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robobun wants to merge 3 commits into from
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bun:test: return an object when a mock is constructed #36497

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51a1fd2
bun:test: return an object when a mock is constructed
robobun Jul 18, 2026
454922a
use newTarget.prototype for mock construct instance
robobun Jul 18, 2026
f32c31e
cover throwing implementation during construct
robobun Jul 31, 2026
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45 changes: 44 additions & 1 deletion src/jsc/bindings/JSMockFunction.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,7 @@ inline To tryJSDynamicCast(JSC::WriteBarrier<WriteBarrierT>& from)
}

JSC_DECLARE_HOST_FUNCTION(jsMockFunctionCall);
JSC_DECLARE_HOST_FUNCTION(jsMockFunctionConstruct);
JSC_DECLARE_CUSTOM_GETTER(jsMockFunctionGetter_protoImpl);
JSC_DECLARE_CUSTOM_GETTER(jsMockFunctionGetter_mock);
JSC_DECLARE_HOST_FUNCTION(jsMockFunctionGetter_mockGetLastCall);
Expand Down Expand Up @@ -463,7 +464,7 @@ class JSMockFunction : public JSC::InternalFunction {
}

JSMockFunction(JSC::VM& vm, JSC::Structure* structure, CallbackKind wrapKind)
: Base(vm, structure, jsMockFunctionCall, jsMockFunctionCall)
: Base(vm, structure, jsMockFunctionCall, jsMockFunctionConstruct)
{
initMock();
}
Expand Down Expand Up @@ -976,6 +977,48 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
return JSValue::encode(jsUndefined());
}

JSC_DEFINE_HOST_FUNCTION(jsMockFunctionConstruct, (JSGlobalObject * lexicalGlobalObject, CallFrame* callframe))
{
Zig::GlobalObject* globalObject = uncheckedDowncast<Zig::GlobalObject>(lexicalGlobalObject);
auto& vm = JSC::getVM(globalObject);
auto scope = DECLARE_THROW_SCOPE(vm);
JSMockFunction* fn = dynamicDowncast<JSMockFunction>(callframe->jsCallee());
if (!fn) [[unlikely]] {
throwTypeError(globalObject, scope, "Expected callee to be mock function"_s);
return {};
}

JSValue newTarget = callframe->newTarget();
JSObject* newTargetObject = newTarget.isObject() ? asObject(newTarget) : fn;
JSValue prototype = newTargetObject->get(globalObject, vm.propertyNames->prototype);
RETURN_IF_EXCEPTION(scope, {});
JSObject* instance = prototype.isObject()
? JSC::constructEmptyObject(globalObject, asObject(prototype))
: JSC::constructEmptyObject(globalObject);
RETURN_IF_EXCEPTION(scope, {});

if (JSC::JSArray* instances = fn->instances.get()) {
instances->push(globalObject, instance);
RETURN_IF_EXCEPTION(scope, {});
} else {
JSC::ObjectInitializationScope object(vm);
instances = JSC::JSArray::tryCreateUninitializedRestricted(
object,
globalObject->arrayStructureForIndexingTypeDuringAllocation(JSC::ArrayWithContiguous),
1);
instances->initializeIndex(object, 0, instance);
fn->instances.set(vm, fn, instances);
}

callframe->setThisValue(instance);
EncodedJSValue encodedResult = jsMockFunctionCall(lexicalGlobalObject, callframe);
RETURN_IF_EXCEPTION(scope, {});
JSValue result = JSValue::decode(encodedResult);
if (result.isObject())
return encodedResult;
return JSValue::encode(instance);
}

Comment on lines +980 to +1021

@coderabbitai coderabbitai Bot Jul 31, 2026

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📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Description: Confirm the vendored JSC CallFrame this-slot/newTarget behavior used by this construct callback.
fd -a CallFrame.h | xargs -I{} rg -n -B2 -A2 'setThisValue|newTarget\(\)' {}

Repository: oven-sh/bun

Length of output: 149

🏁 Script executed:

#!/bin/bash
set -eu

printf '%s\n' '--- CallFrame.h candidates ---'
fd -a -i 'CallFrame\.h$' . | head -20

printf '%s\n' '--- construct callback and callframe access ---'
rg -n -B4 -A8 'jsMockFunctionConstruct|setThisValue|newTarget\(\)|thisValue\(\)' \
  src/jsc/bindings src 2>/dev/null | head -240

Repository: oven-sh/bun

Length of output: 19972

🏁 Script executed:

