fix(ext/node): internal buffer length in readSync (#26064)

Closes https://github.com/denoland/deno/issues/26054

2 files changed, 12 insertions, 1 deletions

diff --git a/ext/node/polyfills/_fs/_fs_read.ts b/ext/node/polyfills/_fs/_fs_read.ts
index dec3a8bbd..df4f5e375 100644
--- a/ext/node/polyfills/_fs/_fs_read.ts
+++ b/ext/node/polyfills/_fs/_fs_read.ts
@@ -173,7 +173,7 @@ export function readSync(
   validateBuffer(buffer);
 
   if (length == null) {
-    length = 0;
+    length = buffer.byteLength;
   }
 
   if (typeof offsetOrOpt === "number") {
diff --git a/tests/unit_node/fs_test.ts b/tests/unit_node/fs_test.ts
index b1f7c53e8..2d1465aec 100644
--- a/tests/unit_node/fs_test.ts
+++ b/tests/unit_node/fs_test.ts
@@ -4,13 +4,16 @@ import { assert, assertEquals, assertThrows } from "@std/assert";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import {
+  closeSync,
   constants,
   createWriteStream,
   existsSync,
   lstatSync,
   mkdtempSync,
+  openSync,
   promises,
   readFileSync,
+  readSync,
   Stats,
   statSync,
   writeFileSync,
@@ -201,3 +204,11 @@ Deno.test(
     assertEquals(res, [0, 1, 2, 3, 4, 5]);
   },
 );
+
+Deno.test("[node/fs] readSync works", () => {
+  const fd = openSync("tests/testdata/assets/hello.txt", "r");
+  const buf = new Uint8Array(256);
+  const bytesRead = readSync(fd!, buf);
+  assertEquals(bytesRead, 12);
+  closeSync(fd!);
+});