code: function tryCmd(flow, callback){
    var command = flow.options.jsPackCmd;
    var processor = {
      name: 'uglify-js',
      process: uglifyProcess
    };
  
    if (!command)
    {
      process.nextTick(function(){
        callback(processor);
      });
      return;
    }
  
    // check command is works
    exec(
      'echo "" | ' + command,
      function(error){
        // if command doesn't work
        if (error)
        {
          if (flow.options.jsPackCmd)
            flow.exit('Pack command: `' + flow.options.jsPackCmd + '`\n' + error);
  
          flow.warn({ message: '[WARN] `google-closure-compiler` is not available, downgrade to uglify.js' });
        }
        else
        {
          processor = {
            name: flow.options.jsPackCmd || 'google-closure-compiler',
            process: cmdProcess
          };
        }
  
        callback(processor);
      }
    );
  }
docs: choose processor