3 files changed, 48 insertions, 0 deletions
diff --git a/cli/js/deno_unstable.ts b/cli/js/deno_unstable.ts
index 192137e1f..991df9955 100644
--- a/cli/js/deno_unstable.ts
+++ b/cli/js/deno_unstable.ts
@@ -12,6 +12,7 @@ export { applySourceMap, formatDiagnostics } from "./ops/errors.ts";
 export { signal, signals, Signal, SignalStream } from "./signals.ts";
 export { setRaw } from "./ops/tty.ts";
 export { utimeSync, utime } from "./ops/fs/utime.ts";
+export { ftruncateSync, ftruncate } from "./ops/fs/truncate.ts";
 export { ShutdownMode, shutdown } from "./net.ts";
 export { listen, listenDatagram, connect } from "./net_unstable.ts";
 export { startTls } from "./tls.ts";
diff --git a/cli/js/lib.deno.unstable.d.ts b/cli/js/lib.deno.unstable.d.ts
index 0a80eb2c6..dd8de2eb6 100644
--- a/cli/js/lib.deno.unstable.d.ts
+++ b/cli/js/lib.deno.unstable.d.ts
@@ -1248,4 +1248,43 @@ declare namespace Deno {
 
   /** **UNSTABLE**: The URL of the file that was originally executed from the command-line. */
   export const mainModule: string;
+
+  /** Synchronously truncates or extends the specified file stream, to reach the
+   * specified `len`.  If `len` is not specified then the entire file contents
+   * are truncated.
+   *
+   * ```ts
+   * // truncate the entire file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, truncate: true, create: true });
+   * Deno.ftruncateSync(file.rid);
+   *
+   * // truncate part of the file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * Deno.write(file.rid, new TextEncoder().encode("Hello World"));
+   * Deno.ftruncateSync(file.rid, 7);
+   * const data = new Uint8Array(32);
+   * Deno.readSync(file.rid, data);
+   * console.log(new TextDecoder().decode(data)); // Hello W
+   * ```
+   */
+  export function ftruncateSync(rid: number, len?: number): void;
+
+  /** Truncates or extends the specified file stream, to reach the specified `len`. If
+   * `len` is not specified then the entire file contents are truncated.
+   *
+   * ```ts
+   * // truncate the entire file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * await Deno.ftruncate(file.rid);
+   *
+   * // truncate part of the file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * await Deno.write(file.rid, new TextEncoder().encode("Hello World"));
+   * await Deno.ftruncate(file.rid, 7);
+   * const data = new Uint8Array(32);
+   * await Deno.read(file.rid, data);
+   * console.log(new TextDecoder().decode(data)); // Hello W
+   * ```
+   */
+  export function ftruncate(rid: number, len?: number): Promise<void>;
 }
diff --git a/cli/js/ops/fs/truncate.ts b/cli/js/ops/fs/truncate.ts
index 861e843f8..2b805e5ac 100644
--- a/cli/js/ops/fs/truncate.ts
+++ b/cli/js/ops/fs/truncate.ts
@@ -13,6 +13,14 @@ function coerceLen(len?: number): number {
   return len;
 }
 
+export function ftruncateSync(rid: number, len?: number): void {
+  sendSync("op_ftruncate", { rid, len: coerceLen(len) });
+}
+
+export async function ftruncate(rid: number, len?: number): Promise<void> {
+  await sendAsync("op_ftruncate", { rid, len: coerceLen(len) });
+}
+
 export function truncateSync(path: string, len?: number): void {
   sendSync("op_truncate", { path, len: coerceLen(len) });
 }