在node.js中异步readFile module.exports

Question

I'm sorry for, what might easily be a naive question, but I`m trying to figure out how node works, especially for a problem like this:

What I need do is to send an object/file from fs.readFile through require and module.exports. This is what I have tried is this

in one file (call it app.js) the code for reading a file:

var fs = require('fs');
var file_contents = undefined;

var callback_reader = function(err, data) {
  if (err) return console.error(err);
  file_contents = data.toString().split('\n');
}

module.exports = {
  parseFile: function(file_path) {
    fs.readFile(file_path.toString(), 'utf-8', callback_reader);
  }
} 

and in some other file, (call it main.js) I need to use the contents of the file read by the readFile like this

var file_importer = require('./app.js')
file_importer.parseFile(real_path_to_file);

but if i try console.log of this last line I always get undefined object. Now I know it is because callback does not execute before the console.log but I`m unsure how to achieve this communication.

Answer 1

So i changed your code a little bit to use callbacks. It seems that you can't use "return" from asyncronous function in module.exports. However, the code bellow works as expected. Hope it helps.

main.js

var file_importer = require('./app.js')
file_importer.parseFile('./time.js', function(err, data){
    if(err) return console.log(err);
    console.log(data);
});

app.js

var fs = require('fs');

module.exports = {
    parseFile: function(file_path, callback) {
        fs.readFile(file_path.toString(), 'utf-8', function(err, data) {
            if (err) return callback(err);
            callback(null, data);
        });
    }
}

// much shorter version
exports.parseFile = function(file_path, callback) {
    fs.readFile(file_path.toString(), 'utf-8', callback);
}

Answer 2

This is javascript work, it don't wait the callback was called to return. You should do your console.log in your callback. Like these :

fs.readFile(pathToFile, 'utf-8', function(err, data) {
  if (err) return err;
  console.log(data);
  // Continue your process here
})