[christianvuerings]

Three differences from Jest's mock functions that shared code trips over:

1. Every mock method and the mock getter required this to be the exact
   Mock cell, so a mock wrapped in a Proxy (a logger that proxies its mocks,
   a module namespace Proxy) threw "The Mock.mock getter can only be used on
   instances of Mock" on proxied.mock.calls, proxied.mockReturnValue(1)
   and expect(proxied).toHaveBeenCalled(). Jest's methods are closures over
   the mock, so any receiver works. Bun now looks through Proxy wrappers to
   the mock (the mock accessor drops the DOMAttribute flag, whose JSC-side
   type check ran before the getter and rejected the Proxy receiver).

2. new mockFn() called the implementation like a plain call and returned
   its result, so a constructor mock returned undefined and mocks had no
   prototype. Jest's mocks are plain functions: new creates an instance
   from mockFn.prototype, runs the implementation with it as this, and
   returns the instance unless the implementation returned an object. Mocks
   get a prototype object with constructor, and a construct callback.

3. mock.instances was never populated. Jest records this for every call
   in both mock.instances and mock.contexts, which is how tests reach the
   objects a mocked constructor produced.

Repro on bun 1.4.0:

const Klass = mock(function (a) { this.a = a; });
new Klass(1); // undefined
Klass.mock.instances; // []
new Proxy(mock(), {}).mock // TypeError

Not changed: a mock method detached from its mock (const { mockReturnValue } = fn)
still throws, since nothing identifies the mock; Jest binds each method per mock.

[claude]

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Walkthrough

Mock functions now support constructor calls, prototype inheritance, object-return rules, and instance tracking. Mock metadata and methods resolve through Proxy wrappers. Tests cover constructor behavior, call contexts, proxy forwarding, state clearing, and detached methods.

Changes

Mock function semantics

Layer / File(s)	Summary
Construction and invocation handling src/jsc/bindings/JSMockFunction.cpp	Mocks expose a constructor-linked prototype, create instances through jsMockFunctionConstruct, record contexts and instances, and preserve constructed instances when implementations return primitives.
Proxy receiver forwarding src/jsc/bindings/JSMockFunction.cpp	Mock accessors and methods unwrap Proxy receivers through thisMockFunction. The mock and _protoImpl accessors accept Proxy receivers.
Constructor and proxy behavior tests test/js/bun/test/mock-fn-jest-semantics.test.ts, test/js/bun/test/mock-fn.test.js	Tests cover prototypes, constructors, return values, inheritance, Proxy forwarding, call contexts, instance tracking, clearing, throwing implementations, and detached-method errors.

Suggested reviewers: dylan-conway, robobun

Merge Risk: ⚪ Minimal · up to fe34b

Mock functions now support constructor semantics, proxy-wrapped metadata access, and call-context tracking with covered Jest-compatible behavior. No current merge-blocking risk is identified.

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Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main changes: Jest-compatible Proxy receiver handling, constructor support, and mock instance recording.
Description check	✅ Passed	The description is detailed, relevant, and explains the behavioral changes, compatibility goals, limitations, and reproduction cases. It does not use the template headings or explicitly describe verif…
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Inline comments:
In `@src/jsc/bindings/JSMockFunction.cpp`:
- Line 603: Remove the DOMAttribute flag from the _protoImpl property definition
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[robobun]

Thanks for this. It overlaps with #37030, which adds a construct callback and mock.instances recording to the same file. This PR covers more (the Proxy receivers and the prototype object), so I am closing #37030 in favour of it.

I built this branch merged with current main and ran the tests from #37030 against it. 189 pass, 1 fails: the fallback when newTarget.prototype is not an object and newTarget comes from another realm. Per GetPrototypeFromConstructor the instance falls back to Object.prototype of the realm of newTarget, and this branch uses the realm of the mock instead.

import { runInNewContext } from "node:vm";
const fn = mock();
const { NewTarget, otherObjectPrototype } = runInNewContext(
  "({ NewTarget: function NewTarget() {}, otherObjectPrototype: Object.prototype })",
);
NewTarget.prototype = null;
const instance = Reflect.construct(fn, [], NewTarget);
expect(Object.getPrototypeOf(instance)).toBe(otherObjectPrototype); // gets Object.prototype of the test realm

#37030 derives the structure the way JSC's own constructors do. It gets the realm right and caches the structure on newTarget:

JSObject* newTarget = asObject(callframe->newTarget());
JSGlobalObject* functionGlobalObject = getFunctionRealm(globalObject, newTarget);
RETURN_IF_EXCEPTION(scope, {});
Structure* structure = InternalFunction::createSubclassStructure(globalObject, newTarget, functionGlobalObject->objectStructureForObjectConstructor());
RETURN_IF_EXCEPTION(scope, {});
JSObject* instance = JSC::constructEmptyObject(vm, structure);

createSubclassStructure reads newTarget.prototype itself, so the prototype object you add in create() still becomes the prototype of new mockFn().

