NOTE: This is using an old version of run. See next comment for new example.

I updated the code to implement callback double call checks for thunk mode.

Suppose you have an evil function like this:

function evil(callback) {
  // I'm evil, I call the callback multiple times.
  callback(null, 1);
  setTimeout(function () {
    // Try to stop me!
    callback(null, 2);  
  }, 10);
}

Then you can call it using thunk mode:

run(function* () {
  console.log("Hello");
  yield evil;
  yield sleep(1000);
  console.log("World");
});

And everything is fine because of the done flag in each thunk's unique callback.

But if you use resume mode with the shared callback, things get messy:

run(function* (resume) {
  console.log("Hello");
  yield evil(resume);
  yield setTimeout(resume, 1000);
  console.log("World");
});

Which crashes with the following output:

Hello
World

/home/tim/Downloads/files-node/files.js:201
    var cont = iterator.next(item).value;
                        ^
Error: Generator has already finished
    at GeneratorFunctionPrototype.next (native)
    at next (/home/tim/Downloads/files-node/files.js:201:25)
    at check (/home/tim/Downloads/files-node/files.js:220:7)
    at null._onTimeout (/home/tim/Downloads/files-node/files.js:193:5)
    at Timer.listOnTimeout [as ontimeout] (timers.js:105:15)

Because the second callback in evil causes sleep's yield to resume early and when sleep is finally done, the generator is already finished.

Using the same evil function as above with the updated version of run, we get the following thunk version:

function sleep(ms) {
  return function (callback) {
    setTimeout(callback, ms);
  };
}


run(function* () {
  console.log("Hello");
  yield evil;
  yield sleep(1000);
  console.log("World");
});

Which runs with no problem, same as before.

Or if we prefer to wrap setTimeout inline, it's written as:

run(function* () {
  console.log("Hello");
  yield evil;
  yield function (callback) {
    setTimeout(callback, 1000);
  };
  console.log("World");
});

But we can also work in resume mode without errors.

run(function* (resume) {
  console.log("Hello");
  yield evil(resume());
  yield setTimeout(resume(), 1000);
  console.log("World");
});

But if you use resume mode with the shared callback, things get messy

I briefly responded on twitter, but to elaborate a bit more... isn't a nasty exception a good thing if you have async code invoking a callback multiple times? If that's happening, there's clearly a bug and I imagine it would be best to "die early and die often" in that case, rather than simply ignore it. Granted, the thunk-style approach could choose to raise a more descriptive error in that case (+1 on that from me, anyway).

Hi, creationix.
May I ask you a question?

In the following code, you pass an argument to iterator.next() method.

10    var cont = iterator.next(item).value;

But whenever I pass any value to iterator.next(), there's nothing to happen.

var x = function()* {
    yield true;
}
var y = x();
y.next('foo'); //return true

What's the meaning of passing an argument to iterator.next() ?