Node spawn multiple shell process issues

Question

I have developed a module to execute a generic command using the child_process spawn. I have two api in this module a single command execution api, CommandLine.execute and a multiple commands execution api, CommandLine.executeCommands, that I call in this way:

// commandItemsArray is a list of commands list
// that is an array of [command, options, arguments]
  commandItemsArray = [ ['ls','-l','./'], ['ls','-a','./'] ];
  command.executeCommands( commandItemsArray
    , function(results) {
      console.log( results );
    }
    , function(error) {
      console.log( error );
    });

or

commandItemsArray = [ ['ls','-l','./'] ];
    command.executeCommands( commandItemsArray
      , function(results) {
        console.log( results );
      }
      , function(error) {
        console.log( error );
      });

This module is standalone (relies only on node built-in modules) and that's the code:

(function() {

  var CommandLine;
  CommandLine = (function() {

    var cp = require('child_process');

    /**
     * Command line helper module
     * This module is standalone
     * @author: loretoparisi at gmail dot com
     */
    function CommandLine(options,logger) {
      var self=this;

      // defaults
      this._options = {
        // true to debug
        debug : false,
        // true to show errors
        error : true
      };

      // override defaults
      for (var attrname in options) { this._options[attrname] = options[attrname]; }

      /**
       * Wrappar around Promise.all
       */
      this.PromiseAll = function(items, block, done, fail) {
          var self = this;
          var promises = [],
              index = 0;
          items.forEach(function(item) {
              promises.push(function(item, i) {
                  return new Promise(function(resolve, reject) {
                      if (block) {
                          block.apply(this, [item, index, resolve, reject]);
                      }
                  });
              }(item, ++index))
          });
          Promise.all(promises).then(function AcceptHandler(results) {
              if (done) done(results);
          }, function ErrorHandler(error) {
              if (fail) fail(error);
          });
      }; //PromiseAll

    }

    /**
     * Execute a command
     * @param commands Array of command, options, arguments
     * @example ['ls','-l','./']
     * @param resolve Block success block
     * @param reject Block rejection block
     */
    CommandLine.prototype.execute = function(command, resolve, reject) {
      var self=this;
      resolve = resolve || function(results) {};
      reject = reject || function(error) {};
      return self.ExecutionBlock(item, index, resolve, reject);
    } //execute

    /**
     * Execute a list of commands
     * @param commands Array of command array of of command, options, arguments
     * @example [ ['ls','-l','./'], ['ls', '-a', './'] ]
     * @param resolve Block success block
     * @param reject Block rejection block
     */
    CommandLine.prototype.executeCommands = function(commands, resolve, reject) {
      var self=this;
      resolve = resolve || function(results) {};
      reject = reject || function(error) {};

      /**
       * Execution block handler
       */
      var ExecutionBlock = function(item, index, resolve, reject) {

        var executable = item[0]; // first elem is command
        var options = item.splice(1,item.length);

        if(self._options.debug) {
          console.log( item );
          console.log( executable, options.join(' ') );
        }

        var data = new Buffer("",'utf-8');
        // LP: now spawn the power!
        var child = cp.spawn(executable, options);
        // Listen for an exit event:
        child.on('exit', function(exitCode) {
          return resolve( { data : data.toString('utf-8'), code : exitCode } );
        });
        // Listen for stdout data
        child.stdout.on('data', function(_data) {
            console.log( ( new Buffer(_data)).toString() );
            data = Buffer.concat([data, _data]);
        });
        // child error
        child.stderr.on('data',
            function(data) {
              if(self._options.error) {
                console.log('err data: ' + data);
              }
              // on error, kill this child
              child.kill();
              return reject(new Error(data.toString()));
            }
        );

      } //ExecutionBlock

      self.PromiseAll(commands
        , function(item, index, _resolve, _reject) {
          ExecutionBlock(item, index, _resolve, _reject);
        }
        , function(results) { // aggregated results
          // all execution done here. The process exitCodes will be returned
            // array index is the index of the processed that exited
            return resolve(results);
          }
        , function(error) { // error
            return reject(error);
        });

    } //executeCommands

    return CommandLine;

  })();

  module.exports = CommandLine;

}).call(this);

I'm using a Promise.all to spawn multiple processes, wait execution, and collect stdout output in a node Buffer like:

    // Listen for stdout data
    child.stdout.on('data', function(_data) {
        console.log( ( new Buffer(_data)).toString() );
        data = Buffer.concat([data, _data]);
    });

At the very exit child event I return the child process exit code and data in a struct:

    child.on('exit', function(exitCode) {
      return resolve( { data : data.toString('utf-8'), code : exitCode } );
    });

While, executing some commands has no problem, executing shell commands like ls give me back unexpected results such as I do not get the results expected to be in the callback of PromiseAll method, and I do not understand why.

self.PromiseAll(commands
        , function(item, index, _resolve, _reject) {
          ExecutionBlock(item, index, _resolve, _reject);
        }
        , function(results) { // aggregated results
          // all execution done here. The process exitCodes will be returned
            // array index is the index of the processed that exited
            return resolve(results);
          }
        , function(error) { // error
            return reject(error);
        }); 

One doubt is about the node Buffer concatenation explained here, that seems to be tricky:

    var data = new Buffer("",'utf-8');
    // LP: now spawn the power!
    var child = cp.spawn(executable, options);
    // Listen for an exit event:
    child.on('exit', function(exitCode) {
      return resolve( { data : data.toString('utf-8'), code : exitCode } );
    });
    // Listen for stdout data
    child.stdout.on('data', function(_data) {
        console.log( ( new Buffer(_data)).toString() );
        data = Buffer.concat([data, _data]);
    });

I will expect { data : data.toString('utf-8')} to keep the whole buffer chained as string, but I wonder if the utf-8 is the right encoding for the stdout output, supposed that we are talking about of shell commands of course.