Fixes https://github.com/oven-sh/bun/issues/2931

2 files changed, 30 insertions, 1 deletions

diff --git a/src/bun.js/node/node_fs.zig b/src/bun.js/node/node_fs.zig
index f7300f28e..f1a0e6a82 100644
--- a/src/bun.js/node/node_fs.zig
+++ b/src/bun.js/node/node_fs.zig
@@ -3502,7 +3502,10 @@ pub const NodeFS = struct {
             }
         }
 
-        _ = ftruncateSync(.{ .fd = fd, .len = @truncate(JSC.WebCore.Blob.SizeType, written) });
+        // https://github.com/oven-sh/bun/issues/2931
+        if ((@enumToInt(args.flag) & std.os.O.APPEND) == 0) {
+            _ = ftruncateSync(.{ .fd = fd, .len = @truncate(JSC.WebCore.Blob.SizeType, written) });
+        }
 
         return Maybe(Return.WriteFile).success;
     }
diff --git a/test/js/node/fs/fs.test.ts b/test/js/node/fs/fs.test.ts
index 03742d843..1edbfa11c 100644
--- a/test/js/node/fs/fs.test.ts
+++ b/test/js/node/fs/fs.test.ts
@@ -47,6 +47,32 @@ function mkdirForce(path: string) {
   if (!existsSync(path)) mkdirSync(path, { recursive: true });
 }
 
+it("writeFileSync in append should not truncate the file", () => {
+  const path = join(tmpdir(), "writeFileSync-should-not-append-" + (Date.now() * 10000).toString(16));
+  var str = "";
+  writeFileSync(path, "---BEGIN---");
+  str += "---BEGIN---";
+  for (let i = 0; i < 10; i++) {
+    const line = "Line #" + i;
+    str += line;
+    writeFileSync(path, line, { flag: "a" });
+  }
+  expect(readFileSync(path, "utf8")).toBe(str);
+});
+
+it("writeFileSync NOT in append SHOULD truncate the file", () => {
+  const path = join(tmpdir(), "writeFileSync-should-not-append-" + (Date.now() * 10000).toString(16));
+  writeFileSync(path, "---BEGIN---");
+  var str = "---BEGIN---";
+  expect(readFileSync(path, "utf8")).toBe(str);
+  for (let i = 0; i < 10; i++) {
+    const line = "Line #" + i;
+    str = line;
+    writeFileSync(path, line);
+    expect(readFileSync(path, "utf8")).toBe(str);
+  }
+});
+
 describe("copyFileSync", () => {
   it("should work for files < 128 KB", () => {
     const tempdir = `${tmpdir()}/fs.test.js/${Date.now()}/1234/hi`;