Streaming File Operations

Sometimes we need more granular control over file operations. Deno provides a low level interface to file operations that may be used for this purpose.

For our first example we will demonstrate using a writeable stream. To handle any low level file operations we must first open the file.

const output = await Deno.open("example.txt", {
  create: true,
  append: true,
});

We are now able to obtain a writer from the output. This automatically handles closing the file when done.

const outputWriter = output.writable.getWriter();

Before we do anything with the writer, we should make sure it's ready.

await outputWriter.ready;

Let's write some text to the file

const outputText = "I love Deno!";
const encoded = new TextEncoder().encode(outputText);
await outputWriter.write(encoded);

Now we close the write stream (and the file)

await outputWriter.close();

For our next example, let's read the text from the file!

const input = await Deno.open("example.txt");

Let's get a reader from the input

const inputReader = input.readable.getReader();
const decoder = new TextDecoder();

Let's read each chunk from the file and print it to the console.

let done = false;
do {
  const result = await inputReader.read();
  done = result.done;

  if (result.value) {
    console.log(`Read chunk: ${decoder.decode(result.value)}`);
  }
} while (!done);

Run this example locally using the Deno CLI:

deno run --allow-read --allow-write https://raw.githubusercontent.com/denoland/deno-docs/main/by-example/streaming-files.ts

Additional resources: