Node.js - Javascript - How to pass variables to a callback function

Question

I am working with Node.js streams and end up with something like this in several places:

var myVariable = 'XXX';
...
.on('end', function() {
    // do something with myVariable
});

The behaviour of the function is the same but depends on a variable so I would like to refactor it to:

var myVariable = 'XXX';
...
.on('end', myFunction);


function myFunction(param){
    // do something with param
}

How can I pass the value of myVariable in the call .on('end', myFunction);?

It is not clear what you actually want. Do you want to pass the variable as a parameter to the function, apart from the value given by the end event? — thefourtheye
– thefourtheye, Commented Jul 10, 2015 at 12:36

JJJ · Accepted Answer · 2015-07-10 12:33:14Z

1

You would make a factory that returns customized functions to be used as the callback.

function myFactory(param) {
    return function(e) {
        console.log( 'param value:', param );
        // do something
    }
}

You would use it like this:

.on('end', myFactory(1));

.on('end', myFactory(2));

If the "end" event passes some parameters to the callback other than or in addition to the event object, add them to the returned function's parameter list (return function(e, foo, bar) { ...)

answered Jul 10, 2015 at 12:33

JJJ

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3 Comments

Cerbrus Over a year ago

I was thinking of answering something like this. +1

codependent Over a year ago

Thanks Juhana, I should've figured out myself!

stdob-- Over a year ago

@Juhana Maybe it would be better if the factory will separate functions, which at the input will accept parameters and a reference to a callback function?

Stainz · Accepted Answer · 2015-07-10 12:53:23Z

1

your callback function should be defined like this way:

function myCallback(myParam) {
  // do some stuff
  return myReturn;
}

when you declare your listener, the "on" method, should take the name of the event, and the reference of your method callback:

.on('eventName', myCallback);

and when you emit the event, there is where you pass params:

emit('eventName', myParam);

then in your callback, you can retrieve params

answered Jul 10, 2015 at 12:53

Stainz

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