toStrictEqual, Bun.deepEquals strict: compare constructors before any per-type comparison by robobun · Pull Request #47837 · oven-sh/bun

robobun · 2026-11-03T09:21:00Z

Supersedes #37776. Merge after #37764.

Problem

expect(new MyMap()).toStrictEqual(new Map()) passes for a subclass MyMap of Map. So does a subclass of Set, Date, Error, RegExp or a typed array, and a Buffer against a Uint8Array. Jest and Vitest reject each pair.
Bun__deepEquals (src/jsc/bindings/bindings.cpp:1298) compares class names only after the comparison of those types has answered.

Fix

toStrictEqual and Bun.deepEquals(a, b, true) compare classes first. The rule is Jest's: one constructor, two arrays, or one built-in of two realms.
Correct because 308 pairs answer as Jest's own equals. 24 named pairs differ.
Verified: test/js/bun/test/expect.test.js and test/js/bun/bun-object/deep-equals.test.ts (main fails 38 and 8 tests). Also 307 test files and --aot.
Self-reviewed: 46 concerns raised, 44 addressed. Not taken: a census test of byte APIs, and the Cookie#toJSON() producer.

Background

Bun__deepEquals is the native walker behind toEqual, toStrictEqual, Bun.deepEquals and node:assert. 2 of its 7 compiled modes change.
Considered the name comparison after each per-type comparison. A subclass with no name stays equal to its base.

Downsides

Breaking for tests and for callers of Bun.deepEquals(a, b, true). A Buffer no longer equals a Uint8Array, nor a null-prototype object a literal. No user reported the false pass, so a maintainer must decide.
Instructions per equal pair: +5 to +23 (Date 338 to 361), +103 to +220 with 8 keys. Another realm or an own constructor costs +582 to +844, a typed pair under --aot +246.
Until bun:test: say when both values print the same, and when printing stopped, instead of an empty diff #37764, a failure for the class alone prints an empty diff. Two Proxies of two classes stay equal (bun:test: type a Proxy against a plain value by its observable constructor in strict equality #45251).

Notes

The rule, in the order the code applies it (src/jsc/bindings/BunStrictClassEquality.h and .cpp)

Only toStrictEqual, strict this.equals and Bun.deepEquals(a, b, true) take the step. toEqual, Bun.deepEquals(a, b), Bun.deepEquals(a, b, true, true) and the node:assert modes keep their code.
Two objects of one Structure that lists no constructor: same class. A Structure is JavaScriptCore's hidden class: the own property names of an object, and its prototype. A type that makes up properties is excluded: a Proxy, a module namespace, process.env.
Two arrays: same class, as in Jest.
Two objects with one prototype and no own constructor: same class. They read constructor from one chain, so the step reads nothing. An object of compiled code (--aot) keeps own properties out of its Structure, so the step asks the object.
A Map against a Set, a Date against another object, an array against an object that is no array: unequal at once. The walker gives these pairs that answer whatever their classes are.
A function, a Proxy or a global object on either side: the step does not type the pair.
Otherwise the step reads both constructor values with a VMInquiry slot, which runs no script. The two values give one class in three cases:
- They are identical.
- They are built-in functions of two realms with one name.
- Each is an own constructor that is no function, and the prototypes are equal. This is a constructor key of plain data.
Where only a getter or a Proxy in the prototype chain can answer, the class names decide.
The class name comparison at the end of the walker stays as it is on main.

What breaks

test/js/bun/bun-object/deep-equals.test.ts holds 332 pairs. 128 of them change, all from equal to unequal. toEqual, Bun.deepEquals(a, b) and util.isDeepStrictEqual do not change for any pair.

An instance of a subclass against an instance of its base. The tests cover Map, Set, Date, RegExp, Error, ArrayBuffer, SharedArrayBuffer, 12 typed arrays, 8 Temporal classes, URL and Headers. So rejects.toStrictEqual(new NotFound("x")) no longer passes for a plain Error.
A Buffer against a Uint8Array. These return a Buffer: fs.readFileSync(), the Bun.zstd* functions, CryptoHasher#digest(), Bun.spawnSync().stdout, the stdout of a shell result, Database#serialize(), Archive#bytes(), redis.getBuffer().
These return a Uint8Array: Bun.gzipSync() and its siblings, Bun.sha(), Response#bytes(), Bun.file().bytes(), TextEncoder#encode(), a bun:sqlite BLOB, and structuredClone() of a Buffer.
A null-prototype object against a literal. Such results of JavaScript and Node: regex groups, querystring.parse(), Object.groupBy(), util.parseArgs().values. Such results of Bun: every map reply of Bun.redis such as hgetall(), Bun.Cookie#toJSON(), and req.params of a route with no named parameter. { __proto__: null, ... } as the expected value passes.
Two classes with one name. A package whose exports name one file for import and another for require is loaded twice. So is a package that is installed twice. Node loads two copies too. A package with main and module stays one class.
With --environment=jsdom or happy-dom, the global URL is a subclass of Bun's. With jsdom, so is Request. So URL.parse(x) against new URL(x) is unequal, and request.clone() against the request.
.not.toStrictEqual between a Buffer and a Uint8Array with the same bytes now passes.
Bun.deepEquals(a, b, true) outside a test returns false for each of these pairs and throws nothing. An example is structuredClone(buffer) against the buffer. The two-argument form does not change.
toEqual passes for every pair above.

A suite can take a pair for equal again with a tester in a preload. This one is for a Buffer and a Uint8Array:

expect.addEqualityTesters([
  (a, b) =>
    a instanceof Uint8Array && b instanceof Uint8Array && a.constructor !== b.constructor
      ? Buffer.compare(a, b) === 0
      : undefined,
]);

What stays equal

Two instances of one class. A class whose prototype.constructor is deleted, against its base. MyArray.from([1]) against [1].
An instance with an own constructor that is the base class. @sinonjs/fake-timers gives its Date such a property.
A built-in or a plain object of another realm against one of the main realm. The tests use a node:vm context, and jsdom and happy-dom with and without --isolate.
On main, an instance of a class with no name equals a literal in plain Bun, but not under --aot. Both are unequal now.

Where Bun differs from Jest 30.5.2

Seven groups of 19 pairs are tests in deep-equals.test.ts, because the class step decides them:

An own constructor key that is no function is data. JSON.parse('{"constructor":{"a":1}}') twice is equal here and in Node, and unequal in Jest.
A constructor getter is not called. A class whose prototype has such a getter goes by its class name. So a Node system error still equals an Error with the same name and message.
A Proxy in the prototype chain is not asked either. The class name is then that of the built-in type. One pair changes from main: a Map whose prototype is a Proxy of MyMap.prototype no longer equals a MyMap.
Bun compares the cause and the own properties of an Error, and the own properties of a typed array. Jest does not.
Two built-in functions of one realm are two classes. A mock function of Bun is native code, so Jest's rule makes two mocks one class.

Two groups of 5 pairs answer as on main:

A Proxy does not equal its target (toStrictEqual and toMatchObject unexpected assertion error #9103).
Two Proxies have no class, so two Proxies of two classes are equal. bun:test: type a Proxy against a plain value by its observable constructor in strict equality #45251, Treat Proxy as transparent in deep equality and toMatchObject #32948 and bun:test: propagate throwing Proxy traps through strict deepEquals and error construction #34649 are about Proxies.

Not in the tests, and as on main:

Two values of one class with different native state are equal. Examples are a DataView over other bytes, a FormData with other entries, and a DOMException with another message. deepEquals: compare DataView bytes in every mode #46676, deepEquals: compare FormData, Blob, File, Cookie and CookieMap by their native state #45613 and deepEquals: compare DOMException name, message and cause #39308 are about those.
toEqual(new MyMap(), new Map()) passes. Jest's toEqual rejects a Map, a Set or another iterable of a subclass. A test that this change turns red and that moves to toEqual then passes on Bun alone.
Strict this.equals(a, b, testers, true) compares classes. Jest's does not.

