From 7c1e2adf8a1fe2667378f734f0cf22ec625ab55c Mon Sep 17 00:00:00 2001
From: davidot <david.tuin@gmail.com>
Date: Sat, 12 Jun 2021 19:23:33 +0200
Subject: [PATCH] LibJS: Make Array.prototype.shift generic

---
 .../LibJS/Runtime/ArrayPrototype.cpp          | 47 ++++++++++++++++---
 .../Array.prototype-generic-functions.js      | 10 ++++
 2 files changed, 51 insertions(+), 6 deletions(-)

diff --git a/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index fd6d271c46..64187d3af6 100644
--- a/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -246,15 +246,50 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::pop)
 // 23.1.3.24 Array.prototype.shift ( ), https://tc39.es/ecma262/#sec-array.prototype.shift
 JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::shift)
 {
-    auto* array = Array::typed_this(vm, global_object);
-    if (!array)
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
         return {};
-    if (array->indexed_properties().is_empty())
-        return js_undefined();
-    auto result = array->indexed_properties().take_first(array);
+    auto length = length_of_array_like(global_object, *this_object);
     if (vm.exception())
         return {};
-    return result.value.value_or(js_undefined());
+    if (length == 0) {
+        this_object->put(vm.names.length, Value(0));
+        if (vm.exception())
+            return {};
+        return js_undefined();
+    }
+    auto first = this_object->get(0).value_or(js_undefined());
+    if (vm.exception())
+        return {};
+
+    for (size_t k = 1; k < length; ++k) {
+        size_t from = k;
+        size_t to = k - 1;
+        bool from_present = this_object->has_property(from);
+        if (vm.exception())
+            return {};
+        if (from_present) {
+            auto from_value = this_object->get(from).value_or(js_undefined());
+            if (vm.exception())
+                return {};
+            this_object->put(to, from_value);
+            if (vm.exception())
+                return {};
+        } else {
+            this_object->delete_property(to);
+            if (vm.exception())
+                return {};
+        }
+    }
+
+    this_object->delete_property(length - 1);
+    if (vm.exception())
+        return {};
+
+    this_object->put(vm.names.length, Value(length - 1));
+    if (vm.exception())
+        return {};
+    return first;
 }
 
 // 23.1.3.30 Array.prototype.toString ( ), https://tc39.es/ecma262/#sec-array.prototype.tostring
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js
index 85ac3bf9e5..b86cc0d4c5 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js
@@ -100,6 +100,16 @@ describe("ability to work with generic non-array objects", () => {
         expect(Array.prototype.includes.call({ length: 5, 2: "foo" }, "foo")).toBeTrue();
     });
 
+    test("shift", () => {
+        expect(Array.prototype.shift.call({})).toBeUndefined();
+        expect(Array.prototype.shift.call({ length: 0 })).toBeUndefined();
+
+        const o = { length: 5, 0: "a", 1: "b", 3: "c" };
+        const front = Array.prototype.shift.call(o);
+        expect(front).toEqual("a");
+        expect(o).toEqual({ length: 4, 0: "b", 2: "c" });
+    });
+
     const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
 
     test("every", () => {