diff --git a/js/src/js.c b/js/src/js.c
index b6986396a3fa..7a1360f2d399 100644
--- a/js/src/js.c
+++ b/js/src/js.c
@@ -998,6 +998,7 @@ Disassemble(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
 
 #define SHOW_FLAG(flag) if (flags & JSFUN_##flag) fputs(" " #flag, stdout);
 
+                SHOW_FLAG(LAMBDA);
                 SHOW_FLAG(SETTER);
                 SHOW_FLAG(GETTER);
                 SHOW_FLAG(BOUND_METHOD);
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index e4d6c21692fc..6a72309e6e15 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -121,11 +121,12 @@ JS_BEGIN_EXTERN_C
 #define JSPROP_INDEX            0x80    /* name is actually (jsint) index */
 
 /* Function flags, set in JSFunctionSpec and passed to JS_NewFunction etc. */
+#define JSFUN_LAMBDA            0x08    /* expressed, not declared, function */
 #define JSFUN_GETTER            JSPROP_GETTER
 #define JSFUN_SETTER            JSPROP_SETTER
 #define JSFUN_BOUND_METHOD      0x40    /* bind this to fun->object's parent */
 #define JSFUN_HEAVYWEIGHT       0x80    /* activation requires a Call object */
-#define JSFUN_FLAGS_MASK        0xf0    /* overlay JSFUN_* attributes */
+#define JSFUN_FLAGS_MASK        0xf8    /* overlay JSFUN_* attributes */
 
 /*
  * Well-known JS values.  The extern'd variables are initialized when the
diff --git a/js/src/jsfun.c b/js/src/jsfun.c
index 2cfba3fd7409..18140b29b53e 100644
--- a/js/src/jsfun.c
+++ b/js/src/jsfun.c
@@ -1419,7 +1419,7 @@ Function(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
     parent = NULL;
 #endif
 
-    fun = js_NewFunction(cx, obj, NULL, 0, 0, parent,
+    fun = js_NewFunction(cx, obj, NULL, 0, JSFUN_LAMBDA, parent,
                          (JSVERSION_IS_ECMA(cx->version))
                          ? cx->runtime->atomState.anonymousAtom
                          : NULL);
diff --git a/js/src/jsopcode.c b/js/src/jsopcode.c
index 9853c57bd842..c1d17a343801 100644
--- a/js/src/jsopcode.c
+++ b/js/src/jsopcode.c
@@ -2268,6 +2268,8 @@ js_DecompileFunction(JSPrinter *jp, JSFunction *fun)
         js_puts(jp, "\n");
 	js_printf(jp, "\t");
     }
+    if (fun->flags & JSFUN_LAMBDA)
+        js_puts(jp, "(");
     if (fun->flags & JSFUN_GETTER)
         js_printf(jp, "%s ", js_getter_str);
     else if (fun->flags & JSFUN_SETTER)
@@ -2326,6 +2328,8 @@ js_DecompileFunction(JSPrinter *jp, JSFunction *fun)
     }
     jp->indent -= 4;
     js_printf(jp, "\t}");
+    if (fun->flags & JSFUN_LAMBDA)
+        js_puts(jp, ")");
     if (jp->pretty)
 	js_puts(jp, "\n");
     return JS_TRUE;
diff --git a/js/src/jsparse.c b/js/src/jsparse.c
index 67056b6d9216..96fa159c2ab6 100644
--- a/js/src/jsparse.c
+++ b/js/src/jsparse.c
@@ -637,7 +637,8 @@ FunctionDef(JSContext *cx, JSTokenStream *ts, JSTreeContext *tc,
 
     /* Find the nearest variable-declaring scope and use it as our parent. */
     parent = cx->fp->varobj;
-    fun = js_NewFunction(cx, NULL, NULL, 0, 0, parent, funAtom);
+    fun = js_NewFunction(cx, NULL, NULL, 0, lambda ? JSFUN_LAMBDA : 0, parent,
+                         funAtom);
     if (!fun)
         return NULL;
 
@@ -2792,7 +2793,7 @@ PrimaryExpr(JSContext *cx, JSTokenStream *ts, JSTreeContext *tc)
                             /* We have to fake a 'function' token here. */
                             CURRENT_TOKEN(ts).t_op = JSOP_NOP;
                             CURRENT_TOKEN(ts).type = TOK_FUNCTION;
-                            pn2 = FunctionDef(cx, ts, tc, JS_TRUE);
+                            pn2 = FunctionExpr(cx, ts, tc);
                             pn2 = NewBinary(cx, TOK_COLON, op, pn3, pn2, tc);
                             goto skip;
                         }