fix(js)!: make BigInt conversions exact and add a shared LossyConversion error

README.md

@@ -172,7 +172,7 @@ Code paths that cannot accept or return DSL classes pass
 | `Number` | `number` | `toI32()`, `toF64()`, `assertI32()`, `from(anytype)` |
 | `String` | `string` | `toSlice(buf)`, `toOwnedSlice(alloc)`, `len()`, `from([]const u8)` |
 | `Boolean` | `boolean` | `toBool()`, `assertBool()`, `from(bool)` |
-| `BigInt` | `bigint` | `toI64()`, `toU64()`, `toI128()`, `from(anytype)` |
+| `BigInt` | `bigint` | `toI64()`, `toU64()`, `toI128()`, `to*LowBits()`, `from(anytype)`, `fromWords(sign, words)` |
 | `Date` | `Date` | `toTimestamp()`, `from(f64)` |
 | `Array` | `Array` | `get(i)`, `getNumber(i)`, `length()`, `set(i, val)` |
 | `Object(T)` | `object` | `get()`, `set(value)` — `T` fields must be DSL types |

examples/js_dsl/mod.test.ts

@@ -67,7 +67,7 @@ describe("primitive types", () => {
 
 	it("exactU32 rejects values outside the u32 range", () => {
 		for (const value of [-1, 1.5, 2 ** 32, Number.MAX_SAFE_INTEGER, Infinity, -Infinity, NaN]) {
-			expect(() => mod.exactU32(value), `value ${value}`).toThrow("InvalidUnsignedInteger");
+			expect(() => mod.exactU32(value), `value ${value}`).toThrow("LossyConversion");
 		}
 	});
 
@@ -79,14 +79,14 @@ describe("primitive types", () => {
 
 	it("safeInteger rejects values that are not non-negative safe integers", () => {
 		for (const value of [-1, 1.5, Number.MAX_SAFE_INTEGER + 1, Infinity, -Infinity, NaN]) {
-			expect(() => mod.safeInteger(value), `value ${value}`).toThrow("InvalidUnsignedInteger");
+			expect(() => mod.safeInteger(value), `value ${value}`).toThrow("LossyConversion");
 		}
 	});
 
 	// 2^32 separates the two: in range for a safe integer, out of range for a u32.
 	it("safeInteger and exactU32 differ exactly at the u32 boundary", () => {
 		expect(mod.safeInteger(2 ** 32)).toEqual(2 ** 32);
-		expect(() => mod.exactU32(2 ** 32)).toThrow("InvalidUnsignedInteger");
+		expect(() => mod.exactU32(2 ** 32)).toThrow("LossyConversion");
 	});
 
 	it("toggleBool", () => {
@@ -156,11 +156,11 @@ describe("primitive types", () => {
 		});
 
 		it("rejects BigInts outside the i128 range instead of crashing", () => {
-			expect(() => mod.bigIntToI128String(2n ** 127n)).toThrow();
-			expect(() => mod.bigIntToI128String(-(2n ** 127n) - 1n)).toThrow();
-			expect(() => mod.bigIntToI128String(2n ** 128n)).toThrow();
-			expect(() => mod.bigIntToI128String(2n ** 200n)).toThrow();
-			expect(() => mod.bigIntToI128String(-(2n ** 200n))).toThrow();
+			expect(() => mod.bigIntToI128String(2n ** 127n)).toThrow("LossyConversion");
+			expect(() => mod.bigIntToI128String(-(2n ** 127n) - 1n)).toThrow("LossyConversion");
+			expect(() => mod.bigIntToI128String(2n ** 128n)).toThrow("LossyConversion");
+			expect(() => mod.bigIntToI128String(2n ** 200n)).toThrow("LossyConversion");
+			expect(() => mod.bigIntToI128String(-(2n ** 200n))).toThrow("LossyConversion");
 		});
 	});
 

