4 files changed, 117 insertions, 0 deletions
diff --git a/js/deno.ts b/js/deno.ts
index 44fba6164..cd290bfa3 100644
--- a/js/deno.ts
+++ b/js/deno.ts
@@ -17,5 +17,6 @@ export { ErrorKind, DenoError } from "./errors";
 export { libdeno } from "./libdeno";
 export { arch, platform } from "./platform";
 export { trace } from "./trace";
+export { truncateSync, truncate } from "./truncate";
 export { setGlobalTimeout } from "./timers";
 export const args: string[] = [];
diff --git a/js/truncate.ts b/js/truncate.ts
new file mode 100644
index 000000000..7b6e2f8e9
--- /dev/null
+++ b/js/truncate.ts
@@ -0,0 +1,42 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+import * as fbs from "gen/msg_generated";
+import { flatbuffers } from "flatbuffers";
+import * as dispatch from "./dispatch";
+
+/**
+ * Truncates or extends the specified file synchronously,
+ * updating the size of this file to become size. 
+ *
+ *     import { truncateSync } from "deno";
+ *
+ *     truncateSync("hello.txt", 10);
+ */
+export function truncateSync(name: string, len?: number): void {
+  dispatch.sendSync(...req(name, len));
+}
+
+/**
+ * Truncates or extends the specified file,
+ * updating the size of this file to become size. 
+ *
+ *     import { truncate } from "deno";
+ *
+ *     await truncate("hello.txt", 10);
+ */
+export async function truncate(name: string, len?: number): Promise<void> {
+  await dispatch.sendAsync(...req(name, len));
+}
+
+function req(
+  name: string,
+  len?: number
+): [flatbuffers.Builder, fbs.Any, flatbuffers.Offset] {
+  const builder = new flatbuffers.Builder();
+  const name_ = builder.createString(name);
+  len = len && len > 0 ? Math.floor(len) : 0;
+  fbs.Truncate.startTruncate(builder);
+  fbs.Truncate.addName(builder, name_);
+  fbs.Truncate.addLen(builder, len);
+  const msg = fbs.Truncate.endTruncate(builder);
+  return [builder, fbs.Any.Truncate, msg];
+}
diff --git a/js/truncate_test.ts b/js/truncate_test.ts
new file mode 100644
index 000000000..4b9cae445
--- /dev/null
+++ b/js/truncate_test.ts
@@ -0,0 +1,73 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+import { testPerm, assertEqual } from "./test_util.ts";
+import * as deno from "deno";
+
+function readDataSync(name: string): string {
+  const data = deno.readFileSync(name);
+  const decoder = new TextDecoder("utf-8");
+  const text = decoder.decode(data);
+  return text;
+}
+
+async function readData(name: string): Promise<string> {
+  const data = await deno.readFile(name);
+  const decoder = new TextDecoder("utf-8");
+  const text = decoder.decode(data);
+  return text;
+}
+
+testPerm({ write: true }, function truncateSyncSuccess() {
+  const enc = new TextEncoder();
+  const d = enc.encode("Hello");
+  const filename = deno.makeTempDirSync() + "/test_truncateSync.txt";
+  deno.writeFileSync(filename, d);
+  deno.truncateSync(filename, 20);
+  let data = readDataSync(filename);
+  assertEqual(data.length, 20);
+  deno.truncateSync(filename, 5);
+  data = readDataSync(filename);
+  assertEqual(data.length, 5);
+  deno.truncateSync(filename, -5);
+  data = readDataSync(filename);
+  assertEqual(data.length, 0);
+  deno.removeSync(filename);
+});
+
+testPerm({ write: true }, async function truncateSuccess() {
+  const enc = new TextEncoder();
+  const d = enc.encode("Hello");
+  const filename = deno.makeTempDirSync() + "/test_truncate.txt";
+  await deno.writeFile(filename, d);
+  await deno.truncate(filename, 20);
+  let data = await readData(filename);
+  assertEqual(data.length, 20);
+  await deno.truncate(filename, 5);
+  data = await readData(filename);
+  assertEqual(data.length, 5);
+  await deno.truncate(filename, -5);
+  data = await readData(filename);
+  assertEqual(data.length, 0);
+  await deno.remove(filename);
+});
+
+testPerm({ write: false }, function truncateSyncPerm() {
+  let err;
+  try {
+    deno.mkdirSync("/test_truncateSyncPermission.txt");
+  } catch (e) {
+    err = e;
+  }
+  assertEqual(err.kind, deno.ErrorKind.PermissionDenied);
+  assertEqual(err.name, "PermissionDenied");
+});
+
+testPerm({ write: false }, async function truncatePerm() {
+  let err;
+  try {
+    await deno.mkdir("/test_truncatePermission.txt");
+  } catch (e) {
+    err = e;
+  }
+  assertEqual(err.kind, deno.ErrorKind.PermissionDenied);
+  assertEqual(err.name, "PermissionDenied");
+});
diff --git a/js/unit_tests.ts b/js/unit_tests.ts
index 3a1fdd9d1..2eea6c17b 100644
--- a/js/unit_tests.ts
+++ b/js/unit_tests.ts
@@ -19,5 +19,6 @@ import "./symlink_test.ts";
 import "./platform_test.ts";
 import "./text_encoding_test.ts";
 import "./trace_test.ts";
+import "./truncate_test.ts";
 import "./v8_source_maps_test.ts";
 import "../website/app_test.js";