Add deno.truncate (#805)

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];
+}