LibJS: Implement BigInt.asUintN

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/cb57475168e
Pull-request: https://github.com/SerenityOS/serenity/pull/12307
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp

@@ -32,7 +32,7 @@ void BigIntConstructor::initialize(GlobalObject& global_object)
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.asIntN, as_int_n, 2, attr);
-    // define_native_function(vm.names.asUintN, as_uint_n, 2, attr);
+    define_native_function(vm.names.asUintN, as_uint_n, 2, attr);
 
     define_direct_property(vm.names.length, Value(1), Attribute::Configurable);
 }
@@ -95,7 +95,16 @@ JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_int_n)
 // 21.2.2.2 BigInt.asUintN ( bits, bigint ), https://tc39.es/ecma262/#sec-bigint.asuintn
 JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_uint_n)
 {
-    TODO();
+    // 1. Set bits to ? ToIndex(bits).
+    auto bits = TRY(vm.argument(0).to_index(global_object));
+
+    // 2. Set bigint to ? ToBigInt(bigint).
+    auto* bigint = TRY(vm.argument(1).to_bigint(global_object));
+
+    // 3. Return the BigInt value that represents ℝ(bigint) modulo 2bits.
+    // FIXME: For large values of `bits`, this can likely be improved with a SignedBigInteger API to
+    //        drop the most significant bits.
+    return js_bigint(vm, modulo(bigint->big_integer(), BIGINT_ONE.shift_left(bits)));
 }
 
 }

Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.asUintN.js

@@ -0,0 +1,77 @@
+describe("errors", () => {
+    test("invalid index", () => {
+        expect(() => {
+            BigInt.asUintN(-1, 0n);
+        }).toThrowWithMessage(RangeError, "Index must be a positive integer");
+
+        expect(() => {
+            BigInt.asUintN(Symbol(), 0n);
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to number");
+    });
+
+    test("invalid BigInt", () => {
+        expect(() => {
+            BigInt.asUintN(1, 1);
+        }).toThrowWithMessage(TypeError, "Cannot convert number to BigInt");
+
+        expect(() => {
+            BigInt.asUintN(1, Symbol());
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to BigInt");
+
+        expect(() => {
+            BigInt.asUintN(1, "foo");
+        }).toThrowWithMessage(SyntaxError, "Invalid value for BigInt: foo");
+    });
+});
+
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        expect(BigInt.asUintN(0, -2n)).toBe(0n);
+        expect(BigInt.asUintN(0, -1n)).toBe(0n);
+        expect(BigInt.asUintN(0, 0n)).toBe(0n);
+        expect(BigInt.asUintN(0, 1n)).toBe(0n);
+        expect(BigInt.asUintN(0, 2n)).toBe(0n);
+
+        expect(BigInt.asUintN(1, -3n)).toBe(1n);
+        expect(BigInt.asUintN(1, -2n)).toBe(0n);
+        expect(BigInt.asUintN(1, -1n)).toBe(1n);
+        expect(BigInt.asUintN(1, 0n)).toBe(0n);
+        expect(BigInt.asUintN(1, 1n)).toBe(1n);
+        expect(BigInt.asUintN(1, 2n)).toBe(0n);
+        expect(BigInt.asUintN(1, 3n)).toBe(1n);
+
+        expect(BigInt.asUintN(2, -3n)).toBe(1n);
+        expect(BigInt.asUintN(2, -2n)).toBe(2n);
+        expect(BigInt.asUintN(2, -1n)).toBe(3n);
+        expect(BigInt.asUintN(2, 0n)).toBe(0n);
+        expect(BigInt.asUintN(2, 1n)).toBe(1n);
+        expect(BigInt.asUintN(2, 2n)).toBe(2n);
+        expect(BigInt.asUintN(2, 3n)).toBe(3n);
+
+        expect(BigInt.asUintN(4, -3n)).toBe(13n);
+        expect(BigInt.asUintN(4, -2n)).toBe(14n);
+        expect(BigInt.asUintN(4, -1n)).toBe(15n);
+        expect(BigInt.asUintN(4, 0n)).toBe(0n);
+        expect(BigInt.asUintN(4, 1n)).toBe(1n);
+        expect(BigInt.asUintN(4, 2n)).toBe(2n);
+        expect(BigInt.asUintN(4, 3n)).toBe(3n);
+
+        const extremelyBigInt = 123456789123456789123456789123456789123456789123456789n;
+
+        expect(BigInt.asUintN(0, extremelyBigInt)).toBe(0n);
+        expect(BigInt.asUintN(1, extremelyBigInt)).toBe(1n);
+        expect(BigInt.asUintN(2, extremelyBigInt)).toBe(1n);
+        expect(BigInt.asUintN(4, extremelyBigInt)).toBe(5n);
+        expect(BigInt.asUintN(128, extremelyBigInt)).toBe(241220992521549204068304577237384191765n);
+        expect(BigInt.asUintN(256, extremelyBigInt)).toBe(extremelyBigInt);
+
+        expect(BigInt.asUintN(0, -extremelyBigInt)).toBe(0n);
+        expect(BigInt.asUintN(1, -extremelyBigInt)).toBe(1n);
+        expect(BigInt.asUintN(2, -extremelyBigInt)).toBe(3n);
+        expect(BigInt.asUintN(4, -extremelyBigInt)).toBe(11n);
+        expect(BigInt.asUintN(128, -extremelyBigInt)).toBe(99061374399389259395070030194384019691n);
+        expect(BigInt.asUintN(256, -extremelyBigInt)).toBe(
+            115792089237316195423570861551898784396480861208851440582668460551124006183147n
+        );
+    });
+});