Using ES6 Generator functions with SailsJS

Question

I love generators in nodejs. They help nodejs look more like server-side code. I'm trying to use generators with my Sails app. This is my controller and it works when I visit 'get /hi':

/**
 * FooController
 *
 * @description :: Server-side logic for managing foos
 * @help        :: See http://sailsjs.org/#!/documentation/concepts/Controllers
 */

module.exports = {
  hi: function (req, res){
    return res.send("Hi there!");
  }
};

However when I change that hi action to a generator function...

module.exports = {
  hi: function* (req, res){
    return res.send("Hi there!");
  }
};

this action never gets to return the response. Like it's waiting for something. How does one utilize ES6 generators within SailsJS controllers and in-fact all of Sails?

Why would you want to do this? Now you've got a function that returns a generator (which is not magic!) instead of a function that calls res.send - of course it behaves differently. — Bergi
It can be done, with a little variation of what you've tried, check out my answer below :) — Lu Roman

Lu Roman Lu Roman · Accepted Answer · 2016-03-07T16:40:00

You can use it, in fact it'll be great if we all use this style, it adds a lot of readability and flexibility in your code (no more callback-hell), but that is not the way to use it.

As @Cohars stated, sails expect a function as controller actions, you can't pass a generator like in Koa, but that does not prevent you from using them, the thing is that a generator by itself is very useless, you need a function that calls it and iterates it and i believe koa does this for you at framework level, but you have some options available to you at library level, like co or yortus/asyncawait/ which is what i use because node-fibers implemented as functions are way more flexible than es6 generators (though they are almost the same) and i'll show you why in a sec.

So here is how you use it:

First npm install co and require it in your controller, or add it as a global in config/bootstrap.js since you will be using it a lot. Once you are done with it, you can use it in your controllers.

module.exports = {
  hi: function(req, res){
    co(function* (){
      // And you can use it with your models calls like this
      let user = yield User.findOne(req.param('id'))
      return res.send("Hi there" + user.name + "!");
    })
  }
};

That's it

Now if you rather use async/await, its pretty similar, it goes like this:

module.exports = {
  hi: function(req, res){
    async(function(){
      // Since await is a function, you can access properties
      // of the returned values like this which is nice to
      // access object properties or array indices
      let userName = await(User.findOne(req.param('id'))).name
      return res.send("Hi there" + userName + "!");
    })();
  }
};

There is a caveat though, if you call other methods of you controller via this, remember that they will refer to the generator fn or the passed regular fn if you use async/await, so be sure to save its context, or since you are already using es6 syntax, you can use fat arrows with async/await, unfortunately not with co, since there is not fat arrow version of generators (..yet?)

So it'll be like this:

module.exports = {
  hi: function(req, res){
    async(() => {
      let params = this._parseParams(req);
      let userName = await(User.findOne(params.id)).name
      return res.send("Hi there" + userName + "!");
    })();
  },
  _parseParams: function(req){
    let params = req.allParams()
    // Do some parsing here...
    return params
  }

};

I've been using the second method for months now, and works perfectly, i've tried with co too and works as well, i just liked more the async/await module (and is supposed to be a little bit faster) and its a perfect match if you are using coffeescript since your sintax will be like do async => await User.find()

UPDATE:

I've created a wrapper that you can use if you use yortus async/await or check the source code and modify it to work with co or something else if you wish.

Here is the link https://www.npmjs.com/package/async-handler, is in alpha but i'm using on my own apps and works as expected, if not submit an issue.

With that the las example would look like this:

module.exports = {
    hi: asyncHandler((req, res)->{
      let params = this._parseParams(req);
      let userName = await(User.findOne(params.id)).name
      return res.send("Hi there" + userName + "!");
    }),
    _parseParams: function(req){
      let params = req.allParams()
      // Do some parsing here...
      return params
    }
};

With this handler you get the extra benefit of having async/promise errors to propagate correctly on sails if they are not caught by a try/catch

Using ES6 Generator functions with SailsJS

4 Answers