#!/bin/bash
set -eu

printf '%s\n' '--- tracked CallFrame files and JSC source roots ---'
git ls-files | rg -i '(^|/)(CallFrame\.h|JavaScriptCore|jsc|webkit)(/|$)' | head -160

printf '%s\n' '--- CallFrame API references ---'
rg -n -S 'class CallFrame|newTarget\(\)|setThisValue\(|thisValue\(\) const|thisValue\(\)' \
  --glob '*.{h,hpp,cpp,cc,mm,inc}' . 2>/dev/null | head -240

printf '%s\n' '--- exact mock constructor context ---'
sed -n '960,1030p' src/jsc/bindings/JSMockFunction.cpp

Repository: oven-sh/bun

Length of output: 38257

🏁 Script executed:

#!/bin/bash
set -eu

printf '%s\n' '--- CallFrame.rs slot definitions ---'
rg -n -B8 -A14 'pub fn (new_target|this_value|set_this_value)|newTarget|setThisValue|thisValue' \
  src/jsc/CallFrame.rs | head -180

printf '%s\n' '--- analogous host-constructor slot reuse ---'
sed -n '1,95p' src/jsc/bindings/NapiClass.cpp
sed -n '1025,1065p' src/jsc/bindings/napi.h

printf '%s\n' '--- constructor implementations using both slots ---'
sed -n '320,350p' src/runtime/webview/JSWebViewConstructor.cpp
sed -n '165,195p' src/jsc/bindings/NodeDirent.cpp

Repository: oven-sh/bun

Length of output: 8808

🏁 Script executed:

#!/bin/bash
set -eu

printf '%s\n' '--- generated CallFrame.rs contents ---'
wc -l src/jsc/CallFrame.rs
cat -n src/jsc/CallFrame.rs | head -220

printf '%s\n' '--- CallFrame generation and WebKit source references ---'
rg -n -S 'CallFrame\.rs|CallFrame|WebKit|oven-sh/WebKit|397dafc9721b8f8046f9448abb6dbc14efe096d3' \
  build.zig build.zig.zon src package.json README.md 2>/dev/null | head -220

Repository: oven-sh/bun

Length of output: 31334

Document the intentional CallFrame slot reuse.

At Line 1013, add a short comment that callframe->newTarget() was captured before setThisValue(instance) repurposes the constructor frame’s this slot for jsMockFunctionCall. src/jsc/CallFrame.rs documents this slot-sharing behavior.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/jsc/bindings/JSMockFunction.cpp` around lines 980 - 1021, Add a brief
comment in jsMockFunctionConstruct immediately before
callframe->setThisValue(instance), documenting that newTarget was captured
earlier because setThisValue repurposes the constructor CallFrame’s this slot
for jsMockFunctionCall, consistent with CallFrame.rs.

Source: Coding guidelines

void JSMockFunctionPrototype::finishCreation(JSC::VM& vm, JSC::JSGlobalObject* globalObject)
{
Base::finishCreation(vm);
Expand Down
34 changes: 34 additions & 0 deletions test/js/bun/test/mock-fn.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -803,6 +803,40 @@ describe("mock()", () => {

expect(bar()()).toBe(true);
});

test("Reflect.construct on a mock returns an object and records the instance", () => {
const fn = jest.fn();
const instance = Reflect.construct(fn, [1, 2]);
expect(typeof instance).toBe("object");
expect(instance).not.toBeNull();
expect(fn.mock.instances).toEqual([instance]);
expect(fn.mock.contexts).toEqual([instance]);
expect(fn.mock.calls).toEqual([[1, 2]]);

const fn2 = jest.fn(() => ({ tag: "impl" }));
const result2 = Reflect.construct(fn2, []);
expect(result2).toEqual({ tag: "impl" });
expect(fn2.mock.instances).toHaveLength(1);
expect(fn2.mock.instances[0]).not.toBe(result2);

const fn3 = jest.fn(() => 42);
const result3 = new fn3();
expect(typeof result3).toBe("object");
expect(fn3.mock.instances).toEqual([result3]);

class NewTarget {}
const fn4 = jest.fn();
const result4 = Reflect.construct(fn4, [], NewTarget);
expect(Object.getPrototypeOf(result4)).toBe(NewTarget.prototype);
expect(result4).toBeInstanceOf(NewTarget);
expect(fn4.mock.instances).toEqual([result4]);

const fn5 = jest.fn(() => {
throw new Error("boom");
});
expect(() => new fn5()).toThrow("boom");
expect(() => Reflect.construct(fn5, [])).toThrow("boom");
});
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});

describe("resetAllMocks", () => {
Expand Down
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