Simple nodejs callback example with fs.readFile

Question

I'm trying to learn async programming and was struggling with lesson 4 of nodeschool.io with the implementation of an async io with callbacks.

Basically, I'm trying to use fs.readFile to count the number of newlines within a file using a callback.

Here's my code:

var fs = require('fs');
var pathToFile = process.argv[2];

function counter(callback) {
    var buffer = fs.readFile(pathToFile, function (err, data) {
    var bufferString = buffer.toString();
    var bufferStringSplit = bufferString.split('\n');
  });
  callback();
}

function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);

I understand that callbacks are executed once the line of code is finished executing, so shouldn't the

var bufferString = buffer.toString();
var bufferStringSplit = bufferString.split('\n');

be called after fs.readFile() finishes reading the file from disk?

Then finally the callback() calls logMyNumber, which should just output the number of lines the file has.

Answer 1

You have several issues going on and I'll try to outline them all as best as possible

Problem 1: Variable scope

var fs = require('fs');
var pathToFile = process.argv[2];

function counter(callback) {
  var buffer = fs.readFile(pathToFile, function (err, data) { 
    // Both of the following variables are scoped to the callback of fs.readFile
    var bufferString = buffer.toString(); 
    var bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

function logMyNumber() {
  // Because the variables are in a closure above, bufferStringSplit is null here
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);

Solution:

Declare the variables in the module's scope:

var fs = require('fs');
var pathToFile = process.argv[2];

// These can now be accessed from anywhere within the module
var bufferString, bufferStringSplit;

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
    callback();
  });
}

// bufferStringSplit should no longer return null here
function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

Problem 2: Callbacks

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = buffer.toString(); 
    bufferStringSplit = bufferString.split('\n'); 

    // Place the callback WITHIN the other callback, otherwise they run in parallel
    callback();
  });
}

Problem 3: fs.readFile API

fs.readFile doesn't return anything, so your buffer variable below is null

function counter(callback) {      
  var buffer = fs.readFile(pathToFile, function (err, data) {
    bufferString = buffer.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

Solution:

function counter(callback) {      
  fs.readFile(pathToFile, function (err, data) {
    // The data argument of the fs.readFile callback is the data buffer
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

Finally, the code should look like:

var fs = require('fs');
var pathToFile = process.argv[2];

var bufferString, bufferStringSplit;

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
    callback();
  });
}

function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);