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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://examples.deno.land/streaming-files.ts

Additional resources: