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I want to run an async operation (for example, wait for a URL call to complete) before I start my app. I don't know how to do that (since it's an upper-level application - no async/await here).

www.js:

var app = require('./app');
var http = require('http');
const port = '3000';
app.set('port', port);
var server = http.createServer(app);
server.listen(port);

app.js:

var express = require('express');
var app = express();
var Promise = require('bluebird');

# HERE IS WHERE I WANT TO "AWAIT" AN ASYNCHRONOUS CALL. 
# I KNOW I CAN'T USE AWAIT BECAUSE I'M NOT WITHIN 
# AN "ASYNC" ANNOTATED FUNCTION. EXAMPLE:
const data = await Promise.promisify(fs.readFile('DATA'));

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
app.use(express.static(path.join(__dirname, 'public')));
app.use('/', routes);
module.exports = app;

I thought about wrapping all of app.js with a Promise.promisify() so I can use async await withing it. But I'm not sure how that would be handled in www.js, since it is not invoked, as in app() but passed on to http.createServer

I want to run an async operation (for example, wait for a URL call to complete) before I start my app. I don't know how to do that (since it's an upper-level application - no async/await here).

www.js:

var app = require('./app');
var http = require('http');
const port = '3000';
app.set('port', port);
var server = http.createServer(app);
server.listen(port);

app.js:

var express = require('express');
var app = express();
var Promise = require('bluebird');

# HERE IS WHERE I WANT TO "AWAIT" AN ASYNCHRONOUS CALL. 
# I KNOW I CAN'T USE AWAIT BECAUSE I'M NOT WITHIN 
# AN "ASYNC" ANNOTATED FUNCTION. EXAMPLE:
const data = await Promise.promisify(fs.readFile('DATA'));

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
app.use(express.static(path.join(__dirname, 'public')));
app.use('/', routes);
module.exports = app;

I thought about wrapping all of app.js with a Promise.promisify() so I can use async await withing it. But I'm not sure how that would be handled in www.js, since it is not invoked, as in app() but passed on to http.createServer

• javascript
• node.js
• express
• promise
• async-await

Share Improve this question Follow edited Dec 29, 2016 at 15:32 eran asked Dec 28, 2016 at 14:29 eraneran 15.1k3737 gold badges104104 silver badges148148 bronze badges 3

• Have you looked up tutorials on promises? Here's a good start: github.com/kriskowal/q – Dave Swersky Commented Dec 28, 2016 at 14:32
• What asynchronous call? Also, app.js doesn't show any exports, and yet you're using it as though it had. – T.J. Crowder Commented Dec 28, 2016 at 14:35
• @T.J.Crowder you are right. My bad. Edited to add export and illustrate reading a file (I know there's a sync version. Let's assume there isn't or I'm using some other Promised call) – eran Commented Dec 29, 2016 at 15:33

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2 Answers 2

Sorted by: Reset to default 18

From what you've said, app.js is presumably exporting something (perhaps app?) but that something isn't ready to be used until an asynchronous call (which I don't think you've shown) is complete.

Assuming that's all true, then what app.js exports needs to provide a way for the things using it to wait until it's ready. One way to do that is to use a promise.

Assuming what you're waiting for isn't promise-ified, then:

app.js:

var express = require('express');
var app = express();
exports.appPromise = new Promise(function(resolve, reject) {
    startTheAsyncOperation(function(err, result) {
        if (err) {
            reject(err);
            return;
        }
        app.set('views', path.join(__dirname, 'views'));
        app.set('view engine', 'jade');
        app.use(express.static(path.join(__dirname, 'public')));
        app.use('/', routes);
        resolve(app);
    });
});

then in www.js:

var appPromise = require('./app').appPromise;
var http = require('http');
const port = '3000';
appPromise.then(function(app) {
    app.set('port', port);
    var server = http.createServer(app);
    server.listen(port);
});

---

Of course, if the async thing you're waiting for provides a promise to you, you don't need new Promise in app.js; instead:

var express = require('express');
var app = express();
exports.appPromise = startTheAsyncOperation().then(function() {
    app.set('views', path.join(__dirname, 'views'));
    app.set('view engine', 'jade');
    app.use(express.static(path.join(__dirname, 'public')));
    app.use('/', routes);
    return app;
});

Take a look at the async package. For your needs, you should go with the waterfall control flow provided by the async package. You can have a look as it is very well documented.

You can wrap your code with the waterfall function and this part of your code will be running synchronously.

Edit: Updated Link, but the accepted answer has a better approach imho.

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