Earlier decisions that this change reverses

Compare prototypes in deepStrictEqual #29037 (closed, not merged) gave Bun.deepEquals(a, b, true) a prototype check and left toStrictEqual out. Its reason: the check "broke cross-realm objects (vm runInContext) and Buffer-vs-Uint8Array content comparisons across the bun:test suite".
node compat batch: callback-throw dispatch, Assert class + native deep-equality parity, Intl gate + URL/buffer fallout, compile cache, watch kill-signal, profilers (+98 tests) #34660 confined the rule of Node "by API ownership" to the node:assert entry points. node:assert: compare constructors like node in deepStrictEqual, not [[Prototype]] identity #40131 says that Bun.deepEquals and expect() "do not change here".
This change handles the realm case with Jest's rule. The Buffer break stays: 8 assertions of Bun's own tests change.
.claude/docs/landing-prs.md asks for a flag on a changed default. bun test: Vitest compatibility #44833 changed equality toward Jest with no flag, and listed each change under "Behaviour changes for existing users".

Measured (release x64 builds of main at 3f3a9ff, without and with this change)

User-mode instructions of one Bun.deepEquals(a, b, true) call, counted by single steps in gdb. The pairs are the strict rows that this change adds to bench/snippets/deep-equals.js.

pair	main	this PR	change
`{ a, b }` twice	2,082	2,101	+19
`{ a, b }` and `{ b, a }`	2,415	2,467	+52
object literal and `JSON.parse()`	2,645	2,697	+52
8 keys, one `Structure`	3,088	3,191	+103
8 keys, literal and `JSON.parse()`	4,266	4,486	+220
`[1, 2, 3]` twice	1,573	1,585	+12
array literal and computed array	1,573	1,578	+5
arrays of 10 `{ id, name }`	23,229	23,387	+158
instances of one class	1,770	1,789	+19
Date	338	361	+23
RegExp	449	472	+23
Map	655	678	+23
Set	575	598	+23
Uint8Array	473	496	+23
Error	6,243	6,352	+109
Sets of 7 values of 7 kinds	20,472	19,177	-1,295
Maps with such keys	20,863	19,514	-1,349
two Dates that own `constructor`	338	1,041	+703
such a Date and a Date	338	920	+582
Date of another realm	338	1,109	+771
Uint8Array of another realm	473	1,317	+844
Map of another realm	655	1,442	+787
`{ a, b }` of another realm	2,415	3,142	+727
instance and object literal (unequal on both)	1,591	863	-728
Map and Set (unequal on both)	290	276	-14
subclass of Set and Set (equal on main)	575	873	+298

A Structure keeps a one-word filter of the names it has seen. With up to 3 keys in these runs, the filter rules out constructor. With more keys, the step does one property-table lookup for each Structure.
The last equal rows need the read of constructor. That happens for a value of another realm and for an own constructor. With jsdom the global typed arrays are the window's, so a Uint8Array that Bun makes is of another realm there.
Unequal Sets of 100 Dates: +23 for each candidate pair, 6.0% in total.
Pairs of typed objects under --aot: 3,216 to 3,462 for one field, 5,601 to 5,847 for 8 fields. bun build --compile --aot and bun test --aot give the same numbers. The step asks each compiled object with AOT::PropertyWalk::getDirect.
toEqual, node:assert and the 4-argument mode: 0. 5 of the 7 Bun__deepEquals instantiations and the 8 per-type functions are identical after address normalisation.
A strict { a: 1 } pair makes 2 allocations and 2 frees on main and here (gdb breakpoint counts).
A subclass of Set with 2,000 objects against a Set with equal members: main enters the walker 4,000,001 times. Then it answers equal. This branch enters the walker once.
The class step enters user code 0 times. A counting constructor getter and counting Proxy traps give the same counts as on main.
Release text: 140,286,972 to 140,291,836 bytes.
File size of each release target against main, from the size check of the build: +0.0 to +16.0 KB. linux-x64 +4.0 KB, darwin-x64 +16.0 KB, windows-x64 +1.0 KB, windows-aarch64 +9.5 KB, linux-aarch64 and darwin-aarch64 0.

