Conditionally call async function in Node

Question

I have the following example code - the first part can result in an async call or not - either way it should continue. I cannot put the rest of the code within the async callback since it needs to run when condition is false. So how to do this?

if(condition) {
    someAsyncRequest(function(error, result)) {
        //do something then continue
    }
}

 //do this next whether condition is true or not

I assume that putting the code to come after in a function might be the way to go and call that function within the async call above or on an else call if condition is false - but is there an alternative way that doesn't require me breaking it up in functions?

Async-await package may help (allows to write async code in a sync style, supports exceptions etc.) npmjs.com/package/asyncawait - such syntax will also be natively supported in future Node versions, see github.com/nodejs/promises/issues/4 — Maxime Pacary
– Maxime Pacary, Commented Dec 7, 2016 at 20:47

ambrons · Accepted Answer · 2015-10-20 14:32:24Z

7

A library I've used in Node quite often is Async (https://github.com/caolan/async). Last I checked this also has support for the browser so you should be able to npm / concat / minify this in your distribution. If you're using this on server-side only you should consider https://github.com/continuationlabs/insync, which is a slightly improved version of Async, with some of the browser support removed.

One of the common patterns I use when using conditional async calls is populate an array with the functions I want to use in order and pass that to async.waterfall.

I've included an example below.

var tasks = [];

if (conditionOne) {
    tasks.push(functionOne);
}

if (conditionTwo) {
    tasks.push(functionTwo);
}

if (conditionThree) {
   tasks.push(functionThree);
}

async.waterfall(tasks, function (err, result) {
    // do something with the result.
    // if any functions in the task throws an error, this function is 
    // immediately called with err == <that error>
});

var functionOne = function(callback) {
    // do something
    // callback(null, some_result);
};

var functionTwo = function(previousResult, callback) {
    // do something with previous result if needed
    // callback(null, previousResult, some_result);
};

var functionThree = function(previousResult, callback) {
    // do something with previous result if needed
    // callback(null, some_result);
};

Of course you could use promises instead. In either case I like to avoid nesting callbacks by using async or promises.

Some of the things you can avoid by NOT using nested callbacks are variable collision, hoisting bugs, "marching" to the right > > > >, hard to read code, etc.

edited Oct 20, 2015 at 14:32

answered Oct 20, 2015 at 12:47

ambrons

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1 Comment

zedd45 Over a year ago

FWIW, Async is the alternative to the first anti-pattern covered here: webapplog.com/seven-things-you-should-stop-doing-with-node-js

Yanick Rochon · Accepted Answer · 2012-10-27 03:08:58Z

3

Just declare some other function to be run whenever you need it :

var otherFunc = function() {
   //do this next whether condition is true or not
}

if(condition) {
    someAsyncRequest(function(error, result)) {
        //do something then continue

        otherFunc();
    }
} else {
    otherFunc();
}

answered Oct 27, 2012 at 3:08

Yanick Rochon

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2 Comments

cyberwombat Over a year ago

yes that's what I have now as I mentioned - just wondering if there was a way that avoided encapsulating the code in a function

Yanick Rochon Over a year ago

@Yashua, no because since your someAsyncRequest is asynchronous, you cannot guarantee when it will be executed, and since JavaScript is not inherently multi threaded, you cannot just "pause" some part of your program until some async call has been made. Thus, you need to protect the part of the code to execute later inside a function and call it at the appropriate time (ie, right away, or only when the async code has been executed).

mikkom · Accepted Answer · 2015-02-04 12:24:15Z

3

Just an another way to do it, this is how I abstracted the pattern. There might be some libraries (promises?) that handle the same thing.

function conditional(condition, conditional_fun, callback) {
    if(condition)
        return conditional_fun(callback);
    return callback();
}

And then at the code you can write

conditional(something === undefined,
            function(callback) {
               fetch_that_something_async(function() {
                  callback();
               });
            },
            function() {

                       /// ... This is where your code would continue


             });

answered Feb 4, 2015 at 12:24

mikkom

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Comments

Asher · Accepted Answer · 2015-08-27 20:26:55Z

0

I would recommend using clojurescript which has an awesome core-async library which makes life super easy when dealing with async calls.

In your case, you would write something like this:

(go
  (when condition
    (<! (someAsyncRequest)))
  (otherCodeToHappenWhetherConditionIsTrueOrNot))

Note the go macro which will cause the body to run asynchronously, and the <! function which will block until the async function will return. Due to the <! function being inside the when condition, it will only block if the condition is true.

edited Aug 27, 2015 at 20:26

answered Aug 27, 2015 at 13:38

Asher

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1 Comment

Asher Over a year ago

@kleopatra Please view my edits to the answer. I hope this is better now.

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