Generator 101

function * HelloGen(){
  yield 100;
  yield 400;
}

var gen = HelloGen();

console.log(gen.next()); // {value: 100, done: false}
console.log(gen.next()); // {value: 400, done: false}
console.log(gen.next()); // {value: undefined, done: true}
`</pre>

<pre>`function * HelloGen2() {
  var a = yield 100;
  var b = yield a + 100;

  console.log(b);
}

var gen2 = HelloGen2();

console.log(gen2.next()); // {value: 100, done: false}
console.log(gen2.nect(500)); // {value: 600, done: false}
console.log(gen2.next(1000)); // {value: undefined, done: done}
// prints 1000
`</pre>

### Preventing Callback Hell

So, how can generators be used to avoid callback hell? First, you need to understand a simple technique that we will be using heavily with generators to write code without callbacks.

#### Understanding Thunks

A thunk is a partially evaluated function which accepts a single callback as the argument. Within generators, we'll be yielding thunks to write without callbacks. A simple thunk is shown below.

<pre>`function(callback) {
  fs.readFile('myfile.md', 'utf8', callback)
}
`</pre>

Thunks can also be created dynamically as shown below:

<pre>`function readFile(filename) {
  return function(callback) {
    fs.readFile(filename, 'utf8', callback);
  };
}
`</pre>

#### Using `co`

`co` is a nice module which helps to use thunks and generators together to create Node.js applications without callbacks.

A simple application which uses `co` and the `readFile()` thunk.

<pre>`var co = require('co');

co(function* (){
  var file1 = yield readFile('file1.md');
  var file2 = yield readFile('file2.md');

  console.log(file1);
  console.log(file2);
})();
`</pre>

As you can see, we are no longer using callbacks. This gives us a simple way to write large modular Node apps easily.

###### How `co` Works Internally

1. First, it calls next(null) and gets a thunk.

2. Then, it evaluate the thunk and saves the result.

3. Then, it calls next(savedResult)(in this case, saveResult == readFile(‘file1.md’), and next(savedResult) pass the content to var file1)

4. Repeat these steps until next() return {done: true}

   A minimal version of co written to show how it works internally.

   function co(generator){
     var gen = generator();
   
   function nextItem(err, result) {
       var item = gen.next(result);
   
   <pre>`if (!item.done) {
     item.value(nextItem); // item.value is a thunk, because yield a thunk
   }

}

nextItem();
}