A few cases from #37030's tests that this PR's test file does not cover yet. They all pass on this branch, so they only need assertions:

• Reflect.construct(fn, [], NewTarget) uses NewTarget.prototype.
• A throwing implementation under new propagates and records { type: "throw", value } in mock.results.
• Reflect.construct(spyOn({}, 9), []) returns an object. This is the crash bun:test: implement proper construct semantics for mock functions #37030 was opened for (new on a mock hit the isCell() assertion in debug builds).

One small thing: the existing tests for mock functions live in test/js/bun/test/mock-fn.test.js. That file also runs under Jest and Vitest, which makes it a good place for Jest parity assertions.

[christianvuerings]

Thanks for running the #37030 suite against this branch. Pushed one commit addressing the review.

Fix. jsMockFunctionConstruct now derives the instance structure the way JSC's own constructors do: getFunctionRealm(globalObject, newTarget) followed by InternalFunction::createSubclassStructure(globalObject, newTarget, functionGlobalObject->objectStructureForObjectConstructor()), then constructEmptyObject(vm, structure). When newTarget.prototype is not an object the instance falls back to Object.prototype of newTarget's realm, and the structure is cached on newTarget. The prototype object created in create() is still what new mockFn() instances inherit from, since createSubclassStructure reads newTarget.prototype itself.

Tests. The cross-realm runInNewContext case you wrote is in mock-fn-jest-semantics.test.ts. I confirmed it fails on the previous head of this branch (the instance got the test realm's Object.prototype) and passes with this commit.

The runner-neutral constructor assertions moved into test/js/bun/test/mock-fn.test.js so they also run under Jest and Vitest: prototype.constructor, new mockFn() recording mock.instances and mock.contexts, object vs primitive return under new, methods defined on mockFn.prototype, Reflect.construct(fn, [], NewTarget) using NewTarget.prototype, and a throwing implementation under new propagating and recording { type: "throw", value }. Reflect.construct(spyOn({}, 9), []) returning an object is there under if (isBun), since Jest's spyOn rejects a missing property. mock-fn-jest-semantics.test.ts keeps the bun-only cases (Proxy receivers, _protoImpl, detached method error, the cross-realm fallback).

With a debug build: mock-fn.test.js 94 pass, 0 fail (571 expect calls); mock-fn-jest-semantics.test.ts 4 pass, 0 fail.

[robobun]

A note on overlap, so that two pull requests do not carry the same change.

#34575 had a construct handler for mocks and the mock.instances change. I removed both from it today. It now covers native functions only, so this pull request is the only open one for new on a mock and for mock.instances.

Three things from that work that can help here:

• Test cases. The removed commit is on the branch robobun/e9fbc481/mock-construct-half. Its constructing a mock block in mock-fn.test.js has cases that this branch does not have: one for each primitive the implementation can return (string, symbol, bigint, number, boolean, null, undefined) through new and Reflect.construct, a spy on a function that returns a symbol, a spy on a property that is not a function, and mockReturnValueOnce(object) followed by mockReturnValue(1). Take any of them.
• mock.instances matches the reference. m(); o.m(); m.call(5) gives [undefined, o, 5] in jest-mock 30.5.2 and in @vitest/spy 5.0.2 on Node v26.3.0. That is what this branch records.
• Earlier feedback on the same call. A maintainer commented on InternalFunction::createSubclassStructure in the construct handler of fix mock function crash when Reflect.construct returns non-object #28315: fix mock function crash when Reflect.construct returns non-object #28315 (comment). NapiClass.cpp makes that call only when newTarget is not the callee.
  What that call does for a mock, read from InternalFunction.cpp at the pinned WebKit commit: a mock is an InternalFunction and not a JSFunction, so every construct takes the slow path. That path reads newTarget.prototype. When the value is an object, it looks up one cached Structure for that prototype in the realm's StructureCache. When it is not an object, it returns the base structure and looks nothing up. So it does not create a structure for each construct. It creates one for each distinct prototype object, on the first new, and each later new costs one property read and one hash lookup.

Measured on release builds for the handler that #34575 had: 101 more instructions for each mock call (1706 to 1807) and 10.6 more bytes kept for each call, from the extra instances entry.

[robobun]

A note so that the overlap is visible. I reopened #37032, which has only the construct handler from this change. #46021 collected the open crash fixes and did not include this one, and the crash still reproduces on main. The lines that create the instance are the same in both.

If #37032 lands first, this pull request needs a rebase in JSMockFunction.cpp. It keeps its other half: mock.instances, the default prototype and the Proxy receivers. If this one lands first, I will close #37032.