Tests

expect.test.js: 40 new tests, 38 fail on main. 37 are not specific to Bun and pass on Jest 30.5.2 and Vitest 4.1.12.
deep-equals.test.ts: 308 pairs answer as equals of @jest/expect-utils 30.5.2 with the testers of its toStrictEqual. Main answers so for 186 of them. test/package.json pins the package. One more test loads a package twice.
test/aot/parallel/typed-objects/deep-equality-of-classes and test-aot-deep-equality.test.ts: 75 pairs answer under --aot as in plain Bun, and a snapshot holds the answers. Both fail on main.
test-environment.test.ts: strict rows under jsdom and happy-dom, with and without --isolate.
dom-nodes.test.ts and dom-nodes-jest.test.js: nodes behind a Proxy, which the step does not type. Jest 30.5.2 passes the second file as it is.
zstd.test.ts and valkey/unit/buffer-operations.test.ts: 8 assertions compared a Buffer with a Uint8Array. They now expect a Buffer. The type of getBuffer() says Buffer now, as its docs do.
307 test files under test/ ran on the release build of this branch. They are every *.test.* file there that calls toStrictEqual, and the tests of the runner. The 9 files that fail there fail the same tests on main.
The 119 files that call toStrictEqual also ran on the debug build, with the same result. The elysia 1.4.28 suite: no change.
BUN_JSC_validateExceptionChecks=1 on the debug build: no report.

Other pull requests on these lines

bun:test: say when both values print the same, and when printing stopped, instead of an empty diff #37764 prints a line where both values print the same. A failure for the class alone needs it.
Make toStrictEqual and strict Bun.deepEquals distinguish null-prototype objects from object literals #37776 makes a null-prototype object unequal to a literal. This change includes that, but not its rows for two Proxies. It also includes the null-prototype part of Bun.deepEquals: fix four false-positive equality classes #32872.
bun:test: compare toString tags in toEqual unless both values are plain objects #42546 and Make deepEquals and toEqual compare built-in constructors, mocks and callable Proxies by identity #38334 change toEqual. They stay separate.
AOT: hangs, wrong answers, failed builds; bun test --aot follows #44833's mocks #47605 changes the class name comparison at the end of the walker. This change does not touch those lines, and git merge-tree finds no conflict. Nobody has built or run the two together.

Not in this change

For two objects with one prototype and no own constructor, the class name comparison at the end repeats work. A pair of plain objects saves about 1,200 instructions without it. That needs its own tests.
Cookie#toJSON() returns a null-prototype object, and CookieMap#toJSON() does not. The types of Archive#bytes() and of the data of a Terminal say Uint8Array for a Buffer.
toStrictEqual of a Map whose prototype is Set.prototype now fails. The printer then throws Set operation called on non-Set object, as toBe does on main.

For a maintainer to decide

Whether to ship the break with no flag, as a listed behaviour change. A text for that list: "toStrictEqual() and Bun.deepEquals(a, b, true) compare the classes of two objects, as in Jest. A Buffer does not equal a Uint8Array, an object with no prototype does not equal an object literal, and an instance of a subclass does not equal an instance of its base. toEqual() does not change."
Bun.deepEquals(a, b, true) takes the same rule as toStrictEqual, because the docs call them one comparison. An enableAsymmetricMatchers condition at the call keeps it on names alone.

