How do I get access to a variable without nested callback?

Question

I have a simple nested callback utilizing the Node FileSystem module and I'm having a hard time trying to get access to a variable that I feel is only made available due to the scope chain. My goal is to reduce nested callbacks as much as possible.

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, function(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, function(err, data) {
      // I have access to filePath variable here.  This works just fine.
      console.log(filePath);
    });
  });
});

But this is what I would like to write instead:

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, processFiles);

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile);
  });
}

function processSingleFile(err, data) {
  // how do I get the filePath variable here?
  console.log(filePath);
}

How do I get the filePath variable here in the second example?

just use console.log(data); in your processSingleFile function. :) — AdityaParab
– AdityaParab, Commented Sep 16, 2015 at 4:29
Use promises to reduce nested callbacks and chain the request — Prashant
– Prashant, Commented Sep 16, 2015 at 4:31
@AdityaParab, data is the contents of the file, not the name of the file. — Kevin
– Kevin, Commented Sep 16, 2015 at 4:34

Raymond Chen · Accepted Answer · 2015-09-16 04:33:48Z

4

You can pass filePath as the first parameter to processSingleFile by binding it.

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile.bind(this, filePath));
  });
}

function processSingleFile(filePath, err, data) {
  console.log(filePath);
}

answered Sep 16, 2015 at 4:33

Raymond Chen

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Comments

Mritunjay · Accepted Answer · 2015-09-16 04:33:14Z

1

Change processSingleFile to a function returning function and pass variable filePath as a variable to that. Like bellow

function processSingleFile(filePath) {
  return function(err, data){
    console.log(filePath);
    console.log(data);
  }
}

call it like bellow

fs.readFile(filePath, processSingleFile(filePath));

answered Sep 16, 2015 at 4:33

Mritunjay

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4 Comments

Raymond Chen Over a year ago

That's a nested callback, you just nested it in a different function.

Mritunjay Over a year ago

@RaymondChen I don't think this nesting is bad. It's just a wrapper.

Patrick Roberts Over a year ago

@RaymondChen technically the solution you accepted which binds a function is just another way of scoping it. What happens internally is it becomes a wrapper function to the original function and passes the arguments differently. So any solution you use will have some sort of scoping.

Raymond Chen Over a year ago

@PatrickRoberts Of course it's scoping. But binding is scoping without using a nested function (because for some reason the OP doesn't like nested functions).

user1496463 · Accepted Answer · 2015-09-16 04:33:31Z

0

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, processFiles);

// declaring filePath outside to allow scope access
var filePath;

function processFiles(err, files) {
  files.forEach(function(file) {
    filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile);
  });
}

function processSingleFile(err, data) {
  // You can access filePath here
  console.log(filePath);
}

answered Sep 16, 2015 at 4:33

user1496463

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2 Comments

user1496463 Over a year ago

Mind you, I have just started working with node, so I can not say if this is the right way of doing it

Raymond Chen Over a year ago

That doesn't work once you have more than one file in the directory.

AdityaParab · Accepted Answer · 2015-09-16 04:35:51Z

0

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, function(err,data){
        processSingleFile(data,filePath);
    });
  });
}

function processSingleFile(data, filePath) {
  // You can access filePath here
  console.log(filePath);
}

answered Sep 16, 2015 at 4:35

AdityaParab

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