diff --git a/dlls/jscript/engine.c b/dlls/jscript/engine.c
index 593294c313d..1ff5eb3ff0c 100644
--- a/dlls/jscript/engine.c
+++ b/dlls/jscript/engine.c
@@ -2028,10 +2028,44 @@ HRESULT instanceof_expression_eval(exec_ctx_t *ctx, expression_t *_expr, DWORD f
 }
 
 /* ECMA-262 3rd Edition    11.8.7 */
-HRESULT in_expression_eval(exec_ctx_t *ctx, expression_t *expr, DWORD flags, jsexcept_t *ei, exprval_t *ret)
+static HRESULT in_eval(exec_ctx_t *ctx, VARIANT *lval, VARIANT *obj, jsexcept_t *ei, VARIANT *retv)
 {
-    FIXME("\n");
-    return E_NOTIMPL;
+    VARIANT_BOOL ret;
+    DISPID id;
+    BSTR str;
+    HRESULT hres;
+
+    if(V_VT(obj) != VT_DISPATCH) {
+        FIXME("throw TypeError");
+        return E_FAIL;
+    }
+
+    hres = to_string(ctx->parser->script, lval, ei, &str);
+    if(FAILED(hres))
+        return hres;
+
+    hres = disp_get_id(V_DISPATCH(obj), str, 0, &id);
+    SysFreeString(str);
+    if(SUCCEEDED(hres))
+        ret = VARIANT_TRUE;
+    else if(hres == DISP_E_UNKNOWNNAME)
+        ret = VARIANT_FALSE;
+    else
+        return hres;
+
+    V_VT(retv) = VT_BOOL;
+    V_BOOL(retv) = ret;
+    return S_OK;
+}
+
+/* ECMA-262 3rd Edition    11.8.7 */
+HRESULT in_expression_eval(exec_ctx_t *ctx, expression_t *_expr, DWORD flags, jsexcept_t *ei, exprval_t *ret)
+{
+    binary_expression_t *expr = (binary_expression_t*)_expr;
+
+    TRACE("\n");
+
+    return binary_expr_eval(ctx, expr, in_eval, ei, ret);
 }
 
 /* ECMA-262 3rd Edition    11.6.1 */
diff --git a/dlls/jscript/tests/lang.js b/dlls/jscript/tests/lang.js
index 354fdb4c255..ae69e71bf47 100644
--- a/dlls/jscript/tests/lang.js
+++ b/dlls/jscript/tests/lang.js
@@ -817,6 +817,17 @@ ok(("" instanceof Object) === false, "'' is instance of Object");
     ok(arguments.toString() === "[object Object]", "arguments.toString() = " + arguments.toString());
 })(1,2);
 
+obj = new String();
+ok(("length" in obj) === true, "length is not in obj");
+ok(("isPrototypeOf" in obj) === true, "isPrototypeOf is not in obj");
+ok(("abc" in obj) === false, "test is in obj");
+obj.abc = 1;
+ok(("abc" in obj) === true, "test is not in obj");
+ok(("1" in obj) === false, "1 is in obj");
+
+obj = [1,2,3];
+ok((1 in obj) === true, "1 is not in obj");
+
 ok(isNaN(NaN) === true, "isNaN(NaN) !== true");
 ok(isNaN(0.5) === false, "isNaN(0.5) !== false");
 ok(isNaN(Infinity) === false, "isNaN(Infinity) !== false");
diff --git a/dlls/jscript/tests/run.c b/dlls/jscript/tests/run.c
index 90b022d95d5..786624cc738 100644
--- a/dlls/jscript/tests/run.c
+++ b/dlls/jscript/tests/run.c
@@ -66,6 +66,8 @@ DEFINE_EXPECT(global_success_i);
 DEFINE_EXPECT(global_notexists_d);
 DEFINE_EXPECT(testobj_delete);
 DEFINE_EXPECT(testobj_value);
+DEFINE_EXPECT(testobj_prop_d);
+DEFINE_EXPECT(testobj_noprop_d);
 DEFINE_EXPECT(GetItemInfo_testVal);
 
 #define DISPID_GLOBAL_TESTPROPGET   0x1000
@@ -78,6 +80,8 @@ DEFINE_EXPECT(GetItemInfo_testVal);
 #define DISPID_GLOBAL_NULL_BSTR     0x1007
 #define DISPID_GLOBAL_NULL_DISP     0x1008
 
+#define DISPID_TESTOBJ_PROP         0x2000
+
 static const WCHAR testW[] = {'t','e','s','t',0};
 static const CHAR testA[] = "test";
 static const WCHAR test_valW[] = {'t','e','s','t','V','a','l',0};
@@ -194,9 +198,21 @@ static HRESULT WINAPI DispatchEx_GetNameSpaceParent(IDispatchEx *iface, IUnknown
     return E_NOTIMPL;
 }
 
-static HRESULT WINAPI DispatchEx_GetDispID(IDispatchEx *iface, BSTR bstrName, DWORD grfdex, DISPID *pid)
+static HRESULT WINAPI testObj_GetDispID(IDispatchEx *iface, BSTR bstrName, DWORD grfdex, DISPID *pid)
 {
-    ok(0, "unexpected call\n");
+    if(!strcmp_wa(bstrName, "prop")) {
+        CHECK_EXPECT(testobj_prop_d);
+        ok(grfdex == fdexNameCaseSensitive, "grfdex = %x\n", grfdex);
+        *pid = DISPID_TESTOBJ_PROP;
+        return S_OK;
+    }
+    if(!strcmp_wa(bstrName, "noprop")) {
+        CHECK_EXPECT(testobj_noprop_d);
+        ok(grfdex == fdexNameCaseSensitive, "grfdex = %x\n", grfdex);
+        return DISP_E_UNKNOWNNAME;
+    }
+
+    ok(0, "unexpected name %s\n", wine_dbgstr_w(bstrName));
     return E_NOTIMPL;
 }
 
@@ -243,7 +259,7 @@ static IDispatchExVtbl testObjVtbl = {
     DispatchEx_GetTypeInfo,
     DispatchEx_GetIDsOfNames,
     DispatchEx_Invoke,
-    DispatchEx_GetDispID,
+    testObj_GetDispID,
     testObj_InvokeEx,
     testObj_DeleteMemberByName,
     DispatchEx_DeleteMemberByDispID,
@@ -884,6 +900,14 @@ static void run_tests(void)
 
     parse_script_a("ok((testObj instanceof Object) === false, 'testObj is instance of Object');");
 
+    SET_EXPECT(testobj_prop_d);
+    parse_script_a("ok(('prop' in testObj) === true, 'prop is not in testObj');");
+    CHECK_CALLED(testobj_prop_d);
+
+    SET_EXPECT(testobj_noprop_d);
+    parse_script_a("ok(('noprop' in testObj) === false, 'noprop is in testObj');");
+    CHECK_CALLED(testobj_noprop_d);
+
     SET_EXPECT(testobj_value);
     parse_script_a("ok(String(testObj) === '1', 'wrong testObj value');");
     CHECK_CALLED(testobj_value);