Async using Generators and Promises

asyncGenerator.js

/** ES 2015 naive implementation of https://github.com/tj/co */
function coAsync(generator) {
  var gen = generator();
  
  return new Promise(function(resolve, reject) {
    (function async({value, done}) {
      if (done) return resolve(value);

      if (Array.isArray(value)) {
        Promise.all(value).then(values => async(gen.next(values)));
      } else {
        value.then(_value => async(gen.next(_value)));
      }
    })(gen.next());
  });
}

function coAsyncWrap(generator) {
  return value => coAsync(generator.bind(null, value));
}

// example usage
function waitThenReturn(value, seconds=1) {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(value), seconds * 1000);
  });
}

coAsync(function*() {
  console.log('coAsync');

  // waits 1 second, then returns value 1
  let a1 = yield waitThenReturn(1);
  console.log(a1);
  
  let a2 = yield waitThenReturn(2);
  console.log(a2);
  
  let a3 = yield waitThenReturn(3);
  console.log(a3);

  return [a1, a2, a3];
})
.then(([b1, b2, b3]) => {
  console.log(b1, b2, b3);
  return fn(3);
})
.then(([b1, b2, b3]) => {
  console.log(b1, b2, b3);
  return fn2(4);
})
.then(([b1, b2, b3]) => {
  console.log(b1, b2, b3);
});

// same as above but allows you to feed a value to the generator
var fn = coAsyncWrap(function*(value) {
  console.log('coAsyncWrap with value', value);
  let a1 = yield waitThenReturn(value + 1);
  console.log(a1);
  let a2 = yield waitThenReturn(2);
  console.log(a2);
  let a3 = yield waitThenReturn(3);
  console.log(a3);

  return [a1, a2, a3];
});


// can return all yielded values at once;
var fn2 = coAsyncWrap(function*(value) {
  console.log('coAsyncWrap 2 with value', value);
  return yield [
    waitThenReturn(value + 1),
    waitThenReturn(value + 2),
    waitThenReturn(value + 3),
  ]
});