async with generators / promises

async.js

// http://pag.forbeslindesay.co.uk/#/22
function async(makeGenerator){
  return function (){
    var generator = makeGenerator.apply(this, arguments)
    
    function handle(result){ // { done: [Boolean], value: [Object] }
      if (result.done) return result.value
      
      return result.value.then(function (res){
        return handle(generator.next(res))
      }, function (err){
        return handle(generator.throw(err))
      })
    }
    
    return handle(generator.next())
  }
}

example.js

// http://pag.forbeslindesay.co.uk/#/28 (parallel operations)
var get = async(function *(){
  var left = yield readJSON('left.json')
  var right = yield readJSON('right.json')
  return {left: left, right: right}
})