Express promise doesn't work

Question

I am quite new in promises and I think I don't understand it correctly because I am trying it in my code without success.

I have a server on NodeJS, using Express library and express-promise

var express = require('express');
var app = express();
app.use(require('express-promise')());

Then I am handling ajax query:

var promise = function() {
    for(var query in req.query ){
        console.log( 'query: ', query );
        switch( query ){
           case 'getobserved':
             results.observedTags = getObserved();
             break; 
           ...
        }
    }
};

And getObserved is getting data from Firebase DB

var getObserved = function() {
    var observedTags = dbRef.child('observedTags');
    observedTags.on('value', function(snap) {
        var allObservedItems = snap.val();
        var list = [];
        for(var ii in allObservedItems ){
            list.push( allObservedItems[ii].name );
        }
        return list;
    });
};

And finally I am trying to send response to client by:

promise.then( res.send( results ), someRejectedMethod );

And I get in console this:

TypeError: undefined is not a function
at d:\wamp\www\soz2\server.js:100:13

Probably method "promise" is undefined. I am not sure if I use express-promise wrong, or it's just my lack of knowledge about whole promises. Need some hint, please

You're defining `promise` to just be a regular anonymous function. Something isn't a promise just because you named it `promise`. Therefore, the `then` property is undefined. — Brad, Apr 19 '15 at 21:10
Ok I understand, but can you give me a hint, what should I do ? — Piotr Wu, Apr 19 '15 at 21:12
trying to send response to user when my function 'promise' will be finished; — Piotr Wu, Apr 19 '15 at 21:14
You don't need a promise for that. At the end of your function, simply call `res.send()`. If you have straightforward logic and want to do something at the end of a function, a promise doesn't help you with anything. — Brad, Apr 19 '15 at 21:15
@PiotrWójcik: Please show us the complete method `promise`. What asynchronous things are you trying to do there, whose "finish" you want to await? — Bergi, Apr 19 '15 at 21:50
I am getting data from Firebase DB, I updated post with proper code — Piotr Wu, Apr 19 '15 at 22:04

score 1 · Accepted Answer · edited May 23 '17 at 11:57

Probably method "promise" is undefined.

No, the promise function seems to be defined. Only it is a function, not a promise object, and has no .then method.

I am not sure if I use express-promise wrong

Yes. From how I understand their docs, all it seems to do is make res.send, res.json, and res.render accept promises or objects that contain promises, which are then recursively expanded and awaited.

or it's just my lack of knowledge about whole promises.

That as well. I recommend the general promise resources as an entry point to learn about promises.

First, you will have to promisify your Firebase methods so that getObserved does return a promise. (Currently, it seems to fail because you try to return from an async callback). You'll probably have to choose a promise libary if you aren't already using one.

Then, in your route you can use it like

for (var query in req.query) {
    console.log('query: ', query);
    switch(query) {
        case 'getobserved':
            results.observedTags = getObserved(); // a promise
            break; 
         …
    }
}
res.json( result ); // here, express-promise does some magic

No need for the var promise = function() { line. And you probably can scrap promise.then( res.send( results ), someRejectedMethod ); as well, as express-promise does handle this for you. If you didn't use it, you could have done it explicitly with something like

var promise = getObserved();
promise.then(function(list) {
    res.json({observedTags: list});
}, someRejectedMethod);

Express promise doesn't work

1 Answers1