Generators -ES2015

generators-es2015.js

# Generators - ES2015

http://riadbenguella.com/how-es6-generators-are-changing-how-we-write-javascript/

Generators are special functions that can be run, paused and resumed at different stages of their execution, thanks to the special keyword `yield`.

```javascript
function* myGenerator() {  
  yield 'first';
  let input = yield 'second';
  yield input;
}

// Getting the generator object
let gen = myGenerator();

// Launching the generator
console.log( gen.next() ): // { value: 'first', done: false }

// First resume (no passed value)
console.log( gen.next() ); // { value: 'second', done: false }

// First resume (pass a value)
console.log( gen.next('third') ); // { value: 'third', done: false }

// Last (no more yield)
console.log( gen.next() ); // { value: undefined, done: true }  
```

## Generators as iterators

```javascript
function* myGenerator() {  
  yield 'first';
  yield 'second';
  yield 'third';
}

for (var v of myGenerator()) {  
  console.log(v);
}
```
## Asynchronous control flow

Having this...

```javascript
function login(username, password) {  
  return fetch('/login', {
    method: 'post',
    body: JSON.stringify({
      username: username
      password: password
    })
  }).then(response => response.json());
}

function getPosts(token) {  
  return fetch('/posts', {
    headers: new Headers({
      'X-Security-Token': token
    })
  }).then(response => response.json());
}
```

We can go from this...

```javascript
const username = '';  
const password = '';

login(username, password)  
  .then(result => getPosts(result.token))
  .then(posts => {
    console.log(posts);
  })
  .catch(err => {
    console.log(err);
  })
;
```

To this...

```javascript
co(function* () {  
  var result = yield login(username, password);
  var posts = yield getPosts(result.token);

  return posts;
}).then(value => {
  console.log(value);
}, err => {
  console.error(err);
});
```

## From express to Koa


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## More info

- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*
- https://ponyfoo.com/articles/es6-generators-in-depth
- https://ponyfoo.com/articles/asynchronous-i-o-with-generators-and-promises
- http://riadbenguella.com/how-es6-generators-are-changing-how-we-write-javascript/
- http://stackoverflow.com/questions/37792303/es6-generator-function-vs-promises
- http://stackoverflow.com/questions/28031289/javascript-asynchronous-programming-promises-vs-generators
- http://tobyho.com/2015/12/27/promise-based-coroutines-nodejs/
- http://codinginadtech.com/posts/es6-and-beyond-part-2/
- http://www.2ality.com/2015/03/es6-generators.html