docs: function(filepath, callback) {
      jsonStr = this.parseFile(filepath);
      if(!jsonStr) {  
        process.stdout.write(filepath+' is not exist');
        throw new Error('file not exist');
        process.exit(1);
      }
      try {
        var sentences = jsonStr.sentences;
        var options = jsonStr.options;
        options.callback = callback;
        this.speak(sentences, options);
        return true;
      } catch(e) {
        throw new Error('error while speaking');
      }
    }