echoGenerator.js

function* echoGenerator() {
  let received = null;

  while (true) {
    received = yield received;

    if (received === null) return;

    received = 'ECHO ' + received;
  }
}

messages = ['hello', 'these', 'are', 'some', 'messages', null];

let echo = echoGenerator();

for (message of messages) {
  console.log(`Sending: ${message}`);
  let received = echo.next(message);

  if (received.done) {
    console.log('Done receiving');
    break;
  }

  console.log(`Received: ${received.value}`);
}

// Sending: hello
// Received: null
// Sending: these
// Received: ECHO these
// Sending: are
// Received: ECHO are
// Sending: some
// Received: ECHO some
// Sending: messages
// Received: ECHO messages
// Sending: null
// Done receiving