LibJS: Implement Array length setter

Author: https://github.com/linusg
Commit: 418092a71a
Pull-request: https://github.com/SerenityOS/serenity/pull/1929

Libraries/LibJS/Tests/array-length-setter.js

@@ -0,0 +1,48 @@
+load("test-common.js");
+
+try {
+    var a = [1, 2, 3];
+
+    assert(a.length === 3);
+    assert(a[0] === 1);
+    assert(a[1] === 2);
+    assert(a[2] === 3);
+
+    a.length = 5;
+    assert(a.length === 5);
+    assert(a[0] === 1);
+    assert(a[1] === 2);
+    assert(a[2] === 3);
+    assert(a[3] === undefined);
+    assert(a[4] === undefined);
+
+    a.length = 1;
+    assert(a.length === 1);
+    assert(a[0] === 1);
+
+    a.length = 0;
+    assert(a.length === 0);
+
+    a.length = "42";
+    assert(a.length === 42);
+
+    a.length = [];
+    assert(a.length === 0);
+
+    a.length = true;
+    assert(a.length === 1);
+
+    [undefined, "foo", -1, Infinity, -Infinity, NaN].forEach(value => {
+        assertThrowsError(() => {
+            a.length = value;
+        }, {
+            error: RangeError,
+            message: "Invalid array length"
+        });
+        assert(a.length === 1);
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}