LibJS: Implement Temporal.Instant.prototype.epochSeconds

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -113,6 +113,7 @@ namespace JS {
     P(endsWith)                              \
     P(entries)                               \
     P(enumerable)                            \
+    P(epochSeconds)                          \
     P(error)                                 \
     P(errors)                                \
     P(escape)                                \

Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.cpp

@@ -4,7 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibCrypto/BigInt/SignedBigInteger.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/Instant.h>
 #include <LibJS/Runtime/Temporal/InstantPrototype.h>
 
 namespace JS::Temporal {
@@ -21,8 +23,42 @@ void InstantPrototype::initialize(GlobalObject& global_object)
 
     auto& vm = this->vm();
 
+    define_native_accessor(vm.names.epochSeconds, epoch_seconds_getter, {}, Attribute::Configurable);
+
     // 8.3.2 Temporal.Instant.prototype[ @@toStringTag ], https://tc39.es/proposal-temporal/#sec-temporal.instant.prototype-@@tostringtag
     define_direct_property(*vm.well_known_symbol_to_string_tag(), js_string(vm.heap(), "Temporal.Instant"), Attribute::Configurable);
 }
 
+static Instant* typed_this(GlobalObject& global_object)
+{
+    auto& vm = global_object.vm();
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return {};
+    if (!is<Instant>(this_object)) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::NotA, "Temporal.Instant");
+        return {};
+    }
+    return static_cast<Instant*>(this_object);
+}
+
+// 8.3.3 get Temporal.Instant.prototype.epochSeconds, https://tc39.es/proposal-temporal/#sec-get-temporal.instant.prototype.epochseconds
+JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::epoch_seconds_getter)
+{
+    // 1. Let instant be the this value.
+    // 2. Perform ? RequireInternalSlot(instant, [[InitializedTemporalInstant]]).
+    auto* instant = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let ns be instant.[[Nanoseconds]].
+    auto& ns = instant->nanoseconds();
+
+    // 4. Let s be RoundTowardsZero(ℝ(ns) / 10^9).
+    auto [s, _] = ns.big_integer().divided_by(Crypto::SignedBigInteger::create_from(1'000'000'000));
+
+    // 5. Return 𝔽(s).
+    return Value((double)s.to_base(10).to_int<i64>().value());
+}
+
 }

Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.h

@@ -17,6 +17,9 @@ public:
     explicit InstantPrototype(GlobalObject&);
     virtual void initialize(GlobalObject&) override;
     virtual ~InstantPrototype() override = default;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(epoch_seconds_getter);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.epochSeconds.js

@@ -0,0 +1,33 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        expect(new Temporal.Instant(0n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(1n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(999_999_999n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(1_000_000_000n).epochSeconds).toBe(1);
+        expect(new Temporal.Instant(1_500_000_000n).epochSeconds).toBe(1);
+        expect(new Temporal.Instant(1_999_999_999n).epochSeconds).toBe(1);
+        expect(new Temporal.Instant(2_000_000_000n).epochSeconds).toBe(2);
+        expect(new Temporal.Instant(8_640_000_000_000_000_000_000n).epochSeconds).toBe(
+            8_640_000_000_000
+        );
+
+        expect(new Temporal.Instant(-0n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(-1n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(-999_999_999n).epochSeconds).toBe(0);
+        expect(new Temporal.Instant(-1_000_000_000n).epochSeconds).toBe(-1);
+        expect(new Temporal.Instant(-1_500_000_000n).epochSeconds).toBe(-1);
+        expect(new Temporal.Instant(-1_999_999_999n).epochSeconds).toBe(-1);
+        expect(new Temporal.Instant(-2_000_000_000n).epochSeconds).toBe(-2);
+        expect(new Temporal.Instant(-8_640_000_000_000_000_000_000n).epochSeconds).toBe(
+            -8_640_000_000_000
+        );
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.Instant object", () => {
+        expect(() => {
+            Reflect.get(Temporal.Instant.prototype, "epochSeconds", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Instant");
+    });
+});