simplified co

simple-co.js

'use strict';

function isGenerator(obj) {
  return typeof obj.next === 'function' && typeof obj.throw === 'function';
}

// co returns a promise that resolves to the return value of the generator arg
function co(generator) {
  return new Promise(resolve => {
    function next(result) {
      const step = generator.next(result);
      const yieldedValue = step.value;
      if (step.done) {
        return yieldedValue;
      }
      if (yieldedValue instanceof Promise) {
        return yieldedValue.then(next);
      }
      if (isGenerator(yieldedValue)) {
        return co(yieldedValue).then(next);
      }
    }
    resolve(next());
  });
}

function square(x) {
  return new Promise(resolve => {
    setTimeout(() => resolve(x * x), 500);
  });
}

function sum(x, y) {
  return new Promise(resolve => {
    setTimeout(() => resolve(x + y), 500);
  });
}

function * sumSquares(x, y) {
  return yield sum(yield square(x), yield square(y));
}

function * main() {
  console.log('2^2 + 3^2 = ' + (yield sumSquares(2, 3)));
  console.log('3^3 + 4^2 = ' + (yield sumSquares(3, 4)));
}

co(main());