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/util/zstd.test.ts, test/js/bun/bun-object/deep-equals.test.ts

… per-type comparison `expect(new MyMap()).toStrictEqual(new Map())` passed, where MyMap extends Map. Bun__deepEquals compared class names only after the comparison for Map, Set, Date, RegExp, Error, ArrayBuffer, the typed arrays and the Temporal classes had answered. toStrictEqual() and Bun.deepEquals(a, b, true) now compare classes first, with the rule of Jest's typeEquality: two objects have one class when they have one `constructor`, when both are arrays, or when their constructors are the same built-in of two realms. The step runs no script. Where only a getter or a Proxy in the prototype chain can answer, the class names decide. An own `constructor` that is no function is a key of the data. Two objects with one prototype and no own `constructor` need no read. toEqual(), Bun.deepEquals(a, b), the fourth argument of Bun.deepEquals() and the node:assert modes do not change. A Buffer is not a Uint8Array for these two: the type of RedisClient#getBuffer() now says Buffer, which is what it returns.

robobun · 2026-11-03T09:21:13Z

^{Updated 3:50 AM PT - Nov 3rd, 2026}

❌ @robobun, your commit 442a2e9 has 40 failures in Build #137216 (All Failures):

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test/aot/a-mock-of-a-built-in-module-that-was-required-first-is-what-require-answers.test.ts - code 1 on 🐧 13 aarch64
test/aot/a-mock-of-a-built-in-module-that-was-required-first-is-what-require-answers.test.ts - code 1 on 🐧 13 x64
test/aot/a-mock-of-a-built-in-module-that-was-required-first-is-what-require-answers.test.ts - code 1 on 🐧 13 x64-asan
test/aot/asking-about-a-function-keeps-none-of-its-code.test.ts - code 1 on 🐧 13 aarch64
test/aot/asking-about-a-function-keeps-none-of-its-code.test.ts - code 1 on 🐧 13 x64
test/aot/asking-about-a-function-keeps-none-of-its-code.test.ts - code 1 on 🐧 13 x64-asan
test/aot/a-test-run-compiles-what-an-executable-compiles.test.ts - code 1 on 🐧 13 aarch64
test/aot/a-test-run-compiles-what-an-executable-compiles.test.ts - code 1 on 🐧 13 x64
test/aot/a-test-run-compiles-what-an-executable-compiles.test.ts - code 1 on 🐧 13 x64-asan
test/aot/the-default-of-a-mocked-built-in-module-is-the-mocks.test.ts - code 1 on 🐧 13 aarch64
test/aot/the-default-of-a-mocked-built-in-module-is-the-mocks.test.ts - code 1 on 🐧 13 x64
test/aot/the-default-of-a-mocked-built-in-module-is-the-mocks.test.ts - code 1 on 🐧 13 x64-asan
test/aot/a-commonjs-file-mocked-again-is-seen-by-who-required-it.test.ts - code 1 on 🐧 13 aarch64
test/aot/a-commonjs-file-mocked-again-is-seen-by-who-required-it.test.ts - code 1 on 🐧 13 x64
test/aot/a-commonjs-file-mocked-again-is-seen-by-who-required-it.test.ts - code 1 on 🐧 13 x64-asan
test/aot/a-missing-export-is-reported-before-a-missing-import-where-a-file-may-be-mocked.test.ts - code 1 on 🐧 13 aarch64
test/aot/a-missing-export-is-reported-before-a-missing-import-where-a-file-may-be-mocked.test.ts - code 1 on 🐧 13 x64
test/aot/a-missing-export-is-reported-before-a-missing-import-where-a-file-may-be-mocked.test.ts - code 1 on 🐧 13 x64-asan
test/aot/test-run-in-parts-failures.test.ts - crash reported on 🐧 13 aarch64
test/aot/test-run-in-parts-failures.test.ts - crash reported on 🐧 13 x64
test/aot/test-run-in-parts-failures.test.ts - code 1 on 🐧 13 x64-asan
test/bundler/compile-aot-size.test.ts - code 1 on 🐧 25.04 x64
test/bundler/compile-aot-size.test.ts - code 1 on 🐧 13 x64
test/aot/a-mock-through-the-global-jest-or-vi-reaches-a-module-of-a-layer-below.test.ts - code 1 on 🐧 13 aarch64
test/aot/a-mock-through-the-global-jest-or-vi-reaches-a-module-of-a-layer-below.test.ts - code 1 on 🐧 13 x64
test/aot/a-mock-through-the-global-jest-or-vi-reaches-a-module-of-a-layer-below.test.ts - code 1 on 🐧 13 x64-asan
test/aot/engine/ir-1.test.ts - code 1 on 🐧 13 aarch64
test/aot/engine/ir-1.test.ts - code 1 on 🐧 13 x64
test/aot/engine/ir-1.test.ts - code 1 on 🐧 13 x64-asan
test/aot/engine/ir-1.test.ts - code 1 on 🪟 11 aarch64
test/aot/engine/ir-1.test.ts - code 1 on 🪟 2019 x64
test/aot/what-require-answered-behind-a-mock-of-a-built-in-module-stays-behind-an-import.test.ts - code 1 on 🐧 13 aarch64
test/aot/what-require-answered-behind-a-mock-of-a-built-in-module-stays-behind-an-import.test.ts - code 1 on 🐧 13 x64
test/aot/what-require-answered-behind-a-mock-of-a-built-in-module-stays-behind-an-import.test.ts - code 1 on 🐧 13 x64-asan
test/aot/engine/native-6.test.ts - pid 1423 sigabrt on 🐧 3.23 aarch64
test/aot/engine/native-6.test.ts - pid 2366 sigabrt on 🐧 3.23 x64
test/aot/engine/native-6.test.ts - pid 21241 sigabrt on 🐧 13 aarch64
test/aot/engine/native-6.test.ts - pid 21520 sigabrt on 🐧 13 x64
test/aot/engine/native-6.test.ts - pid 37724 sigabrt on 🐧 13 x64-asan
test/aot/engine/native-6.test.ts - code 1 on 🍎 any x64
test/aot/parallel/module-loaders/raw-behind-a-path-asks-for-the-text-of-the-file/main.mjs - compare: with --aot-erase-types: it does not exit and print as plain Bun does on 🐧 25.04 aarch64
test/aot/parallel/module-loaders/raw-behind-a-path-asks-for-the-text-of-the-file/main.mjs - compare: with --aot-erase-types: it does not exit and print as plain Bun does on 🐧 25.04 x64
test/aot/parallel/module-loaders/raw-behind-a-path-asks-for-the-text-of-the-file/main.mjs - compare: it does not exit and print as plain Bun does on 🐧 13 x64-asan

