1 files changed, 21 insertions, 0 deletions
diff --git a/js/src/tests/ecma_6/Function/configurable-length-builtins.js b/js/src/tests/ecma_6/Function/configurable-length-builtins.js
new file mode 100644
index 000000000..f28eaae15
--- /dev/null
+++ b/js/src/tests/ecma_6/Function/configurable-length-builtins.js
@@ -0,0 +1,21 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/ */
+
+// Deleting .length from a variety of builtin functions works as expected.
+for (var fun of [Math.sin, Array.prototype.map, eval]) {
+    assertEq(delete fun.length, true);
+    assertEq(fun.hasOwnProperty("length"), false);
+    assertEq("length" in fun, true);  // still inheriting Function.prototype.length
+    assertEq(fun.length, 0);
+
+    // The inherited property is nonwritable, so assigning still fails
+    // (silently, in sloppy mode).
+    fun.length = Math.hypot;
+    assertEq(fun.length, 0);
+
+    // It can be shadowed via defineProperty.
+    Object.defineProperty(fun, "length", {value: Math.hypot});
+    assertEq(fun.length, Math.hypot);
+}
+
+reportCompare(0, 0, 'ok');