examples/js_dsl/mod.zig

@@ -107,8 +107,7 @@ pub fn reverseString(s: String) !String {
 
 /// Double a BigInt value.
 pub fn doubleBigInt(n: BigInt) !BigInt {
-    var lossless: bool = false;
-    const val = try n.toI64(&lossless);
+    const val = try n.toI64();
     return BigInt.from(val * 2);
 }
 

src/Value.zig

@@ -274,7 +274,10 @@ pub fn getValueBigintUint64(self: Value, lossless: ?*bool) NapiError!u64 {
 ///
 /// Returns `error.Overflow` if the BigInt needs more words than `words` can hold:
 /// napi sets the out `word_count` to the *required* count, which may exceed the
-/// buffer, so slicing by it unchecked would read out of bounds.
+/// buffer, so slicing by it unchecked would read out of bounds. napi still fills
+/// `words` with the low-order words and `sign_bit` is set before the error is
+/// returned, so callers that want truncation semantics may catch `error.Overflow`
+/// and read the buffer directly.
 ///
 /// NOTE: napi's C entry takes `int*` (4-byte aligned). Casting a u1 to int* is UB, since that
 /// is 4-bytes aligned. We use a local `c_int` for the napi call and narrow back to `u1` for the caller.
@@ -286,9 +289,9 @@ pub fn getValueBigintWords(self: Value, sign_bit: ?*u1, words: []u64) (NapiError
     try status.check(
         c.napi_get_value_bigint_words(self.env, self.value, &raw_sign, &word_count, words.ptr),
     );
-    if (word_count > words.len) return error.Overflow;
     // napi guarantees raw_sign ∈ {0, 1}
     if (sign_bit) |s| s.* = @intCast(raw_sign);
+    if (word_count > words.len) return error.Overflow;
     return words[0..word_count];
 }
 

src/js/bigint.zig

@@ -14,78 +14,146 @@ pub const BigInt = struct {
         if ((try val.typeof()) != .bigint) return error.TypeMismatch;
     }
 
-    /// Attempts to convert the JavaScript BigInt to a Zig `i64`.
+    /// Attempts to convert the JavaScript BigInt exactly to a Zig `i64`.
     ///
-    /// If `lossless` is provided, it will be set to `true` if the BigInt can be
-    /// represented exactly as an `i64`, and `false` otherwise (e.g., if the
-    /// BigInt is too large or too small for `i64`).
-    /// Returns an error if the conversion fails or the `napi_env` is invalid.
-    pub fn toI64(self: BigInt, lossless: ?*bool) !i64 {
+    /// Returns `error.LossyConversion` if the value is outside the `i64` range. Use
+    /// `toI64LowBits` when low-64-bit conversion is intended.
+    pub fn toI64(self: BigInt) !i64 {
+        var lossless = false;
+        const value = try self.toI64LowBits(&lossless);
+        if (!lossless) return error.LossyConversion;
+        return value;
+    }
+
+    /// Converts the JavaScript BigInt to the low 64 bits as a Zig `i64`.
+    ///
+    /// If `lossless` is provided, it is set to whether the returned `i64`
+    /// represents the original BigInt exactly. Out-of-range values still return
+    /// their low 64 bits, matching N-API BigInt conversion semantics, but set
+    /// `lossless` to false.
+    pub fn toI64LowBits(self: BigInt, lossless: ?*bool) !i64 {
         return self.val.getValueBigintInt64(lossless);
     }
 
