LibJS: Add Object.defineProperty() and start caring about attributes

We now care (a little bit) about the "configurable" and "writable"
property attributes.

Property attributes are stored together with the property name in
the Shape object. Forward transitions are not attribute-savvy and will
cause poor Shape reuse in the case of multiple same-name properties
with different attributes.

Oh, and this patch also adds Object.getOwnPropertyDescriptor() :^)

1 files changed, 39 insertions, 0 deletions

diff --git a/Libraries/LibJS/Tests/Object.defineProperty.js b/Libraries/LibJS/Tests/Object.defineProperty.js
new file mode 100644
index 0000000000..4f3b7049a6
--- /dev/null
+++ b/Libraries/LibJS/Tests/Object.defineProperty.js
@@ -0,0 +1,39 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+
+    var o = {};
+    Object.defineProperty(o, "foo", { value: 1, writable: false, enumerable: false });
+
+    assert(o.foo === 1);
+    o.foo = 2;
+    assert(o.foo === 1);
+
+    var d = Object.getOwnPropertyDescriptor(o, "foo");
+    assert(d.configurable === false);
+    assert(d.enumerable === false);
+    assert(d.writable === false);
+    assert(d.value === 1);
+
+    Object.defineProperty(o, "bar", { value: "hi", writable: true, enumerable: true });
+
+    assert(o.bar === "hi");
+    o.bar = "ho";
+    assert(o.bar === "ho");
+
+    d = Object.getOwnPropertyDescriptor(o, "bar");
+    assert(d.configurable === false);
+    assert(d.enumerable === true);
+    assert(d.writable === true);
+    assert(d.value === "ho");
+
+    try {
+        Object.defineProperty(o, "bar", { value: "xx", enumerable: false });
+    } catch (e) {
+        assert(e.name === "TypeError");
+    }
+
+    console.log("PASS");
+} catch (e) {
+    console.log(e)
+}