3 files changed, 52 insertions, 5 deletions

diff --git a/js/deno.ts b/js/deno.ts
index 060b2d526..ff9939b1b 100644
--- a/js/deno.ts
+++ b/js/deno.ts
@@ -1,4 +1,4 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
 // Public deno module.
-export { exit, readFileSync } from "./os";
+export { exit, readFileSync, writeFileSync } from "./os";
 export { libdeno } from "./globals";
diff --git a/js/os.ts b/js/os.ts
index 22ef6ef4f..22bbd7748 100644
--- a/js/os.ts
+++ b/js/os.ts
@@ -121,15 +121,32 @@ export function readFileSync(filename: string): Uint8Array {
 export function writeFileSync(
   filename: string,
   data: Uint8Array,
-  perm: number
+  perm = 0o666
 ): void {
-  util.notImplemented();
-  /*
-  pubInternal("os", {
+  /* Ideally we could write:
+  const res = send({
     command: fbs.Command.WRITE_FILE_SYNC,
     writeFileSyncFilename: filename,
     writeFileSyncData: data,
     writeFileSyncPerm: perm
   });
   */
+  const builder = new flatbuffers.Builder();
+  const filename_ = builder.createString(filename);
+  const dataOffset = fbs.WriteFileSync.createDataVector(builder, data);
+  fbs.WriteFileSync.startWriteFileSync(builder);
+  fbs.WriteFileSync.addFilename(builder, filename_);
+  fbs.WriteFileSync.addData(builder, dataOffset);
+  fbs.WriteFileSync.addPerm(builder, perm);
+  const msg = fbs.WriteFileSync.endWriteFileSync(builder);
+  fbs.Base.startBase(builder);
+  fbs.Base.addMsg(builder, msg);
+  fbs.Base.addMsgType(builder, fbs.Any.WriteFileSync);
+  builder.finish(fbs.Base.endBase(builder));
+  const resBuf = libdeno.send(builder.asUint8Array());
+  if (resBuf != null) {
+    const bb = new flatbuffers.ByteBuffer(new Uint8Array(resBuf!));
+    const baseRes = fbs.Base.getRootAsBase(bb);
+    maybeThrowError(baseRes);
+  }
 }
diff --git a/js/unit_tests.ts b/js/unit_tests.ts
index f203444fa..6eebf8860 100644
--- a/js/unit_tests.ts
+++ b/js/unit_tests.ts
@@ -5,6 +5,7 @@
 
 import { test, assert, assertEqual } from "./testing/testing.ts";
 import { readFileSync } from "deno";
+import * as deno from "deno";
 
 import "./compiler_test.ts";
 
@@ -109,6 +110,35 @@ test(function tests_readFileSync_NotFound() {
 });
 */
 
+/* TODO(ry) Add this once we can create a tmpDir to write the file into.
+test(function writeFileSyncSuccess() {
+  const enc = new TextEncoder();
+  const dataWritten = enc.encode("Hello");
+  const filename = "TEMPDIR/test.txt";
+  deno.writeFileSync(filename, dataWritten, 0o666);
+  const dataRead = readFileSync(filename);
+  assertEqual(dataRead, dataWritten);
+});
+*/
+
+// For this test to pass we need --allow-write permission.
+// Otherwise it will fail with deno.PermissionDenied instead of deno.NotFound.
+test(function writeFileSyncFail() {
+  const enc = new TextEncoder();
+  const data = enc.encode("Hello");
+  const filename = "/baddir/test.txt";
+  // The following should fail because /baddir doesn't exist (hopefully).
+  let caughtError = false;
+  try {
+    deno.writeFileSync(filename, data);
+  } catch (e) {
+    caughtError = true;
+    // TODO assertEqual(e, deno.NotFound);
+    assertEqual(e.name, "deno.NotFound");
+  }
+  assert(caughtError);
+});
+
 test(async function tests_fetch() {
   const response = await fetch("http://localhost:4545/package.json");
   const json = await response.json();