diff --git a/src/twocore/backend/emit_core.gleam b/src/twocore/backend/emit_core.gleam
index 9fd2405..c215004 100644
--- a/src/twocore/backend/emit_core.gleam
+++ b/src/twocore/backend/emit_core.gleam
@@ -4220,6 +4220,7 @@ fn resolve_js(op: String) -> Option(String) {
     "set_prop" -> Some("set_prop")
     "gen_make" -> Some("gen_make")
     "gen_next" -> Some("gen_next")
+    "gen_return" -> Some("gen_return")
     "iter_array" -> Some("iter_array")
     "iter_to_array" -> Some("iter_to_array")
     "iter_take" -> Some("iter_take")
diff --git a/src/twocore/frontend/js/lower.gleam b/src/twocore/frontend/js/lower.gleam
index 6b71797..69bddf5 100644
--- a/src/twocore/frontend/js/lower.gleam
+++ b/src/twocore/frontend/js/lower.gleam
@@ -116,7 +116,11 @@
 //// future direction could hand a dynamic string off to an embedded arc VM, but that
 //// is explicitly out of scope — the focus is making the AOT path excellent.)
 ////
-//// Not yet (a clean `Unsupported` error / panic): generators/async, and a
+//// Generators (`function* g(){ … }`) lower to a resumable state machine driven by
+//// `.next()` — both the DECLARATION form and the function-EXPRESSION form (e.g. an
+//// immediately-invoked `(function*(){ … })()`), which the pre-pass rewrites the same
+//// way; async is still unsupported.
+//// Not yet (a clean `Unsupported` error / panic): async, and a
 //// `return`/`break`/`continue` inside a `try`/`finally` body (it would bypass the
 //// finally, so it is rejected). Static
 //// field initializers run when the module's `main/0` runs (like any top-level
@@ -5792,6 +5796,9 @@ fn lower_instance_method_generic(
     // generator `.next(v)` — advances a generator, or delegates to a user
     // `next` method on an ordinary iterator object.
     "next", _ -> coll("gen_next")
+    // generator `.return(v)` — closes a generator (§27.5.1.4), yielding
+    // `{value: v, done: true}`; delegates to a user `return` method otherwise.
+    "return", _ -> coll("gen_return")
     // Object.prototype.hasOwnProperty(key) — own-property test (no prototype
     // chain in this model), returning a JS boolean.
     "hasOwnProperty", [k, ..] -> host("object_has_own", [k])
@@ -7213,6 +7220,18 @@ fn lambda_nodes_expr(e: ast.Expression) -> List(RawLambda) {
       let stmts = arrow_body_stmts(body)
       [RawLambda(span, params, stmts), ..lambda_nodes_stmts(stmts)]
     }
+    ast.FunctionExpression(span:, params:, body:, is_generator: True, ..) -> {
+      // A generator function EXPRESSION (e.g. `(function*(){ … })()`): rewrite its
+      // body to the resumable state machine, exactly as a generator DECLARATION is
+      // rewritten in `as_function_decl`. On an unsupported generator shape, fall
+      // back to the original body so the surviving `yield` reports a clean
+      // Unsupported error during lowering (rather than silently mis-collecting).
+      let stmts = case transform_generator(params, body) {
+        Ok(new_body) -> block_stmts(new_body)
+        Error(_) -> block_stmts(body)
+      }
+      [RawLambda(span, params, stmts), ..lambda_nodes_stmts(stmts)]
+    }
     ast.FunctionExpression(span:, params:, body:, ..) -> {
       let stmts = block_stmts(body)
       [RawLambda(span, params, stmts), ..lambda_nodes_stmts(stmts)]
diff --git a/src/twocore/runtime/rt_js.gleam b/src/twocore/runtime/rt_js.gleam
index bffd603..50632ce 100644
--- a/src/twocore/runtime/rt_js.gleam
+++ b/src/twocore/runtime/rt_js.gleam
@@ -326,6 +326,11 @@ pub fn gen_make(step: Dynamic) -> Dynamic
 @external(erlang, "twocore_rt_js_ffi", "gen_next")
 pub fn gen_next(gen: Dynamic, args: Dynamic) -> Dynamic
 
+/// `gen.return(v)` — close a generator and yield `{value: v, done: true}` (or
+/// delegate to a user `return` method on a plain iterator object).
