Working with async in node.js

Question

I'm a little confused to as how to properly use the async module. Say I have:

result = long_sync_io_function();
short_sync_function(result);
... //other stuff dependent on result

Generally, in Node you convert long_sync_io_function() to its asynchronous counterpart long_async_io_function(callback(err, result)) and do something like:

long_async_io_function(function(err, result) {
    if (!err) {
         short_sync_function(result);
         ...//other stuff dependent on result
    }
});

But the constant embedding of callbacks quickly means lots and lots of indentation. Is the correct way to use async:

//1
async.waterfall([
    function(callback) {
        result = long_sync_io_function();
        callback(null, result);
    },
    function(result, callback) {
        short_sync_function(result);
        //other stuff dependent on result
    }
]);

or

//2
async.waterfall([
    function(callback) {
        long_async_io_function(callback);
    },
    function(result, callback) {
        short_sync_function(result);
        ...//other stuff dependent on result
    }
]);

Are these equivalent? I can't tell if aysnc helps create asynchronous code as in 1, or just aids in structuring existing asynchronous code as in 2.

slebetman · Accepted Answer · 2013-08-13 18:14:47Z

2

The async library has no ability to create asynchronous functions/code. Instead it is meant as helpers for higher order structure/organisation of code that's already asynchronous.

So it's number 2.

Additional answer

For simply avoiding nesting callbacks you don't need to use the async library. Simply name your functions instead of declaring them inline. So, instead of:

long_async_io_function(function(err, result) {
    if (!err) {
         //..do stuff dependent on result
         another_async_function(function(err,result) {
            //..do other stuff
         });
    }
});

You can write it like this:

function do_other_stuff (err, result) {
    //..
}

function do_stuff_with_io_result (err, result) {
    //..
    another_async_function(do_other_stuff);
}

long_async_io_function(do_stuff_with_io_result );

This makes the code self documenting and much easier to debug (especially if you're stepping through in a debugger or looking at a stack trace) and you can therefore remove the redundant comments like do stuff with result and do other stuff.

edited Aug 13, 2013 at 18:14

answered Aug 13, 2013 at 18:03

slebetman

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7 Comments

Colin Over a year ago

OK. So if long_sync_io_function() came from a 3rd party module, I'd still need to create my own long_async_io_function() wrapper function then use async

slebetman Over a year ago

Yes. For wrappers, you can't convert a monolithic long sync functions to async in pure javascript. You'll need to use threading libraries (which are themselves implemented in C) or write a wrapper in C using the node API and use pthreads. The only way to convert sync code to async in pure javascript is to break up the inner loop into recursive calls to setTimeout().

Colin Over a year ago

Can't I just wrap it in a function which calls the long sync function using process.nextTick(long_sync_io_function)?

slebetman Over a year ago

All that will do is wait roughly 1ms (or 10ms, depends on your OS's tick) and then execute the long sync function which will then block all other code from executing. Yes, it prevents it from blocking it now so that you can run other code first but it doesn't prevent it from blocking.

Colin Over a year ago

Interesting. Is there node modules out there that help make things truly asynchronous?

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