LibJS: Add Date.prototype.setMonth()

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -220,6 +220,7 @@ namespace JS {
     P(setHours)                              \
     P(setMilliseconds)                       \
     P(setMinutes)                            \
+    P(setMonth)                              \
     P(setPrototypeOf)                        \
     P(setSeconds)                            \
     P(setYear)                               \

Userland/Libraries/LibJS/Runtime/DatePrototype.cpp

@@ -52,6 +52,7 @@ void DatePrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.getMinutes, get_minutes, 0, attr);
     define_native_function(vm.names.setMinutes, set_minutes, 3, attr);
     define_native_function(vm.names.getMonth, get_month, 0, attr);
+    define_native_function(vm.names.setMonth, set_month, 2, attr);
     define_native_function(vm.names.getSeconds, get_seconds, 0, attr);
     define_native_function(vm.names.setSeconds, set_seconds, 2, attr);
     define_native_function(vm.names.getTime, get_time, 0, attr);
@@ -409,6 +410,40 @@ JS_DEFINE_NATIVE_FUNCTION(DatePrototype::get_month)
     return Value(this_object->month());
 }
 
+JS_DEFINE_NATIVE_FUNCTION(DatePrototype::set_month)
+{
+    auto* this_object = typed_this(vm, global_object);
+    if (!this_object)
+        return {};
+
+    auto arg_or = [&vm, &global_object](size_t i, i32 fallback) { return vm.argument_count() > i ? vm.argument(i).to_number(global_object) : Value(fallback); };
+
+    auto& datetime = this_object->datetime();
+
+    auto new_month_value = vm.argument(0).to_number(global_object);
+    if (vm.exception())
+        return {};
+    if (!new_month_value.is_finite_number()) {
+        this_object->set_is_invalid(true);
+        return js_nan();
+    }
+    auto new_month = new_month_value.as_i32() + 1; // JS Months: 0 - 11, DateTime months: 1-12
+
+    auto new_date_value = arg_or(1, this_object->date());
+    if (vm.exception())
+        return {};
+    if (!new_date_value.is_finite_number()) {
+        this_object->set_is_invalid(true);
+        return js_nan();
+    }
+    auto new_date = new_date_value.as_i32();
+
+    this_object->set_is_invalid(false);
+
+    datetime.set_time(datetime.year(), new_month, new_date, datetime.hour(), datetime.minute(), datetime.second());
+    return Value(this_object->time());
+}
+
 JS_DEFINE_NATIVE_FUNCTION(DatePrototype::get_seconds)
 {
     auto* this_object = typed_this(vm, global_object);

Userland/Libraries/LibJS/Runtime/DatePrototype.h

@@ -33,6 +33,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(get_minutes);
     JS_DECLARE_NATIVE_FUNCTION(set_minutes);
     JS_DECLARE_NATIVE_FUNCTION(get_month);
+    JS_DECLARE_NATIVE_FUNCTION(set_month);
     JS_DECLARE_NATIVE_FUNCTION(get_seconds);
     JS_DECLARE_NATIVE_FUNCTION(set_seconds);
     JS_DECLARE_NATIVE_FUNCTION(get_time);

Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.setMonth.js

@@ -0,0 +1,87 @@
+test("no arguments", () => {
+    let date = new Date(2021, 0, 1);
+    date.setMonth();
+    expect(date.getTime()).toBe(NaN);
+});
+
+test("NaN or undefined as only argument", () => {
+    let date = new Date(2021, 0, 1);
+    date.setMonth(NaN);
+    expect(date.getTime()).toBe(NaN);
+
+    date = new Date(2021, 0, 1);
+    date.setMonth(undefined);
+    expect(date.getTime()).toBe(NaN);
+
+    date = new Date(2021, 0, 1);
+    date.setMonth("a");
+    expect(date.getTime()).toBe(NaN);
+});
+
+test("Only month as argument", () => {
+    let date = new Date(2021, 0, 1);
+
+    date.setMonth(7);
+    expect(date.getFullYear()).toBe(2021);
+    expect(date.getMonth()).toBe(7);
+    expect(date.getDate()).toBe(1);
+    expect(date.getHours()).toBe(0);
+    expect(date.getMinutes()).toBe(0);
+    expect(date.getSeconds()).toBe(0);
+    expect(date.getMilliseconds()).toBe(0);
+});
+
+test("Month and date as arguments", () => {
+    let date = new Date(2021, 0, 1);
+
+    date.setMonth(7, 3);
+    expect(date.getFullYear()).toBe(2021);
+    expect(date.getMonth()).toBe(7);
+    expect(date.getDate()).toBe(3);
+    expect(date.getHours()).toBe(0);
+    expect(date.getMinutes()).toBe(0);
+    expect(date.getSeconds()).toBe(0);
+    expect(date.getMilliseconds()).toBe(0);
+});
+
+test("NaN or undefined in any arguments", () => {
+    let date = new Date(2021, 0, 1);
+    date.setMonth(NaN, 3);
+    expect(date.getTime()).toBe(NaN);
+
+    date = new Date(2021, 0, 1);
+    date.setMonth(2021, NaN);
+    expect(date.getTime()).toBe(NaN);
+
+    date = new Date(2021, 0, 1);
+    date.setMonth(undefined, 3);
+    expect(date.getTime()).toBe(NaN);
+
+    date = new Date(2021, 0, 1);
+    date.setMonth(2021, undefined);
+    expect(date.getTime()).toBe(NaN);
+
+    date.setMonth(3, 16);
+    expect(date.getFullYear()).toBe(2021);
+    expect(date.getMonth()).toBe(3);
+    expect(date.getDate()).toBe(16);
+    expect(date.getHours()).toBe(0);
+    expect(date.getMinutes()).toBe(0);
+    expect(date.getSeconds()).toBe(0);
+    expect(date.getMilliseconds()).toBe(0);
+});
+
+test("Make Invalid Date valid again", () => {
+    let date = new Date(2021, 0, 1);
+    date.setMonth(NaN, 3, 16);
+    expect(date.getTime()).toBe(NaN);
+
+    date.setMonth(3, 16);
+    expect(date.getFullYear()).toBe(2021);
+    expect(date.getMonth()).toBe(3);
+    expect(date.getDate()).toBe(16);
+    expect(date.getHours()).toBe(0);
+    expect(date.getMinutes()).toBe(0);
+    expect(date.getSeconds()).toBe(0);
+    expect(date.getMilliseconds()).toBe(0);
+});