-    /// Attempts to convert the JavaScript BigInt to a Zig `u64`.
+    /// Attempts to convert the JavaScript BigInt exactly to a Zig `u64`.
     ///
-    /// If `lossless` is provided, it will be set to `true` if the BigInt can be
-    /// represented exactly as a `u64`, and `false` otherwise.
-    /// Returns an error if the conversion fails (e.g., BigInt is negative) or
-    /// the `napi_env` is invalid.
-    pub fn toU64(self: BigInt, lossless: ?*bool) !u64 {
+    /// Returns `error.LossyConversion` if the value is negative or outside the `u64`
+    /// range. Use `toU64LowBits` when low-64-bit conversion is intended.
+    pub fn toU64(self: BigInt) !u64 {
+        var lossless = false;
+        const value = try self.toU64LowBits(&lossless);
+        if (!lossless) return error.LossyConversion;
+        return value;
+    }
+
+    /// Converts the JavaScript BigInt to the low 64 bits as a Zig `u64`.
+    ///
+    /// If `lossless` is provided, it is set to whether the returned `u64`
+    /// represents the original BigInt exactly. Out-of-range or negative values
+    /// still return their low 64 bits, matching N-API BigInt conversion
+    /// semantics, but set `lossless` to false.
+    pub fn toU64LowBits(self: BigInt, lossless: ?*bool) !u64 {
         return self.val.getValueBigintUint64(lossless);
     }
 
-    /// Attempts to convert the JavaScript BigInt to a Zig `i128`.
+    /// Attempts to convert the JavaScript BigInt exactly to a Zig `i128`.
+    ///
+    /// Returns `error.LossyConversion` if the value is outside `[-2^127, 2^127)`.
+    /// Use `toI128LowBits` when low-128-bit conversion is intended.
     ///
-    /// This function reads the BigInt as two 64-bit words and reconstructs it
-    /// into a Zig `i128`. It handles both positive and negative BigInts.
-    /// Returns `error.Overflow` if the value is outside the i128 range, and an
-    /// error if N-API operations fail.
+    /// Returns an error if N-API operations fail.
     pub fn toI128(self: BigInt) !i128 {
         var sign_bit: u1 = 0;
         var words: [2]u64 = .{ 0, 0 };
-        const result = try self.val.getValueBigintWords(&sign_bit, &words);
-        const lo: u128 = if (result.len > 0) result[0] else 0;
-        const hi: u128 = if (result.len > 1) result[1] else 0;
+        // BigInts needing more than two words are always outside the i128
+        // range; getValueBigintWords reports them as error.Overflow.
+        _ = self.val.getValueBigintWords(&sign_bit, &words) catch |err| switch (err) {
+            error.Overflow => return error.LossyConversion,
+            else => |e| return e,
+        };
+        const lo: u128 = words[0];
+        const hi: u128 = words[1];
         const magnitude: u128 = (hi << 64) | lo;
         const max_positive: u128 = std.math.maxInt(i128);
         if (sign_bit == 1) {
-            if (magnitude > max_positive + 1) return error.Overflow;
+            if (magnitude > max_positive + 1) return error.LossyConversion;
             if (magnitude == max_positive + 1) return std.math.minInt(i128);
             return -@as(i128, @intCast(magnitude));
         }
-        if (magnitude > max_positive) return error.Overflow;
+        if (magnitude > max_positive) return error.LossyConversion;
         return @intCast(magnitude);
     }
 