🧪 To try this PR locally:

bunx bun-pr 47837

That installs a local version of the PR into your bun-47837 executable, so you can run:

bun-47837 --bun

robobun · 2026-11-03T09:21:29Z

Status

Reproduced with the Bun 1.4.2 release and with a debug build of main: bun test test/js/bun/test/expect.test.js -t "compares classes" fails 38 of the 40 new tests. A build of this branch passes all 40.
CI: build 137216 finished red for 40 test files. All are under test/aot/, plus test/bundler/compile-aot-size.test.ts. The same 40 files are red in build 137162 of main at 3bf1a06, which this branch merges. No test file that this change touches fails on any lane. That includes the ASAN, Windows and macOS lanes.
This is a draft. It waits for bun:test: say when both values print the same, and when printing stopped, instead of an empty diff #37764, and for a maintainer's decision on the behaviour change.

…ct-equal-builtin-subclass

github-actions Bot added the claude label Nov 3, 2026

Merge remote-tracking branch 'origin/main' into robobun/68746faa/stri…

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…ct-equal-builtin-subclass

github-actions Bot reviewed Nov 3, 2026

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Comment thread src/jsc/bindings/BunStrictClassEquality.h Resolved

BunStrictClassEquality.h: state the class rule in one comment line

442a2e9

This was referenced Nov 3, 2026

Make toStrictEqual and strict Bun.deepEquals distinguish null-prototype objects from object literals #37776

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bun:test: toEqual reads a Proxy around a Set or a Map through the Proxy #47888

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toStrictEqual, Bun.deepEquals strict: compare constructors before any per-type comparison - #47837

toStrictEqual, Bun.deepEquals strict: compare constructors before any per-type comparison#47837
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