test: property-based fuzz testing for js->zig->js roundtrip

.github/workflows/ci.yml

@@ -36,6 +36,19 @@ jobs:
       - name: Test
         run: pnpm test:js
 
+  test-fuzz:
+    name: Fuzz Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: ./.github/actions/setup-env
+      - name: Build
+        run: pnpm build
+      - name: Fuzz
+        run: pnpm test:fuzz:round_trip
+        env:
+          FUZZ_RUNS: "1000"
+
   lint:
     name: Lint
     runs-on: ubuntu-latest

README.md

@@ -84,7 +84,7 @@ c.count; // 1 (getter, not a method call)
 | `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 |

build.zig.zon

@@ -54,6 +54,11 @@
             .imports = .{.zapi},
             .link_libc = true,
         },
+        .test_fuzz_numeric = .{
+            .root_source_file = "tests/fuzz/mod.zig",
+            .imports = .{.zapi},
+            .link_libc = true,
+        },
     },
     .libraries = .{
         .example_hello_world = .{
@@ -80,6 +85,12 @@
             .linker_allow_shlib_undefined = true,
             .dest_sub_path = "example_register_decls.node",
         },
+        .test_fuzz_numeric = .{
+            .root_module = .test_fuzz_numeric,
+            .linkage = .dynamic,
+            .linker_allow_shlib_undefined = true,
+            .dest_sub_path = "test_fuzz_numeric.node",
+        },
     },
     .tests = .{
         .napi = .{ .root_module = .napi },

examples/js_dsl/mod.zig

@@ -95,8 +95,7 @@ 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);
 }
 

package.json

@@ -15,6 +15,7 @@
 		"build:zig": "zig build",
 		"test:zig": "zig build test:zapi",
 		"test:js": "vitest run examples/**/*.test.ts",
+		"test:fuzz:round_trip": "pnpm build:zig && vitest run tests/**/*.test.ts",
 		"test": "pnpm test:zig && pnpm test:js",
 		"lint:zig": "zig fmt src",
 		"lint:js": "pnpm biome check",
@@ -34,6 +35,7 @@
 		"@biomejs/biome": "^2.4.6",
 		"@chainsafe/biomejs-config": "^1.0.0",
 		"@types/node": "^24.1.0",
+		"fast-check": "^4.8.0",
 		"typescript": "^5.8.3",
 		"vitest": "^4.0.18"
 	},

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/create_callback.zig

@@ -38,7 +38,7 @@ pub fn createCallback(
                 const is_value_arg = arg_hint == .value or (arg_hint == .auto and param_type == Value);
 
                 if (arg_hint == .data) {
-                    args[i] = @alignCast(@ptrCast(info.data));
+                    args[i] = @ptrCast(@alignCast(info.data));
                 } else if (is_env_arg) {
                     args[i] = env;
                 } else if (is_value_arg) {

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.InvalidArg` 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.InvalidArg;
+        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.InvalidArg` 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.InvalidArg;
+        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.InvalidArg` 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.InvalidArg,
+            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.InvalidArg;
             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.InvalidArg;
         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;