+    /// Converts the JavaScript BigInt to the low 128 bits as a Zig `i128`.
+    ///
+    /// This returns `BigInt.asIntN(128, b)`, i.e. the low 128 bits
+    /// of the magnitude reinterpreted as a signed i128. This matches the
+    /// ECMAScript `BigInt.asIntN` semantics defined at
+    /// https://tc39.es/ecma262/#sec-bigint.asintn.
+    ///
+    /// Returns an error if N-API operations fail.
+    pub fn toI128LowBits(self: BigInt) !i128 {
+        var sign_bit: u1 = 0;
+        // Pre-zeroed: NAPI writes only as many words as the BigInt has; unused
+        // words stay 0. When the value is 0n NAPI returns word_count == 0, so
+        // both words[0] and words[1] remain 0 — magnitude correctly becomes 0.
+        // When the BigInt exceeds 128 bits, getValueBigintWords returns
+        // error.Overflow but NAPI has still filled the two lower words, so
+        // swallowing the error gives BigInt.asIntN(128) truncation semantics.
+        var words: [2]u64 = .{ 0, 0 };
+        _ = self.val.getValueBigintWords(&sign_bit, &words) catch |err| switch (err) {
+            error.Overflow => {},
+            else => |e| return e,
+        };
+        const lo: u128 = words[0];
+        const hi: u128 = words[1];
+        const magnitude: u128 = (hi << 64) | lo;
+        if (sign_bit == 1) {
+            // Negative: result = -magnitude interpreted as i128.
+            // Use wrapping subtraction + bitcast to handle both minInt(i128)
+            // (magnitude == 2^127, which doesn't fit in i128 as positive) and
+            // out-of-range values (magnitude > 2^127, truncated to low 128 bits).
+            // Guard: out-of-range negatives whose low 128 bits are all zero
+            // (e.g. -2^128n → words [0, 0]) give magnitude == 0. asIntN(128)
+            // of such values is 0n; return early rather than relying on the
+            // `0 -% 0 == 0` coincidence below.
+            if (magnitude == 0) return 0;
+            return @bitCast(0 -% magnitude);
+        }
+        // Positive: bitcast u128 → i128 gives BigInt.asIntN(128) semantics.
+        // In-range values have magnitude ≤ 2^127-1 so the sign bit is clear;
+        // out-of-range values have the sign bit set, matching JS asIntN(128).
+        return @bitCast(magnitude);
+    }
+
     /// Converts the JavaScript BigInt to a Zig `i64`, panicking on failure.
     ///
     /// This is a convenience method for cases where the BigInt is guaranteed
-    /// to be representable as an `i64` without loss.
+    /// to be exactly representable as an `i64`.
     pub fn assertI64(self: BigInt) i64 {
-        return self.toI64(null) catch @panic("BigInt.assertI64 failed");
+        return self.toI64() catch @panic("BigInt.assertI64 failed");
     }
 
     /// Converts the JavaScript BigInt to a Zig `u64`, panicking on failure.
     ///
     /// This is a convenience method for cases where the BigInt is guaranteed
-    /// to be representable as a `u64` without loss.
+    /// to be exactly representable as a `u64`.
     pub fn assertU64(self: BigInt) u64 {
-        return self.toU64(null) catch @panic("BigInt.assertU64 failed");
+        return self.toU64() catch @panic("BigInt.assertU64 failed");
     }
 
     /// Converts the JavaScript BigInt to a Zig `i128`, panicking on failure.
     ///
     /// This is a convenience method for cases where the BigInt is guaranteed
-    /// to be representable as an `i128`.
+    /// to be exactly representable as an `i128`.
     pub fn assertI128(self: BigInt) i128 {
         return self.toI128() catch @panic("BigInt.assertI128 failed");
     }
 
     /// Creates a JavaScript `BigInt` from a Zig integer value.
     ///
     /// Accepts `i64`, `u64`, and `comptime_int` values within this range.
-    /// For `i128`/`u128` values, use `fromWords` (not currently exposed but
-    /// available at the `napi.Env` level).
+    /// For `i128`/`u128` values, use `fromWords`.
     ///
     /// Panics if N-API operations fail (e.g., invalid environment) or for Zig
     /// integer types larger than `u64` that cannot be converted to `i64`/`u64`.
@@ -118,6 +186,23 @@ pub const BigInt = struct {
         return .{ .val = val };
     }
 
