Promises in mongoose

Question

Can anyone please explain why this:

// this works
router.route('/videos')
  .get((req, res)=>{
    Video.find()
      .exec()
      .then(console.log);
  });

// this also works
router.route('/videos')
  .get((req, res)=>{
    Video.find()
      .exec()
      .then(videos=>{
        res.json(videos)
      });
  });

works, and this:

router.route('/videos')
  .get((req, res)=>{
    Video.find()
      .exec()
      .then(res.json);
  });

doesn't?

The res object represents the HTTP response that an Express.js app sends when it gets an HTTP request. The console.log method outputs the videos data while res.json does not seem to be called.

Are you losing an error due to the promises? Tap a .catch() on the end: `.then(res.json).catch(console.log);` — Adrian Lynch, Apr 20 '15 at 13:39
Try `.then(res.json.bind(res))`, as you might be losing the binding context. — robertklep, Apr 20 '15 at 13:41
This is about how `this` works, not promises, either bind or wrap it in a lambda — Benjamin Gruenbaum, Apr 20 '15 at 13:42
@BenjaminGruenbaum: Isn't this a duplicate? I just can't find it right now — Bergi, Apr 20 '15 at 13:43
It is, probably "how does this work in JavaScript", I'm on mobile — Benjamin Gruenbaum, Apr 20 '15 at 13:44
Thank you, res.json.bind(res) solved it. Sorry if it's duplicate. — Andrei Rosca, Apr 20 '15 at 13:44
@BenjaminGruenbaum: Nah, I meant of another [promise] question that had this problem with express — Bergi, Apr 20 '15 at 13:45

score 1 · Accepted Answer · edited May 23 '17 at 12:06

1

res.json does expect to be called as a method, with res being the this value. It works with console.log because that is already bound to the console object in node.

You can use

.then(res.json.bind(res))

or continue to use that arrow function. See also How to access the correct `this` context inside a callback?

edited May 23 '17 at 12:06

Community

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answered Apr 20 '15 at 13:42

Bergi

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Promises in mongoose

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