generators with async calls.js

// Prone to bugs! This is for learning and demonstration purposes only

let prev = 0, curr = 1;

// the generator
function* fibGen(n) {
  const pos = 1;

  for(let i=pos; i < n; ++i) {
    [prev, curr] = yield asyncFetchPrevAndCurrValues(n); // returns a promise
    [prev, curr] = [curr, prev + curr];
  }
}

// the async service
asyncFetchPrevAndCurrValues = (n) => (
  new Promise(res => {
    if (n === 1)
      [prev, curr] = [undefined, 0];

    setTimeout(() => res([prev, curr]), 50);
  })
);

// the generator runner
function runFibGen(gen=fibGen(50), args, fibs=[0]) { // generate an array of 50 fibs asynchronously
  let item = gen.next(args);

  if(!item.done) {
    item.value.then(
      res => {
        fibs = fibs.concat([res[1]]);
        /* res contains prev, curr as resolved by the Promise. These will be the basis upon which the generator computes the 
            next values by using runFibGen to pass them back to the generator with gen.next as follows
        */
        runFibGen(gen, res, fibs)
      }
    );
  } else
    console.log(fibs);
}

// run it
runFibGen();


// 0112358

// let memo = {
//   0: 0,
//   1: 1
// };
//
// function fib (n) {
//
//   if(memo[n] === undefined){
//     memo[n] = fib(n-1) + fib(n-2);
//     return memo[n]
//   }
//
//   return memo[n]
// }
//
// console.log(fib(500));