+    /// Creates a JavaScript `BigInt` from a sign bit and word array.
+    ///
+    /// `sign_bit` is 0 for non-negative, 1 for negative. `words` is the
+    /// little-endian unsigned magnitude. The resulting BigInt equals
+    /// `(sign_bit == 1 ? -1 : 1) * sum(words[i] << (64 * i))`.
+    ///
+    /// Use this for constructing BigInts whose magnitude exceeds `u64` (e.g.,
+    /// i128 or larger). For `i64`/`u64` values, prefer `BigInt.from`.
+    ///
+    /// Panics if N-API operations fail (e.g., invalid environment).
+    pub fn fromWords(sign_bit: u1, words: []const u64) BigInt {
+        const e = context.env();
+        const val = e.createBigintWords(sign_bit, words) catch
+            @panic("BigInt.fromWords: createBigintWords failed");
+        return .{ .val = val };
+    }
+
     /// Returns the underlying `napi.Value` representation of this JavaScript BigInt.
     pub fn toValue(self: BigInt) napi.Value {
         return self.val;

src/js/number.zig

@@ -27,19 +27,24 @@ pub const Number = struct {
 
     /// Attempts to convert the JavaScript number to a Zig `i32`.
     ///
-    /// Returns an error if the conversion fails (e.g., number is too large).
+    /// Follows N-API int32 conversion semantics: finite values outside the
+    /// 32-bit range are truncated to the equivalent low 32 bits, and non-finite
+    /// values (`NaN`, `+Infinity`, `-Infinity`) become 0.
+    ///
+    /// Returns an error if the underlying value is not a number or the N-API
+    /// call fails.
     pub fn toI32(self: Number) !i32 {
         return self.val.getValueInt32();
     }
 
     /// Attempts to convert the JavaScript number to a Zig `u32`.
     ///
-    /// Safety: The underlying napi implementation does not reject numbers out of
-    /// the range [0, 2^32). The caller is responsible for bounds checking.
-    ///
-    /// Returns an error if the given `self.val` is not a number.
+    /// Follows N-API uint32 conversion semantics: finite values outside the
+    /// 32-bit range are truncated to the equivalent low 32 bits, and non-finite
+    /// values (`NaN`, `+Infinity`, `-Infinity`) become 0.
     ///
-    /// Source: https://tc39.es/ecma262/#sec-touint32
+    /// Returns an error if the underlying value is not a number or the N-API
+    /// call fails.
     pub fn toU32(self: Number) !u32 {
         return self.val.getValueUint32();
     }
@@ -60,7 +65,7 @@ pub const Number = struct {
             value > max or
             @trunc(value) != value)
         {
-            return error.InvalidUnsignedInteger;
+            return error.LossyConversion;
         }
         return @intFromFloat(value);
     }
@@ -69,7 +74,7 @@ pub const Number = struct {
     ///
     /// Exact across the whole `u32` range.
     ///
-    /// Returns `error.InvalidUnsignedInteger` if the number is negative,
+    /// Returns `error.LossyConversion` if the number is negative,
     /// fractional, non-finite, or greater than `std.math.maxInt(u32)`.
     pub fn toU32Exact(self: Number) !u32 {
         return self.toUnsignedExact(u32, std.math.maxInt(u32));
@@ -78,7 +83,7 @@ pub const Number = struct {
     /// Attempts to convert the JavaScript number to a non-negative JS safe
     /// integer, returned as a `u64`.
     ///
-    /// Returns `error.InvalidUnsignedInteger` if the number is negative,
+    /// Returns `error.LossyConversion` if the number is negative,
     /// fractional, non-finite, or greater than `Number.MAX_SAFE_INTEGER`
     /// (2^53 - 1).
     ///
@@ -98,7 +103,13 @@ pub const Number = struct {
 
     /// Attempts to convert the JavaScript number to a Zig `i64`.
     ///
-    /// Returns an error if the conversion fails (e.g., number is too large).
+    /// Follows N-API int64 conversion semantics: non-finite values (`NaN`,
+    /// `+Infinity`, `-Infinity`) become 0. Values outside JavaScript's safe
+    /// integer range may lose precision, and finite values outside the `i64`
+    /// range are clamped by Node to `minInt(i64)`/`maxInt(i64)`.
+    ///
+    /// Returns an error if the underlying value is not a number or the N-API
+    /// call fails.
     pub fn toI64(self: Number) !i64 {
         return self.val.getValueInt64();
     }