title: Performing basic math on an object project but value returns NaN
body: <p>I'm currently learning about JS objects and trying to perform basic subtraction on a property that is then outputted via a method. However, the object method returns value of NaN even though <code>typeof</code> tells me <code>currentAge</code> and <code>currentYear</code>are numbers. What am I doing wrong and How can I get my below method to output the correct number?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var today = new Date();
  var year = today.getFullYear();
  
  var Sam = {
    age: 27, //Number
    birthMonth: "March", //March
    currentYear: year, //number
    birthYear: this.currentYear - this.age,
  
    // Method to say birth year
    sayBirthYear: function(){
      console.log("I was born in the year " + this.birthYear );     
      //return NANS
    }
  }
  
  Sam.sayBirthYear(); // Outputs Birth year</code></pre>
  </div>
  </div>
  </p>