlyPropertyChangeError. Neither caller checks the result before mutating the structure: JSObject.cpp:2353-2360 and JSObject.cpp:2386-2392.

[dylan-conway]

On the review summary's finding that had no inline thread (a put that starts on another object and lands on a receiver with immutable properties, while the receiver has static properties that are not reified): fixed in 1c22663. definePropertyOnReceiver() now sends such a receiver to definePropertyOnReceiverSlow(), the specification's steps, so the receiver's [[DefineOwnProperty]] decides on every route (ordinary key, special key, index, Proxy, super), nothing reports success for a change that did not happen, and a native setter of the receiver is not called. Reflect.set({}, 'parse', 1, JSON) returns false. Tests in immutable-properties-inheritors.js, including a class with native static setters.

🤖 Addressed by Claude Code

[dylan-conway]

On the two additional findings of the last review summary, which had no inline thread:

• Array.prototype.concat with a species constructor that returns a packed array with immutable properties (the RELEASE_ASSERT in convertFromCopyOnWrite()): fixed in 1c22663. The appendMemcpy() overload that takes another array returns false for such an array before ensureLength(), so concat takes its generic path and the define is refused with a TypeError. Tested for four storage kinds in immutable-properties-array-storage.js.
• putDirectCustomAccessor() and putDirectNonIndexAccessor() recording flags on the shared Structure after a refused put: fixed in e64f3d3. Both return before they touch the Structure (and before the first one's assertion) when the put was refused.

🤖 Addressed by Claude Code

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🔴 Source/JavaScriptCore/runtime/JSObject.cpp — Scripts get true back from a refused assignment when the receiver is an immutable object holding a not-yet-reified static property. In putInlineSlow's prototype walk at JSObject.cpp:909-926 the CustomValue and BuiltinOrFunctionOrLazyProperty shortcuts fire whenever slot.thisValue() == obj, with no check of obj->structure()->hasImmutableProperties(); the putDirect at JSObject.cpp:924 is refused (result ignored) and return true follows, or at JSObject.cpp:914 the native setter runs on the immutable receiver. Fix: in both branches, skip the shortcut (fall through to break so definePropertyOnReceiver/definePropertyOnReceiverSlow decides) when obj has immutable properties, matching the receiver check already added to definePropertyOnReceiver at JSObject.cpp:1003.

  Why this was flagged

  Trigger: $vm.makePropertiesImmutable(JSON); const base = Object.create(JSON, { q: { get() { return 1; } } }); Reflect.set(base, "stringify", 42, JSON) before JSON.stringify has ever been read (so the static entry is unreified). base's own getter makes canPerformFastPutInlineExcludingProto() false (JSObjectInlines.h:204), so putInlineForJSObject calls putInlineSlow(base) (JSObjectInlines.h:434). base is not immutable, so the new refusal at JSObject.cpp:851 does not apply. At JSObject.cpp:922 isThisValueAltered(slot, obj) is false because the receiver is JSON itself, so JSObject.cpp:924 calls obj->putDirect, which putDirectInternal refuses with ReadonlyPropertyChangeError (JSObjectInlines.h:508-511); the bool is discarded and line 925 returns true. Reflect.set therefore reports true while JSON.stringify is unchanged. On the base branch the putDirect succeeds, so true was correct there; the PR's own contract says such a [[Set]] fails unless it changes nothing.

  Verification: The only immutability check the diff adds to putInlineSlow is for this (lines 850-851: if (structure()->hasImmutableProperties() && !isThisValueAltered(slot, this))). Lines 921-926 then run with !isThisValueAltered(slot, obj) true because slot.thisValue() == JSON == obj: obj->putDirect(vm, propertyName, value, attributesForStructure(attributes), slot); return true;.

[dylan-conway]

On the last review summary's finding without an inline thread (putInlineSlow()'s shortcuts for a static property that is not reified, taken when the prototype walk meets the receiver itself): fixed in d84ab50. Both shortcuts are skipped for a receiver with immutable properties, which goes on to definePropertyOnReceiver(). Reflect.set(base, "stringify", 42, JSON) returns false, and a native static setter is not called. Tested in immutable-properties-inheritors.js.

🤖 Addressed by Claude Code

[claude]

Code review found no issues

No high-confidence issues detected in this change.