CODEWITHSUNDEEP
node.js
cathy.sasaki
cathy.sasaki

Reputation: 4025

Why is fs.readdir returning `undefined`?

The below code builds a dictionary (associative array) of the files in a folder, skipping directories and hidden (.*) files. The coe works because the console.log(file_dict) shows the dictionary before being returned. However, when I call the get_files function, the return value from return file_dict is undefined

I'm pretty sure this problem is because readdir is async. So, I need to switch, to readdirSync. BUT conceptually I'm not getting why the returned value in the async version is undefined.

What am I missing?

file-explorer.js

    var fs = require('fs');

    var get_files = function() {
        fs.readdir(__dirname + '/content/', function (err, files) { 
          if (!files.length) {
            return console.log('    \033[31m No files to show\033[39m\n');
          }

          // called for each file walked in the directory
          var file_dict = {};
          var file_index = 0;

          function file(i) {
            var filename = files[i];

            fs.stat(__dirname + '/' + filename, function (err, stat) {

              if (stat.isDirectory() || filename[0] == '.'  ) {
                    // do nothing, skip these dictionaries and dot files
              } else {
                ++file_index;
                file_dict[file_index] = filename;
              };

              if (++i == files.length) {
                            console.log('right before returning');
                console.log(file_dict);
                return file_dict;
              } else {
                // continue getting files
                file(i);
              };
            });
          }
          file(0);
        });
    }


console.log ( get_files() ); // returns undefined

Thanks.

Upvotes: 2

Views: 9007

Answers (1)

Dan D.
Dan D.

Reputation: 74655

Asynchronous functions don't return. They call their callback with their return value.

Consider:

var get_files = function (cb) {
    fs.readdir(__dirname + '/content/', function (err, files) {
        if (!files.length) {
            return console.log('    \033[31m No files to show\033[39m\n');
        }

        // called for each file walked in the directory
        var file_dict = {};
        var file_index = 0;

        function file(i) {
            var filename = files[i];

            fs.stat(__dirname + '/' + filename, function (err, stat) {

                if (stat.isDirectory() || filename[0] == '.') {
                    // do nothing, skip these dictionaries and dot files
                } else {
                    ++file_index;
                    file_dict[file_index] = filename;
                };

                if (++i == files.length) {
                    console.log('right before returning');
                    console.log(file_dict);
                    return cb(file_dict);
                } else {
                    // continue getting files
                    return file(i);
                };
            });
        }
        return file(0);
    });
}


get_files(function (v) { console.log(v); });

Upvotes: 7

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