即使原型方法中正确，JavaScript原型返回值也未定义

Question

I'm trying to create a simple method to convert xml to json. I'm trying to do it using javascript's prototype because i'm trying to learn how to use prototype and because I want to be able to do something like myXmlString.toJson();

I have the script below which demonstrates that the xml string gets converted to a json object just fine inside the method, but when i try to call the method it is undefined.

var parseString = require('xml2js').parseString;  // Used to parse the returned xml (ick) to json (yum)

String.prototype.toJson = function() {
  parseString(this, {explicitArray: false}, function (err, jsonResult) {

    if(err) {
      console.log('err');
      console.log(err);
      return err;
    }

    console.log('inside toJSON');
    console.log(jsonResult); // <-- Prints a json object as expected

    return jsonResult;
  });
}

var mystring = '<parent><child>3</child></parent>';
console.log(mystring); // Prints an xml string

mystring.toJson(); // Should this convert the variable mystring to a json structure?
console.log(mystring); // Prints the xml string
console.log(mystring.toJson()); // Prints undefined. Why?

Answer 1

Inside your toJson method, you should return the value parseString returns. At the moment, you just execute parseString without returning the value it returns.