Return value from Node.js module with asynchronous function

Question

I wrote a module for my Node.js project which processes some data and is supposed to return the result, like that:

var result = require('analyze').analyzeIt(data);

The problem is that analyze.js depends on an asynchronous function. Basically it looks like this:

var analyzeIt = function(data) {
    someEvent.once('fired', function() {
        // lots of code ...
    });
    return result;
};
exports.analyzeIt = analyzeIt;

Of course, this cannot work because result is still empty when it is returned. But how can I solve that?

Community · Accepted Answer · 2017-05-23 12:12:51Z

10

You solve it the same way Node solves it in its API: With a callback, which might be a simple callback, an event callback, or a callback associated with a promise library of some kind. The first two are more Node-like, the promise stuff is very au currant.

Here's the simple callback way:

var analyzeIt = function(data, callback) {
    someEvent.once('fired', function() {
        // lots of code ...

        // Done, send result (or of course send an error instead)
        callback(null, result); // By Node API convention (I believe),
                                // the first arg is an error if any,
                                // the second data if no error
    });
};
exports.analyzeIt = analyzeIt;

Usage:

require('analyze').analyzeIt(data, function(err, result) {
    // ...use err and/or result here
});

But as Kirill points out, you might want to have analyzeIt return an EventEmitter and then emit a data event (or whatever event you like, really), or error on error:

var analyzeIt = function(data) {
    var emitter = new EventEmitter();

    // I assume something asynchronous happens here, so
    someEvent.once('fired', function() {
        // lots of code ...

        // Emit the data event (or error, of course)
        emitter.emit('data', result);
    });

    return emitter;
};

Usage:

require('analyze').analyzeIt(data)
    .on('error', function(err) {
        // ...use err here...
    })
    .on('data', function(result) {
        // ...use result here...
    });

Or, again, some kind of promises library.

edited May 23, 2017 at 12:12

CommunityBot

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answered Feb 4, 2015 at 15:10

T.J. Crowder

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2 Comments

wortwart Over a year ago

Thanks! I didn't get the callback solution to work, so I fixed it with custom elements with which I have already some experience. I thought I could do without changing the caller syntax but I guess that's not possible. I hope Promises will get implemented in Io.js soon, that seems to be a rather elegant way to do it.

T.J. Crowder Over a year ago

@wortwart: ES6 will have promises, but you can have them today, there are several promise libs out there.

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