coroutine_1.js

//Considering getValueFromFile1, getValueFromFile2 and getValueFromFile3
//all return promises
function* coroutine(){                      //this part looks
    let a = yield getValueFromFile1();      //very similar to
    let b = yield getValueFromFile2();      //the synchronous
    let sum = a + b;                        //equivalent
    yield printSumToFile3(sum);
    console.log('Success');
}

let co = coroutine();                       
co.next().value                             //getValueFromFile1() called; which returns a promise
.then(result => co.next(result).value)      //result stored in variable 'a' and getValueFromFile2() called
.then(result => co.next(result).value)      //result stored in variable 'b' and printSumToFile3() called
.then(result => co.next(result).value);     //print 'Success'