+@external(erlang, "twocore_rt_js_ffi", "gen_return")
+pub fn gen_return(gen: Dynamic, args: Dynamic) -> Dynamic
+
 /// Materialize the source of a `for-of` as an array: arrays and strings pass
 /// through, a generator is drained to an array, anything else becomes empty.
 @external(erlang, "twocore_rt_js_ffi", "iter_array")
diff --git a/src/twocore_rt_js_ffi.erl b/src/twocore_rt_js_ffi.erl
index b102e66..dfda0fc 100644
--- a/src/twocore_rt_js_ffi.erl
+++ b/src/twocore_rt_js_ffi.erl
@@ -63,7 +63,7 @@
     bit_and/2, bit_or/2, bit_xor/2, bit_not/1, shl/2, shr/2, ushr/2, pow/2,
     math_unary/2, math_binary/3, math_reduce/2, math_random/0,
     cell_new/1, cell_get/1, cell_set/2,
-    new_object/0, globalthis_new/0, wrapper_new/2, error_make/2, error_ctor/1, error_is_error/1, js_error_to_value/1, gen_make/1, gen_next/2, iter_array/1, iter_to_array/1, iter_take/2, iter_drop/2, get_prop/2, set_prop/3, define_data/3, define_accessor/4,
+    new_object/0, globalthis_new/0, wrapper_new/2, error_make/2, error_ctor/1, error_is_error/1, js_error_to_value/1, gen_make/1, gen_next/2, gen_return/2, iter_array/1, iter_to_array/1, iter_take/2, iter_drop/2, get_prop/2, set_prop/3, define_data/3, define_accessor/4,
     builtin_ctor/1, builtin_prototype/1, to_object/1,
     static_get/2, static_get_chain/2, static_set/3, has_prop/2, delete_prop/2,
     new_array/1, array_construct/1, array_push/2, array_pop/1, is_array/1, array_spread_into/2,
@@ -1304,6 +1304,36 @@ gen_next(Recv, Args) when is_reference(Recv) ->
 gen_next(Recv, Args) ->
     delegate(Recv, <<"next">>, Args).
 
+%% `gen.return(v)` — Generator.prototype.return (§27.5.1.4). Closes the generator
+%% (as if a `return v` ran at the current suspend point) and returns the result
+%% object `{value: v, done: true}`; `v` defaults to `undefined`. A subsequent
+%% `.next()`/iterator-helper on the closed generator then sees it exhausted. The
+%% generators modelled here have no observable `try`/`finally`, so closing is a
+%% simple state transition to DONE. A non-generator receiver delegates to a user
+%% `return` method (a plain-object iterator implementing the optional return step);
+%% a receiver without one is a non-reference/absent-method — left to `delegate`.
+gen_return(Recv, Args) when is_reference(Recv) ->
+    case erlang:get(?CELL_KEY(Recv)) of
+        Tag when
+            Tag =:= js_gen_done orelse
+                (is_tuple(Tag) andalso
+                    (element(1, Tag) =:= js_gen orelse
+                        element(1, Tag) =:= js_gen_running))
+        ->
+            %% Replace the step with a permanently-DONE step and keep the `{js_gen,_}`
+            %% tag, so both `.next()` and the iterator helpers (which dispatch on that
+            %% tag) treat the closed generator as an empty iterator.
+            erlang:put(
+                ?CELL_KEY(Recv),
+                {js_gen, fun(_Sent) -> iter_result(undefined, true) end}
+            ),
+            iter_result(arg(Args, 0), true);
+        _ ->
+            delegate(Recv, <<"return">>, Args)
+    end;
+gen_return(Recv, Args) ->
+    delegate(Recv, <<"return">>, Args).
+
 %% IteratorClose for a generator receiver (§7.4.11 as used by the Iterator
 %% helpers): permanently mark the generator DONE so any later `.next()` returns
 %% `{value: undefined, done: true}`. Called when a helper short-circuits (e.g.
diff --git a/test/js_compiler_test.gleam b/test/js_compiler_test.gleam
index 7011cc1..1e95633 100644
--- a/test/js_compiler_test.gleam
+++ b/test/js_compiler_test.gleam
@@ -6851,3 +6851,99 @@ pub fn array_map_thisarg_arrow_unaffected_test() {
   val("[1, 2, 3].map((x) => x * 2, { k: 9 }).join(',')")
   |> should.equal(dyn("2,4,6"))
 }
+
+// ==== Iterator (wave 13) ====
+
+pub fn generator_expression_iife_yields_test() {
+  // A generator FUNCTION EXPRESSION invoked immediately (`(function*(){…})()`)
+  // must produce a real generator object (not `undefined`, as it did before the
+  // expression form was transformed). Driving it with `.next()` yields each value
+  // in order, then `{done:true}` (§27.5). Regression for `(function*(){…})()`.
