I am using Koa and Bluebird and is trying out coroutines. I find that the below gives me an error. But if I replace yield initRouters() with the contents of initRouters() it works. Why is that?

'use strict';

const fs = require('fs');
const Promise = require('bluebird');
const readdir = Promise.promisify(fs.readdir);

let app = require('koa')();

let initRouters = function *() {
  const routerDir = `${__dirname}/routers`;
  let routerFiles = yield readdir(routerDir);
  return Promise.map(routerFiles, function *(file) {
    var router = require(`${routerDir}/${file}`);

    app
      .use(router.routes())
      .use(router.allowedMethods());
  });
}

Promise.coroutine(function *() {
  yield initRouters();

  const PORT = process.env.PORT || 8000;
  app.listen(PORT);
  console.log(`Server listening on ${PORT}`);
})();

The error I get is

Unhandled rejection TypeError: A value [object Generator] was yielded that could not be treated as a promise

    See 

From coroutine:
    at Function.Promise.coroutine (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:176:17)
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:21:9)
    at PromiseSpawn._continue (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:145:21)
    at PromiseSpawn._promiseFulfilled (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:92:10)
    at /home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:183:15
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:27:3)
    at Module._pile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:141:18)
    at node.js:933:3

I am using Koa and Bluebird and is trying out coroutines. I find that the below gives me an error. But if I replace yield initRouters() with the contents of initRouters() it works. Why is that?

'use strict';

const fs = require('fs');
const Promise = require('bluebird');
const readdir = Promise.promisify(fs.readdir);

let app = require('koa')();

let initRouters = function *() {
  const routerDir = `${__dirname}/routers`;
  let routerFiles = yield readdir(routerDir);
  return Promise.map(routerFiles, function *(file) {
    var router = require(`${routerDir}/${file}`);

    app
      .use(router.routes())
      .use(router.allowedMethods());
  });
}

Promise.coroutine(function *() {
  yield initRouters();

  const PORT = process.env.PORT || 8000;
  app.listen(PORT);
  console.log(`Server listening on ${PORT}`);
})();

The error I get is

Unhandled rejection TypeError: A value [object Generator] was yielded that could not be treated as a promise

    See http://goo.gl/MqrFmX

From coroutine:
    at Function.Promise.coroutine (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:176:17)
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:21:9)
    at PromiseSpawn._continue (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:145:21)
    at PromiseSpawn._promiseFulfilled (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:92:10)
    at /home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:183:15
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:27:3)
    at Module._pile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:141:18)
    at node.js:933:3

• javascript
• ecmascript-6
• bluebird
• koa

Share Improve this question Follow asked Mar 12, 2016 at 4:24 Jiew MengJiew Meng 88.6k192192 gold badges529529 silver badges833833 bronze badges 2

• I think you need to wrap your initRouters generator function in Promise.coroutine as well – laggingreflex Commented Mar 12, 2016 at 10:15
• In initRouters try yielding the Promise.map() call instead of returning it. – mikefrey Commented Mar 16, 2016 at 16:35

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2 Answers 2

Sorted by: Reset to default 6

You have reach an egg/chicken problem.

Promise.coroutine let's you yield a promise:

Returns a function that can use yield to yield promises.

The problem here is that you trying to yield a generator function, not a promise. This results in the error:

Unhandled rejection TypeError: A value [object Generator] was yielded that could not be treated as a promise

Why?

Well, the generator function returns a promise, but it's only a promise when you yield it. And because you can't yield a generator function, you never get the value (the promise).

You can (and should) use coroutine and yield initRouters();. BUT, that initRouters() should be a normal function that returns a promise, not a generator function.

As a side note, you don't need Promise.map() because everything you do inside the .map is synchronous, so you can perfectly do:

let initRouters = function() {
  const routerDir = `${__dirname}/routers`;

  return readdir(routerDir).then(function (files) {
    files.map(function (file) {
      var router = require(`${routerDir}/${file}`);

      app
        .use(router.routes())
        .use(router.allowedMethods());
    })
  })
}

Here you are returning a promise that resolves when you yield it. But as you see, you are returning already a promise, that's why Promise.coroutine doesn't give you any error. In the case of the generator, you are not returning a promise.

initRouters() returns a generator. You can't yield that to Promise.coroutine, which only allows you to yield promises (as the error clearly indicates).

You either need to wrap initRouters in Promise.coroutine itself, or use

yield* initRouters();

so that all promises from the generator are yielded and not the generator itself. Notice that the co library does allow such quirks, in contrast to Bluebird.