VM and CLI

• dart:io works on the Dart VM (CLI, servers, Flutter desktop). It does not run in the browser.

Step 1 - Write then read

import 'dart:io';

void main() {
  final f = File('hello.txt');
  f.writeAsStringSync('hello\n');
  final text = f.readAsStringSync();
  print(text);
}

Step 2 - existsSync and append

import 'dart:io';

void main() {
  final f = File('log.txt');
  if (f.existsSync()) {
    f.writeAsStringSync('line\n', mode: FileMode.append);
  } else {
    f.writeAsStringSync('line\n');
  }
}

Step 3 - Bytes (optional)

• readAsBytesSync / writeAsBytesSync for binary data.

Good habit

• Sync I/O blocks the isolate; prefer async in real apps (next guide).

Practice tasks

• Write a function void ensureFileWithDefault(File f, String defaultText) that creates the file with default content only if missing.
• Delete the test file with deleteSync() after experiments or use a dedicated data/ folder.