Reading Files

Many applications need to read files from disk. Deno provides a simple interface for reading files.

The easiest way to read a file is to just read the entire contents into memory as bytes.

const bytes = await Deno.readFile("hello.txt");

Instead of reading the file as bytes, there is a convenience function to read the file as a string.

const text = await Deno.readTextFile("hello.txt");

Often you need more control over when what parts of the file are read. For this you start by opening a file to get a `Deno.FsFile` object.

const file = await Deno.open("hello.txt");

Read some bytes from the beginning of the file. Allow up to 5 to be read but also note how many actually were read.

const buffer = new Uint8Array(5);
const bytesRead = await file.read(buffer);
console.log(`Read ${bytesRead} bytes`);

You can also seek to a known location in the file and read from there.

const pos = await file.seek(6, Deno.SeekMode.Start);
console.log(`Seeked to position ${pos}`);
const buffer2 = new Uint8Array(2);
const bytesRead2 = await file.read(buffer2);
console.log(`Read ${bytesRead2} bytes`);

You can use rewind back to the start using seek as well.

await file.seek(0, Deno.SeekMode.Start);

Make sure to close the file when you are done.

file.close();

Synchronous reading is also supported.

Deno.readFileSync("hello.txt");
Deno.readTextFileSync("hello.txt");
const f = Deno.openSync("hello.txt");
f.seekSync(6, Deno.SeekMode.Start);
const buf = new Uint8Array(5);
f.readSync(buf);
f.close();

The `--allow-read` permission is required to read files.

Run this example locally using the Deno CLI:

deno run --allow-read https://docs.deno.com/by-example/reading-files.ts

Additional resources: