Deno.FsFile - Deno

class Deno.FsFile

implements Disposable

The Deno abstraction for reading and writing files.

This is the most straight forward way of handling files within Deno and is recommended over using the discrete functions within the Deno namespace.

using file = await Deno.open("/foo/bar.txt", { read: true });
const fileInfo = await file.stat();
if (fileInfo.isFile) {
  const buf = new Uint8Array(100);
  const numberOfBytesRead = await file.read(buf); // 11 bytes
  const text = new TextDecoder().decode(buf);  // "hello world"
}

Properties

readonly



readable: ReadableStream<Uint8Array>

A ReadableStream instance representing to the byte contents of the file. This makes it easy to interoperate with other web streams based APIs.

using file = await Deno.open("my_file.txt", { read: true });
const decoder = new TextDecoder();
for await (const chunk of file.readable) {
  console.log(decoder.decode(chunk));
}

readonly



writable: WritableStream<Uint8Array>

A WritableStream instance to write the contents of the file. This makes it easy to interoperate with other web streams based APIs.

const items = ["hello", "world"];
using file = await Deno.open("my_file.txt", { write: true });
const encoder = new TextEncoder();
const writer = file.writable.getWriter();
for (const item of items) {
  await writer.write(encoder.encode(item));
}

Methods



[Symbol.dispose](): void



close(): void

Close the file. Closing a file when you are finished with it is important to avoid leaking resources.

using file = await Deno.open("my_file.txt");
// do work with "file" object



isTerminal(): boolean

Checks if the file resource is a TTY (terminal).

// This example is system and context specific
using file = await Deno.open("/dev/tty6");
file.isTerminal(); // true



lock(exclusive?: boolean): Promise<void>

Acquire an advisory file-system lock for the file.



lockSync(exclusive?: boolean): void

Synchronously acquire an advisory file-system lock synchronously for the file.



read(p: Uint8Array): Promise<number | null>

Read the file into an array buffer (p).

Resolves to either the number of bytes read during the operation or EOF (null) if there was nothing more to read.

It is possible for a read to successfully return with 0 bytes. This does not indicate EOF.

It is not guaranteed that the full buffer will be read in a single call.

// if "/foo/bar.txt" contains the text "hello world":
using file = await Deno.open("/foo/bar.txt");
const buf = new Uint8Array(100);
const numberOfBytesRead = await file.read(buf); // 11 bytes
const text = new TextDecoder().decode(buf);  // "hello world"



readSync(p: Uint8Array): number | null

Synchronously read from the file into an array buffer (p).

Returns either the number of bytes read during the operation or EOF (null) if there was nothing more to read.

It is possible for a read to successfully return with 0 bytes. This does not indicate EOF.

It is not guaranteed that the full buffer will be read in a single call.

// if "/foo/bar.txt" contains the text "hello world":
using file = Deno.openSync("/foo/bar.txt");
const buf = new Uint8Array(100);
const numberOfBytesRead = file.readSync(buf); // 11 bytes
const text = new TextDecoder().decode(buf);  // "hello world"



seek(offset: number | bigint,
whence: SeekMode,
): Promise<number>

Seek to the given offset under mode given by whence. The call resolves to the new position within the resource (bytes from the start).

// Given the file contains "Hello world" text, which is 11 bytes long:
using file = await Deno.open(
  "hello.txt",
  { read: true, write: true, truncate: true, create: true },
);
await file.write(new TextEncoder().encode("Hello world"));

// advance cursor 6 bytes
const cursorPosition = await file.seek(6, Deno.SeekMode.Start);
console.log(cursorPosition);  // 6
const buf = new Uint8Array(100);
await file.read(buf);
console.log(new TextDecoder().decode(buf)); // "world"

The seek modes work as follows:

// Given the file contains "Hello world" text, which is 11 bytes long:
const file = await Deno.open(
  "hello.txt",
  { read: true, write: true, truncate: true, create: true },
);
await file.write(new TextEncoder().encode("Hello world"));

// Seek 6 bytes from the start of the file
console.log(await file.seek(6, Deno.SeekMode.Start)); // "6"
// Seek 2 more bytes from the current position
console.log(await file.seek(2, Deno.SeekMode.Current)); // "8"
// Seek backwards 2 bytes from the end of the file
console.log(await file.seek(-2, Deno.SeekMode.End)); // "9" (i.e. 11-2)



seekSync(offset: number | bigint,
whence: SeekMode,
): number

Synchronously seek to the given offset under mode given by whence. The new position within the resource (bytes from the start) is returned.

using file = Deno.openSync(
  "hello.txt",
  { read: true, write: true, truncate: true, create: true },
);
file.writeSync(new TextEncoder().encode("Hello world"));

// advance cursor 6 bytes
const cursorPosition = file.seekSync(6, Deno.SeekMode.Start);
console.log(cursorPosition);  // 6
const buf = new Uint8Array(100);
file.readSync(buf);
console.log(new TextDecoder().decode(buf)); // "world"

The seek modes work as follows:

// Given the file contains "Hello world" text, which is 11 bytes long:
using file = Deno.openSync(
  "hello.txt",
  { read: true, write: true, truncate: true, create: true },
);
file.writeSync(new TextEncoder().encode("Hello world"));

