ES7 async functions with ES6 generators and promises

async.js

/*

ES7

```
async function f() {
  await ...
}
```

ES6

```
const f = async(function* () {
  yield ...
});
```

*/

function async(f) {
  return function () {
    let generator = f.apply(this, arguments);
    return new Promise((resolve, reject) => {
      let onValue = generator.next.bind(generator);
      let onError = generator.throw.bind(generator);

      (function step(callback) {
        let next;

        try {
          next = callback();
        } catch (err) {
          return reject(err);
        }

        if (next.done) resolve(next.value);
        else if (next.value && next.value.then) Promise.resolve(next.value)
          .then((value) => step(() => onValue(value)), (err) => step(() => onError(err)));
        else step(() => onValue(value));
      }(onValue));
    });
  };
}