JavaScript variables hoisting in nodejs/async

Question

In the next example I don't have access to variable "locals" inside the functions "fetcher", "parser" and "saveToDb".

var parser = require('parser.js');
var fetcher = require('fetcher.js');
var saveToDb = require('models/model.js');
var async = require('async');


function task() {
    var locals = []    //<-- declared here
    async.series([
        fetcher,    //<--  can not access "locals"
        parser,     //<--  can not access "locals"
        saveToDb    //<--  can not access "locals"
    ],
            function (err) {
                if (err) return callback(err);
                callback(null);
    });
}

In the next example "local"s is accessible. I just copyed the functions declarations from the requested modules, and pasted them straight inside "async.series".

var async = require('async');

function task() {
    var locals = []    //<-- declared here
    async.series([
        function(callback) {// <-- can access "locals"},  
        function(callback) {// <-- can access "locals"},
        function(callback) {// <-- can access "locals"}
    ],
            function (err) {
                if (err) return callback(err);
                callback(null);
    });
}

While this works - I do want to keep my code modular. How can I fix that ? Or - what I forgot here about the fundamentals of JavaScript ?

Thanks.

It could be said that you are forgetting one thing: That your code won't be very modular if it could access more or less random variables (in this case the locals variable) defined elsewhere. Making something modular usually takes more than just spreading things around files, it's also about making relationships between different parts of your code explicit, for example by passing all data a function needs as arguments. — Myrne Stol
– Myrne Stol, Commented Jun 7, 2013 at 16:29

robertklep · Accepted Answer · 2013-06-07 14:47:43Z

3

In the first example, the callbacks live in another scope so can't access locals.

You could create partial functions that get the locals variable passed as first argument, but that would require you to rewrite your callbacks.

// creating a partial
async.series([
  fetcher.bind(fetcher, locals),
  parser.bind(parser, locals),
  saveToDb.bind(saveToDb, locals)
], ...)

// new function signatures
function fetcher (locals, callback) { ... }
function parser  (locals, callback) { ... }
function saveToDb(locals, callback) { ... }

edited Jun 7, 2013 at 14:47

answered Jun 7, 2013 at 14:42

robertklep

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14 Comments

Daniel Over a year ago

Why the bind? I guess that because I don't use "this" inside my specific callback - it does not really matter - but I do want to understand why you added the bind. Thanks robert.

robertklep Over a year ago

bind serves two purposes: determine what this is in the function, but also to create partial functions which have pre-specified arguments 'attached' to them (see the MDN explanation, and here a simple gist to demonstrate the principle).

Daniel Over a year ago

I think I got this part right now, but will that work even if "async.series" takes no arguments except "callback" ?

robertklep Over a year ago

Sure :) I updated my gist to show how you can use it with async.

Myrne Stol Over a year ago

I think the fetcher.bind(fetcher, locals) could better be fetcher.bind(undefined, locals) Specifying the original function as value for this looks weird to me.

|

matehat · Accepted Answer · 2013-06-07 14:40:23Z

2

You can have the 3 mentionned functions defined like this :

function fetcher(locals) {
  return function _fetcher(callback) {
    // this function body will have access to locals
  }
}

and rewrite your code as:

function task() {
    var locals = []    //<-- declared here
    async.series([
        fetcher(locals),    //<--  can now access "locals"
        parser(locals),     //<--  can now access "locals"
        saveToDb(locals)    //<--  can now access "locals"
    ],
            function (err) {
                if (err) return callback(err);
                callback(null);
    });
}

answered Jun 7, 2013 at 14:40

matehat

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1 Comment

Daniel Over a year ago

I'm not sure if "async.series" takes arguments except "callback", I'll check that. Thanks matehat.

mpm · Accepted Answer · 2013-06-07 14:41:20Z

0

IF i understand your problem correctly this is a javascript 101 question.

fetcher can be a factory that creates a concrete fetcher.

var fetcher = function(locals){
 locals=locals||[] // optional , assign a default , whatever ...
 return function(){
   //do some operation with locals.
   doSomething(locals);
 }
}

then in your script

 async.series([
        fetcher(locals),    //<--  will return the callback to use
        parser(locals),     //<--  will return the callback to use
        saveToDb(locals)    //<--  will return the callback to use
    ],

i believe it is a kind of currying.

answered Jun 7, 2013 at 14:41

mpm

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1 Comment

Daniel Over a year ago

"async.series" takes no arguments except "callback" (I think), and also "locals" is my main "data" - I cannot delete it each time. Thanks mpm.

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