bun:test: toBeEmpty() takes one iterator step, like jest-extended

src/jsc/JSValue.rs

@@ -2432,6 +2432,13 @@ impl JSValue {
     pub fn is_iterable(self, global: &JSGlobalObject) -> JsResult<bool> {
         host_fn::from_js_host_call_generic(global, || JSC__JSValue__isIterable(self, global))
     }
+    /// True when the first iterator step reports done. Closes an iterator that has more.
+    pub fn is_iterable_empty(self, global: &JSGlobalObject) -> JsResult<bool> {
+        unsafe extern "C" {
+            safe fn JSC__JSValue__isIterableEmpty(this: JSValue, global: &JSGlobalObject) -> bool;
+        }
+        host_fn::from_js_host_call_generic(global, || JSC__JSValue__isIterableEmpty(self, global))
+    }
     /// `JSValue.forEach` — invoke `callback` for each iterable element.
     pub fn for_each(
         self,

src/jsc/bindings/IteratorOperationsBinding.cpp

@@ -0,0 +1,24 @@
+#include "root.h"
+
+#include <JavaScriptCore/IteratorOperations.h>
+
+using namespace JSC;
+
+// True when the first iterator step reports done. Closes an iterator that has more, like `break` in a for-of.
+extern "C" bool JSC__JSValue__isIterableEmpty(EncodedJSValue encodedIterable, JSGlobalObject* globalObject)
+{
+    auto& vm = getVM(globalObject);
+    auto scope = DECLARE_THROW_SCOPE(vm);
+
+    IterationRecord iterationRecord = iteratorForIterable(globalObject, JSValue::decode(encodedIterable));
+    RETURN_IF_EXCEPTION(scope, false);
+
+    JSValue next = iteratorStep(globalObject, iterationRecord);
+    RETURN_IF_EXCEPTION(scope, false);
+    if (next.isFalse())
+        return true;
+
+    scope.release();
+    iteratorClose(globalObject, iterationRecord.iterator);
+    return false;
+}

src/runtime/test_runner/expect/toBeEmpty.rs

@@ -1,6 +1,4 @@
-use core::ffi::c_void;
-
-use bun_jsc::{CallFrame, JSGlobalObject, JSPropertyIterator, JSPropertyIteratorOptions, JSValue, JsResult, VM};
+use bun_jsc::{CallFrame, JSGlobalObject, JSPropertyIterator, JSPropertyIteratorOptions, JSValue, JsResult};
 
 use super::{throw, Expect};
 
@@ -21,24 +19,7 @@ pub(crate) fn to_be_empty(
     if actual_length == f64::INFINITY {
         if value.js_type_loose().is_object() {
             if value.is_iterable(global)? {
-                let mut any_properties_in_iterator = false;
-
-                extern "C" fn anything_in_iterator(
-                    _: *mut VM,
-                    _: &JSGlobalObject,
-                    any_: *mut c_void,
-                    _: JSValue,
-                ) {
-                    // SAFETY: `any_` is the `&mut bool` passed to `for_each` below.
-                    unsafe { *any_.cast::<bool>() = true };
-                }
-
-                value.for_each(
-                    global,
-                    (&raw mut any_properties_in_iterator).cast::<c_void>(),
-                    anything_in_iterator,
-                )?;
-                pass = !any_properties_in_iterator;
+                pass = value.is_iterable_empty(global)?;
             } else {
                 let Some(_cell) = value.to_cell() else {
                     return Err(global.throw_type_error(format_args!(

test/js/bun/test/expect.test.js

@@ -3811,6 +3811,88 @@ describe("expect()", () => {
     }
   });
 
+  // jest-extended takes the same one step. Bun also closes an iterator that has more, like `break` in a for-of.
+  // The iterators are finite and count their steps, so a full walk fails the count and does not hang the runner.
+  describe("toBeEmpty() takes one step of an iterable", () => {
+    const counted = (/** @type {number} */ length) => {
+      const seen = { steps: 0, closed: 0 };
+      const iterable = {
+        [Symbol.iterator]: () => ({
+          next: () => ({ done: ++seen.steps > length, value: seen.steps }),
+          return: () => (seen.closed++, {}),
+        }),
+      };
+      return { seen, iterable };
+    };
+
+    test("an iterable with elements", () => {
+      const { seen, iterable } = counted(10);
+      expect(iterable).not.toBeEmpty();
+      expect(seen).toEqual({ steps: 1, closed: isBun ? 1 : 0 });
+    });
+
+    test("an iterable with no elements", () => {
+      const { seen, iterable } = counted(0);
+      expect(iterable).toBeEmpty();
+      expect(seen).toEqual({ steps: 1, closed: 0 });
+    });
+
+    test("a failed assertion", () => {
+      const { seen, iterable } = counted(10);
+      expect(() => expect(iterable).toBeEmpty()).toThrow("Expected value to be empty");
+      expect(seen).toEqual({ steps: 1, closed: isBun ? 1 : 0 });
+    });
+
+    test("a Map iterator keeps the entries after the first", () => {
+      const entries = new Map([
+        ["a", 1],
+        ["b", 2],
+        ["c", 3],
+      ]).entries();
+      expect(entries).not.toBeEmpty();
+      expect([...entries]).toEqual([
+        ["b", 2],
+        ["c", 3],
+      ]);
+    });
+
+    if (isBun) {
+      test("a generator runs its finally block", () => {
+        const seen = { steps: 0, cleanedUp: false };
+        const generator = (function* () {
+          try {
+            while (++seen.steps <= 10) yield seen.steps;
+          } finally {
+            seen.cleanedUp = true;
+          }
+        })();
+        expect(generator).not.toBeEmpty();
+        expect(seen).toEqual({ steps: 1, cleanedUp: true });
+      });
+
+      test("an error from next() or return() reaches the caller", () => {
+        const throwsFromNext = {
+          [Symbol.iterator]: () => ({
+            next() {
+              throw new Error("from next");
+            },
+          }),
+        };
+        let steps = 0;
+        const throwsFromReturn = {
+          [Symbol.iterator]: () => ({
+            next: () => ({ done: ++steps > 10, value: steps }),
+            return() {
+              throw new Error("from return");
+            },
+          }),
+        };
+        expect(() => expect(throwsFromNext).not.toBeEmpty()).toThrow("from next");
+        expect(() => expect(throwsFromReturn).not.toBeEmpty()).toThrow("from return");
+      });
+    }
+  });
+
   test("toBeEmptyObject()", () => {
     // Map and Set are not considered as object in jest-extended
     // https://github.com/jestjs/jest/blob/main/packages/jest-get-type/src/index.ts#L26