// Seek 6 bytes from the start of the file
console.log(file.seekSync(6, Deno.SeekMode.Start)); // "6"
// Seek 2 more bytes from the current position
console.log(file.seekSync(2, Deno.SeekMode.Current)); // "8"
// Seek backwards 2 bytes from the end of the file
console.log(file.seekSync(-2, Deno.SeekMode.End)); // "9" (i.e. 11-2)



setRaw(mode: boolean,
options?: SetRawOptions,
): void

Set TTY to be under raw mode or not. In raw mode, characters are read and returned as is, without being processed. All special processing of characters by the terminal is disabled, including echoing input characters. Reading from a TTY device in raw mode is faster than reading from a TTY device in canonical mode.

using file = await Deno.open("/dev/tty6");
file.setRaw(true, { cbreak: true });



stat(): Promise<FileInfo>

Resolves to a Deno.FileInfo for the file.

import { assert } from "jsr:@std/assert";

using file = await Deno.open("hello.txt");
const fileInfo = await file.stat();
assert(fileInfo.isFile);



statSync(): FileInfo

Synchronously returns a Deno.FileInfo for the file.

import { assert } from "jsr:@std/assert";

using file = Deno.openSync("hello.txt")
const fileInfo = file.statSync();
assert(fileInfo.isFile);



sync(): Promise<void>

Flushes any pending data and metadata operations of the given file stream to disk.

const file = await Deno.open(
  "my_file.txt",
  { read: true, write: true, create: true },
);
await file.write(new TextEncoder().encode("Hello World"));
await file.truncate(1);
await file.sync();
console.log(await Deno.readTextFile("my_file.txt")); // H



syncData(): Promise<void>

Flushes any pending data operations of the given file stream to disk.

using file = await Deno.open(
 "my_file.txt",
 { read: true, write: true, create: true },
);
await file.write(new TextEncoder().encode("Hello World"));
await file.syncData();
console.log(await Deno.readTextFile("my_file.txt")); // Hello World



syncDataSync(): void

Synchronously flushes any pending data operations of the given file stream to disk.

using file = Deno.openSync(
 "my_file.txt",
 { read: true, write: true, create: true },
);
file.writeSync(new TextEncoder().encode("Hello World"));
file.syncDataSync();
console.log(Deno.readTextFileSync("my_file.txt")); // Hello World



syncSync(): void

Synchronously flushes any pending data and metadata operations of the given file stream to disk.

const file = Deno.openSync(
  "my_file.txt",
  { read: true, write: true, create: true },
);
file.writeSync(new TextEncoder().encode("Hello World"));
file.truncateSync(1);
file.syncSync();
console.log(Deno.readTextFileSync("my_file.txt")); // H



truncate(len?: number): Promise<void>

Truncates (or extends) the file to reach the specified len. If len is not specified, then the entire file contents are truncated.

Truncate the entire file

using file = await Deno.open("my_file.txt", { write: true });
await file.truncate();

Truncate part of the file

// if "my_file.txt" contains the text "hello world":
using file = await Deno.open("my_file.txt", { write: true });
await file.truncate(7);
const buf = new Uint8Array(100);
await file.read(buf);
const text = new TextDecoder().decode(buf); // "hello w"



truncateSync(len?: number): void

Synchronously truncates (or extends) the file to reach the specified len. If len is not specified, then the entire file contents are truncated.

Truncate the entire file

using file = Deno.openSync("my_file.txt", { write: true });
file.truncateSync();

Truncate part of the file

// if "my_file.txt" contains the text "hello world":
using file = Deno.openSync("my_file.txt", { write: true });
file.truncateSync(7);
const buf = new Uint8Array(100);
file.readSync(buf);
const text = new TextDecoder().decode(buf); // "hello w"



unlock(): Promise<void>

Release an advisory file-system lock for the file.



unlockSync(): void

Synchronously release an advisory file-system lock for the file.



utime(atime: number | Date,
mtime: number | Date,
): Promise<void>

Changes the access (atime) and modification (mtime) times of the file stream resource. Given times are either in seconds (UNIX epoch time) or as Date objects.

using file = await Deno.open("file.txt", { create: true, write: true });
await file.utime(1556495550, new Date());



utimeSync(atime: number | Date,
mtime: number | Date,
): void

Synchronously changes the access (atime) and modification (mtime) times of the file stream resource. Given times are either in seconds (UNIX epoch time) or as Date objects.

using file = Deno.openSync("file.txt", { create: true, write: true });
file.utime(1556495550, new Date());



write(p: Uint8Array): Promise<number>

Write the contents of the array buffer (p) to the file.

Resolves to the number of bytes written.

It is not guaranteed that the full buffer will be written in a single call.

const encoder = new TextEncoder();
const data = encoder.encode("Hello world");
using file = await Deno.open("/foo/bar.txt", { write: true });
const bytesWritten = await file.write(data); // 11



writeSync(p: Uint8Array): number

Synchronously write the contents of the array buffer (p) to the file.

Returns the number of bytes written.

It is not guaranteed that the full buffer will be written in a single call.

const encoder = new TextEncoder();
const data = encoder.encode("Hello world");
using file = Deno.openSync("/foo/bar.txt", { write: true });
const bytesWritten = file.writeSync(data); // 11