Why am I getting undefined error? : <p>the syntax seems to be correct, I dont get it, here is 
error: app.js:73 Uncaught ReferenceError: getBirthyear is not defined
Code:  </p>

<pre><code>const person = {
  name: 'Tommy',
  age: 32,
  location: {
    state: 'Missouri',
    city: 'louisisana',
    street: '1 marcia drive',
    job: 'web dev'
  },
   getBirthyear: function() {
    return 2018 - this.age;
  }
}
let val;
val = getBirthyear();
document.write(val);
</code></pre>
