1 files changed, 44 insertions, 0 deletions
diff --git a/js/src/tests/non262/lexical-environment/unscopables-mutation.js b/js/src/tests/non262/lexical-environment/unscopables-mutation.js
new file mode 100644
index 0000000000..2f35e1dd3c
--- /dev/null
+++ b/js/src/tests/non262/lexical-environment/unscopables-mutation.js
@@ -0,0 +1,44 @@
+// When obj[@@unscopables].x changes, bindings appear and disappear accordingly.
+
+let x = "global";
+function getX() { return x; }
+
+let unscopables = {x: true};
+let obj = {x: "obj", [Symbol.unscopables]: unscopables};
+
+with (obj) {
+    assertEq(x, "global");
+    x = "global-1";
+    assertEq(x, "global-1");
+    assertEq(obj.x, "obj");
+
+    unscopables.x = false;  // suddenly x appears in the with-environment
+
+    assertEq(x, "obj");
+    x = "obj-1";
+    assertEq(getX(), "global-1");  // unchanged
+    assertEq(obj.x, "obj-1");
+
+    unscopables.x = true;  // *poof*
+
+    assertEq(x, "global-1");
+    x = "global-2";
+    assertEq(getX(), "global-2");
+    assertEq(obj.x, "obj-1");  // unchanged
+
+    // The determination of which binding is assigned happens when the LHS of
+    // assignment is evaluated, before the RHS. This is observable if we make
+    // the binding appear or disappear during evaluation of the RHS, before
+    // assigning.
+    x = (unscopables.x = false, "global-3");
+    assertEq(getX(), "global-3");
+    assertEq(obj.x, "obj-1");
+
+    x = (unscopables.x = true, "obj-2");
+    assertEq(getX(), "global-3");
+    assertEq(obj.x, "obj-2");
+}
+
+assertEq(x, "global-3");
+
+reportCompare(0, 0);