diff --git a/src/bun.js/bindings/napi.cpp b/src/bun.js/bindings/napi.cpp
index 0c8bdec56da47a..2929ae9e23fdc1 100644
--- a/src/bun.js/bindings/napi.cpp
+++ b/src/bun.js/bindings/napi.cpp
@@ -2501,6 +2501,82 @@ extern "C" napi_status napi_typeof(napi_env env, napi_value val,
     return napi_generic_failure;
 }
 
+static_assert(std::is_same_v<JSBigInt::Digit, uint64_t>, "All NAPI bigint functions assume that bigint words are 64 bits");
+#if USE(BIGINT32)
+#error All NAPI bigint functions assume that BIGINT32 is disabled
+#endif
+
+extern "C" napi_status napi_get_value_bigint_int64(napi_env env, napi_value value, int64_t* result, bool* lossless)
+{
+    NAPI_PREMABLE
+    JSValue jsValue = toJS(value);
+    if (jsValue.isEmpty() || !result) {
+        return napi_invalid_arg;
+    }
+    if (!jsValue.isHeapBigInt()) {
+        return napi_bigint_expected;
+    }
+
+    JSBigInt* bigint = jsValue.asHeapBigInt();
+
+    if (bigint->isZero()) {
+        *result = 0;
+        if (lossless != nullptr) *lossless = true;
+    } else {
+        uint64_t digit = bigint->digit(0);
+        uint64_t signed_digit = digit;
+        if (bigint->sign()) {
+            // negate, while keeping it unsigned because i don't trust signed overflow
+            signed_digit = ~signed_digit + 1;
+        }
+        memcpy(result, &signed_digit, sizeof(signed_digit));
+
+        if (lossless != nullptr) {
+            if (bigint->length() > 1) {
+                *lossless = false;
+            } else if (bigint->sign()) {
+                // negative
+                // lossless if numeric value is >= -2^63,
+                // for which digit will be <= 2^63
+                *lossless = (digit <= (1ull << 63));
+            } else {
+                // positive
+                // lossless if numeric value is <= 2^63 - 1
+                *lossless = (digit <= static_cast<uint64_t>(INT64_MAX));
+            }
+        }
+    }
+
+    return napi_ok;
+}
+
+extern "C" napi_status napi_get_value_bigint_uint64(napi_env env, napi_value value, uint64_t* result, bool* lossless)
+{
+    NAPI_PREMABLE
+    JSValue jsValue = toJS(value);
+    if (jsValue.isEmpty() || !result) {
+        return napi_invalid_arg;
+    }
+    if (!jsValue.isHeapBigInt()) {
+        return napi_bigint_expected;
+    }
+
+    JSBigInt* bigint = jsValue.asHeapBigInt();
+
+    if (bigint->isZero()) {
+        *result = 0;
+        if (lossless != nullptr) *lossless = true;
+    } else {
+        *result = bigint->digit(0);
+        if (lossless != nullptr) {
+            // lossless if and only if only one digit and positive
+            *lossless = (bigint->length() == 1 && bigint->sign() == false);
+        }
+    }
+
+    return napi_ok;
+}
+
 extern "C" napi_status napi_get_value_bigint_words(napi_env env,
     napi_value value,
     int* sign_bit,
diff --git a/src/napi/napi.zig b/src/napi/napi.zig
index c8d735d0b23f37..ab089d9b6d1f0c 100644
--- a/src/napi/napi.zig
+++ b/src/napi/napi.zig
@@ -1032,26 +1032,8 @@ pub export fn napi_create_bigint_uint64(env: napi_env, value: u64, result_: ?*na
     return .ok;
 }
 pub extern fn napi_create_bigint_words(env: napi_env, sign_bit: c_int, word_count: usize, words: [*c]const u64, result: *napi_value) napi_status;
-// TODO: lossless
-pub export fn napi_get_value_bigint_int64(_: napi_env, value_: napi_value, result_: ?*i64, _: *bool) napi_status {
-    log("napi_get_value_bigint_int64", .{});
-    const result = result_ orelse {
-        return invalidArg();
-    };
-    const value = value_.get();
-    result.* = value.toInt64();
-    return .ok;
-}
-// TODO: lossless
-pub export fn napi_get_value_bigint_uint64(_: napi_env, value_: napi_value, result_: ?*u64, _: *bool) napi_status {
-    log("napi_get_value_bigint_uint64", .{});
-    const result = result_ orelse {
-        return invalidArg();
-    };
-    const value = value_.get();
-    result.* = value.toUInt64NoTruncate();
-    return .ok;
-}
+pub extern fn napi_get_value_bigint_int64(env: napi_env, value: napi_value, result: ?*i64, lossless: ?*bool) napi_status;
+pub extern fn napi_get_value_bigint_uint64(env: napi_env, value: napi_value, result: ?*u64, lossless: ?*bool) napi_status;
 
 pub extern fn napi_get_value_bigint_words(env: napi_env, value: napi_value, sign_bit: [*c]c_int, word_count: [*c]usize, words: [*c]u64) napi_status;
 pub extern fn napi_get_all_property_names(env: napi_env, object: napi_value, key_mode: napi_key_collection_mode, key_filter: napi_key_filter, key_conversion: napi_key_conversion, result: *napi_value) napi_status;