Node.js v19.1.0 Release Highlights and New Features

Node.js v19.1.0 has been released, bringing several notable updates for backend developers. The release adds function mocking support to the node:test module, allowing top‑level mock objects to replace methods during testing.

Example of mocking an object method:

test('spies on an object method', (t) => {
  const number = {
    value: 5,
    add(a) {
      return this.value + a;
    },
  };
  t.mock.method(number, 'add');

  assert.strictEqual(number.add(3), 8);
  assert.strictEqual(number.add.mock.calls.length, 1);
});

The release also enables recursive watching on Linux via fs.watch with the recursive: true option.

const watcher = fs.watch(testDirectory, { recursive: true });
watcher.on('change', function(event, filename) {
  // handle change
});

Other notable changes include updating ICU to version 72.1, adding new collaborators to the documentation, removing the experimental fetch warning, adding MIME utilities, and improving text decoder performance.