LibJS: Basic implementation of most of Date's constructor arguments

The constructor with a string argument isn't implemented yet, but
this implements the other variants.

The timestamp constructor doens't handle negative timestamps correctly.

Out-of-bound and invalid arguments aren't handled correctly.

Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -60,13 +60,48 @@ Value DateConstructor::call(Interpreter& interpreter)
     return js_string(interpreter, static_cast<Date&>(date.as_object()).string());
 }
 
-Value DateConstructor::construct(Interpreter&, Function&)
+Value DateConstructor::construct(Interpreter& interpreter, Function&)
 {
-    // TODO: Support args
-    struct timeval tv;
-    gettimeofday(&tv, nullptr);
-    auto datetime = Core::DateTime::now();
-    auto milliseconds = static_cast<u16>(tv.tv_usec / 1000);
+    if (interpreter.argument_count() == 0) {
+        struct timeval tv;
+        gettimeofday(&tv, nullptr);
+        auto datetime = Core::DateTime::now();
+        auto milliseconds = static_cast<u16>(tv.tv_usec / 1000);
+        return Date::create(global_object(), datetime, milliseconds);
+    }
+    if (interpreter.argument_count() == 1 && interpreter.argument(0).is_string()) {
+        // FIXME: Parse simplified ISO8601-like string, like Date.parse() will do.
+        struct timeval tv;
+        gettimeofday(&tv, nullptr);
+        auto datetime = Core::DateTime::now();
+        auto milliseconds = static_cast<u16>(tv.tv_usec / 1000);
+        return Date::create(global_object(), datetime, milliseconds);
+    }
+    if (interpreter.argument_count() == 1) {
+        // A timestamp since the epoch, in UTC.
+        // FIXME: Date() probably should use a double as internal representation, so that NaN arguments and larger offsets are handled correctly.
+        // FIXME: DateTime::from_timestamp() seems to not support negative offsets.
+        double value = interpreter.argument(0).to_double(interpreter);
+        auto datetime = Core::DateTime::from_timestamp(static_cast<time_t>(value / 1000));
+        auto milliseconds = static_cast<u16>(fmod(value, 1000));
+        return Date::create(global_object(), datetime, milliseconds);
+    }
+    // A date/time in components, in local time.
+    // FIXME: This doesn't construct an "Invalid Date" object if one of the parameters is NaN.
+    // FIXME: This doesn't range-check args and convert months > 12 to year increments etc.
+    auto arg_or = [&interpreter](size_t i, i32 fallback) { return interpreter.argument_count() > i ? interpreter.argument(i).to_i32(interpreter) : fallback; };
+    int year = interpreter.argument(0).to_i32(interpreter);
+    int month_index = interpreter.argument(1).to_i32(interpreter);
+    int day = arg_or(2, 1);
+    int hours = arg_or(3, 0);
+    int minutes = arg_or(4, 0);
+    int seconds = arg_or(5, 0);
+    int milliseconds = arg_or(6, 0);
+
+    if (year >= 0 && year <= 99)
+      year += 1900;
+    int month = month_index + 1;
+    auto datetime = Core::DateTime::create(year, month, day, hours, minutes, seconds);
     return Date::create(global_object(), datetime, milliseconds);
 }
 

Libraries/LibJS/Tests/builtins/Date/Date.js

@@ -3,3 +3,45 @@ test("basic functionality", () => {
     expect(Date.name === "Date");
     expect(Date.prototype).not.toHaveProperty("length");
 });
+
+test("timestamp constructor", () => {
+    // The timestamp constructor takes a timestamp in milliseconds since the start of the epoch, in UTC.
+
+    // 50 days and 1234 milliseconds after the start of the epoch.
+    // Most Date methods return values in local time, but since timezone offsets are less than 17 days,
+    // these checks will pass in all timezones.
+    let timestamp = 50 * 24 * 60 * 60 * 1000 + 1234;
+
+    let date = new Date(timestamp);
+    expect(date.getTime()).toBe(timestamp); // getTime() returns the timestamp in UTC.
+    expect(date.getMilliseconds()).toBe(234);
+    expect(date.getSeconds()).toBe(1);
+    expect(date.getFullYear()).toBe(1970);
+    expect(date.getMonth()).toBe(1); // Feb
+});
+
+test("tuple constructor", () => {
+    // The tuple constructor takes a date in local time.
+    expect(new Date(2019, 11).getFullYear()).toBe(2019);
+    expect(new Date(2019, 11).getMonth()).toBe(11);
+    expect(new Date(2019, 11).getDate()).toBe(1); // getDay() returns day of week, getDate() returnsn day in month
+    expect(new Date(2019, 11).getHours()).toBe(0);
+    expect(new Date(2019, 11).getMinutes()).toBe(0);
+    expect(new Date(2019, 11).getSeconds()).toBe(0);
+    expect(new Date(2019, 11).getMilliseconds()).toBe(0);
+
+    let date = new Date(2019, 11, 15,  9, 16, 14, 123); // Note: Month is 0-based.
+    expect(date.getFullYear()).toBe(2019);
+    expect(date.getMonth()).toBe(11);
+    expect(date.getDate()).toBe(15);
+    expect(date.getHours()).toBe(9);
+    expect(date.getMinutes()).toBe(16);
+    expect(date.getSeconds()).toBe(14);
+    expect(date.getMilliseconds()).toBe(123);
+
+    // getTime() returns a time stamp in UTC, but we can at least check it's in the right interval, which will be true independent of the local timezone if the range is big enough.
+    let timestamp_lower_bound = 1575072000000;  // 2019-12-01T00:00:00Z
+    let timestamp_upper_bound = 1577750400000;  // 2019-12-31T00:00:00Z
+    expect(date.getTime()).toBeGreaterThan(timestamp_lower_bound);
+    expect(date.getTime()).toBeLessThan(timestamp_upper_bound);
+});