明确承诺不起作用

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

Answer 1

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);