LibJS: Fix Array.prototype.lastIndexOf() implementation

2 files changed, 8 insertions, 10 deletions

diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index edfa930ed2..983e4c36b6 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -414,22 +414,19 @@ Value ArrayPrototype::last_index_of(Interpreter& interpreter)
     if (array_size == 0)
         return Value(-1);
 
-    i32 from_index = 0;
+    i32 from_index = array_size - 1;
     if (interpreter.argument_count() >= 2) {
         from_index = interpreter.argument(1).to_i32(interpreter);
         if (interpreter.exception())
             return {};
-        if (from_index >= array_size)
-            return Value(-1);
-        auto negative_min_index = ((array_size - 1) * -1);
-        if (from_index < negative_min_index)
-            from_index = 0;
-        else if (from_index < 0)
+        if (from_index >= 0)
+            from_index = min(from_index, array_size - 1);
+        else
             from_index = array_size + from_index;
     }
 
     auto search_element = interpreter.argument(0);
-    for (i32 i = array_size - 1; i >= from_index; --i) {
+    for (i32 i = from_index; i >= 0; --i) {
         auto& element = array->elements().at(i);
         if (strict_eq(interpreter, element, search_element))
             return Value(i);
diff --git a/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js b/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
index 1ae10c4e55..c2b1df8d4b 100644
--- a/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
+++ b/Libraries/LibJS/Tests/Array.prototype.lastIndexOf.js
@@ -6,11 +6,12 @@ try {
     var array = [1, 2, 3, 1, "hello"];
 
     assert(array.lastIndexOf("hello") === 4);
+    assert(array.lastIndexOf("hello", 1000) === 4);
     assert(array.lastIndexOf(1) === 3);
-    assert(array.lastIndexOf(1, -1) === -1);
+    assert(array.lastIndexOf(1, -1) === 3);
     assert(array.lastIndexOf(1, -2) === 3);
     assert(array.lastIndexOf(2) === 1);
-    assert(array.lastIndexOf(2, -3) === -1);
+    assert(array.lastIndexOf(2, -3) === 1);
     assert(array.lastIndexOf(2, -4) === 1);
     assert([].lastIndexOf('hello') === -1);
     assert([].lastIndexOf('hello', 10) === -1);