3 files changed, 65 insertions, 0 deletions
diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index 674d500fa3..f208973621 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -58,6 +58,7 @@ ArrayPrototype::ArrayPrototype()
     put_native_function("lastIndexOf", last_index_of, 1);
     put_native_function("includes", includes, 1);
     put_native_function("find", find, 1);
+    put_native_function("findIndex", find_index, 1);
     put("length", Value(0));
 }
 
@@ -458,4 +459,38 @@ Value ArrayPrototype::find(Interpreter& interpreter)
     return js_undefined();
 }
 
+Value ArrayPrototype::find_index(Interpreter& interpreter)
+{
+    auto* array = array_from(interpreter);
+    if (!array)
+        return {};
+
+    auto* callback = callback_from_args(interpreter, "findIndex");
+    if (!callback)
+        return {};
+
+    auto this_value = interpreter.argument(1);
+    auto array_size = array->elements().size();
+
+    for (size_t i = 0; i < array_size; ++i) {
+        auto value = array->elements().at(i);
+        if (value.is_empty())
+            continue;
+
+        MarkedValueList arguments(interpreter.heap());
+        arguments.append(value);
+        arguments.append(Value((i32)i));
+        arguments.append(array);
+
+        auto result = interpreter.call(callback, this_value, move(arguments));
+        if (interpreter.exception())
+            return {};
+
+        if (result.to_boolean())
+            return Value((i32)i);
+    }
+
+    return Value(-1);
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.h b/Libraries/LibJS/Runtime/ArrayPrototype.h
index b5912ee96f..30f4c6f000 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.h
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.h
@@ -55,5 +55,6 @@ private:
     static Value last_index_of(Interpreter&);
     static Value includes(Interpreter&);
     static Value find(Interpreter&);
+    static Value find_index(Interpreter&);
 };
 }
diff --git a/Libraries/LibJS/Tests/Array.prototype.findIndex.js b/Libraries/LibJS/Tests/Array.prototype.findIndex.js
new file mode 100644
index 0000000000..0795a66474
--- /dev/null
+++ b/Libraries/LibJS/Tests/Array.prototype.findIndex.js
@@ -0,0 +1,29 @@
+load("test-common.js");
+
+try {
+    assert(Array.prototype.findIndex.length === 1);
+
+    assertThrowsError(() => {
+        [].findIndex(undefined);
+    }, {
+        error: TypeError,
+        message: "undefined is not a function"
+    });
+
+    var array = ["hello", "friends", 1, 2, false];
+
+    assert(array.findIndex(value => value === "hello") === 0);
+    assert(array.findIndex((value, index, arr) => index === 1) === 1);
+    assert(array.findIndex(value => value == "1") === 2);
+    assert(array.findIndex(value => value === 1) === 2);
+    assert(array.findIndex(value => typeof(value) !== "string") === 2);
+    assert(array.findIndex(value => typeof(value) === "boolean") === 4);
+    assert(array.findIndex(value => value > 1) === 3);
+    assert(array.findIndex(value => value > 1 && value < 3) === 3);
+    assert(array.findIndex(value => value > 100) === -1);
+    assert([].findIndex(value => value === 1) === -1);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}