generator-reduce.js

'use strict';

const co = require('co');

// Like `reduce()` but accepting a generator function, such that it can be used asynchronously
// within a generator function invoked by `co()`.
function* reduceg(arr, genfn, initial) {
  let acc = initial;
  for (let x of arr) {
    acc = yield genfn(acc, x);
  }
  return acc;
}

// For the sake of illustration: Promise returning function that might do something asynchronous.
function process(x, reduction) {
  return new Promise(resolve => {
    if (x % 2 == 0) { //
      setTimeout(() => {
        reduction.push(`${x} hello async`);
        resolve(true);
      }, 1);
    } else if (x % 3 == 0) {
      reduction.push(`${x} hello sync`);
      resolve(true);
    } else {
      resolve(false);
    }
  });
}

co(function *() {
  let result = yield reduceg([1, 2, 3, 4, 5], function* (reduction, x) {
    let processed = yield process(x, reduction);
    if (!processed) {
      reduction.push(`${x} NOT PROCESSED`);
    }
    return reduction;
  }, []);
  console.log(result);
});