+  let m =
+    compile(
+      "function f() { let it = (function*(){ yield 10; yield 20; })(); "
+      <> "let a = it.next(); let b = it.next(); let c = it.next(); "
+      <> "return a.value + b.value * 10 + (c.done === true ? 1000 : 0); }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(1210.0)
+}
+
+pub fn generator_expression_empty_exhausted_some_test() {
+  // Mirrors test/built-ins/Iterator/prototype/some/iterator-already-exhausted.js:
+  // an already-exhausted iterator returns `false` from `.some` for any predicate,
+  // because there are no remaining values to test (§27.1.4.18).
+  let m =
+    compile(
+      "function f() { let it = (function*(){})(); it.next(); "
+      <> "let a = it.some(function(x){ return true; }); "
+      <> "let b = it.some(function(x){ return false; }); "
+      <> "return (a === false && b === false) ? 1 : 0; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(1.0)
+}
+
+pub fn generator_expression_empty_exhausted_toarray_test() {
+  // Mirrors Iterator/prototype/toArray/iterator-already-exhausted.js: `.toArray`
+  // on an exhausted iterator yields an empty array (§27.1.4.19).
+  let m =
+    compile(
+      "function f() { let it = (function*(){})(); it.next(); "
+      <> "return it.toArray().length; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(0.0)
+}
+
+pub fn generator_expression_exhausted_every_find_reduce_test() {
+  // every → true (vacuously) and find → undefined on an exhausted iterator
+  // (§27.1.4.6 / .8); reduce with an initial value returns that seed unchanged
+  // (§27.1.4.16).
+  let m =
+    compile(
+      "function f() { let it = (function*(){})(); it.next(); "
+      <> "let e = it.every(function(x){ return false; }); "
+      <> "let g = (function*(){})(); g.next(); "
+      <> "let fd = g.find(function(x){ return true; }); "
+      <> "let h = (function*(){})(); h.next(); "
+      <> "let rd = h.reduce(function(a, x){ return a + x; }, 42); "
+      <> "return (e === true && fd === undefined && rd === 42) ? 1 : 0; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(1.0)
+}
+
+pub fn generator_expression_map_take_toarray_test() {
+  // The lazy helpers compose over a generator-expression source: map doubles each
+  // value, take limits to two, toArray collects them (§27.1.4.12 / .17 / .19).
+  let m =
+    compile(
+      "function f() { let it = (function*(){ yield 1; yield 2; yield 3; yield 4; })(); "
+      <> "let arr = it.map(function(x){ return x * 2; }).take(2).toArray(); "
+      <> "return arr.length * 100 + arr[0] * 10 + arr[1]; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(224.0)
+}
+
+pub fn generator_return_closes_test() {
+  // Generator.prototype.return (§27.5.1.4): `.return(v)` closes the generator and
+  // yields `{value: v, done: true}`; a following `.next()` sees it exhausted.
+  let m =
+    compile(
+      "function f() { let it = (function*(){ yield 1; yield 2; })(); "
+      <> "let r = it.return(99); let n = it.next(); "
+      <> "return (r.value === 99 && r.done === true && n.done === true) ? 1 : 0; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(1.0)
+}
+
+pub fn generator_return_underlying_closed_map_test() {
+  // Mirrors Iterator/prototype/map/underlying-iterator-closed.js: closing the
+  // source with `.return()` BEFORE building a `.map` helper means the mapped
+  // iterator produces nothing — its first `.next()` is `{value:undefined,done:true}`.
+  let m =
+    compile(
+      "function f() { let it = (function*(){ yield 0; yield 1; yield 2; })(); "
+      <> "it.return(); let mapped = it.map(function(){ return 0; }); "
+      <> "let r = mapped.next(); "
+      <> "return (r.value === undefined && r.done === true) ? 1 : 0; }",
+    )
+  to_float(call(m, "f", [])) |> should.equal(1.0)
+}