diff --git a/Libraries/LibJS/Runtime/ReflectObject.cpp b/Libraries/LibJS/Runtime/ReflectObject.cpp
index a36be0520e..3fd2dddb04 100644
--- a/Libraries/LibJS/Runtime/ReflectObject.cpp
+++ b/Libraries/LibJS/Runtime/ReflectObject.cpp
@@ -211,12 +211,12 @@ Value ReflectObject::has(Interpreter& interpreter)
     return Value(target->has_property(property_key));
 }
 
-Value ReflectObject::is_extensible(Interpreter&)
+Value ReflectObject::is_extensible(Interpreter& interpreter)
 {
-    // FIXME: For this to be useful we need one of these:
-    // Object.seal(), Object.freeze(), Reflect.preventExtensions()
-    // For now we just return true, as that's always the case.
-    return Value(true);
+    auto* target = get_target_object_from(interpreter, "isExtensible");
+    if (!target)
+        return {};
+    return Value(target->is_extensible());
 }
 
 Value ReflectObject::own_keys(Interpreter& interpreter)
@@ -227,10 +227,12 @@ Value ReflectObject::own_keys(Interpreter& interpreter)
     return target->get_own_properties(*target, GetOwnPropertyMode::Key);
 }
 
-Value ReflectObject::prevent_extensions(Interpreter&)
+Value ReflectObject::prevent_extensions(Interpreter& interpreter)
 {
-    // FIXME: Implement me :^)
-    ASSERT_NOT_REACHED();
+    auto* target = get_target_object_from(interpreter, "preventExtensions");
+    if (!target)
+        return {};
+    return Value(target->prevent_extensions());
 }
 
 Value ReflectObject::set(Interpreter& interpreter)
diff --git a/Libraries/LibJS/Tests/Reflect.isExtensible.js b/Libraries/LibJS/Tests/Reflect.isExtensible.js
new file mode 100644
index 0000000000..104e725d7d
--- /dev/null
+++ b/Libraries/LibJS/Tests/Reflect.isExtensible.js
@@ -0,0 +1,29 @@
+load("test-common.js");
+
+try {
+    assert(Reflect.isExtensible.length === 1);
+
+    [null, undefined, "foo", 123, NaN, Infinity].forEach(value => {
+        assertThrowsError(() => {
+            Reflect.isExtensible(value);
+        }, {
+            error: TypeError,
+            message: "First argument of Reflect.isExtensible() must be an object"
+        });
+    });
+
+    assert(Reflect.isExtensible({}) === true);
+
+    var o = {};
+    o.foo = "foo";
+    assert(o.foo === "foo");
+    assert(Reflect.isExtensible(o) === true);
+    Reflect.preventExtensions(o);
+    o.bar = "bar";
+    assert(o.bar === undefined);
+    assert(Reflect.isExtensible(o) === false);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}
diff --git a/Libraries/LibJS/Tests/Reflect.preventExtensions.js b/Libraries/LibJS/Tests/Reflect.preventExtensions.js
new file mode 100644
index 0000000000..efbda277d4
--- /dev/null
+++ b/Libraries/LibJS/Tests/Reflect.preventExtensions.js
@@ -0,0 +1,23 @@
+load("test-common.js");
+
+try {
+    assert(Reflect.preventExtensions.length === 1);
+
+    [null, undefined, "foo", 123, NaN, Infinity].forEach(value => {
+        assertThrowsError(() => {
+            Reflect.preventExtensions(value);
+        }, {
+            error: TypeError,
+            message: "First argument of Reflect.preventExtensions() must be an object"
+        });
+    });
+
+    var o = {};
+    assert(Reflect.isExtensible(o) === true);
+    assert(Reflect.preventExtensions(o) === true);
+    assert(Reflect.isExtensible(o) === false);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}