title: get json field from string identifier
body: <p>Suppose I have a json that looks like this:</p>
  
  <pre><code>let jsonExample = {
      "name": "foo",
      "age": 43,
      "number" : 13,
      "id": "osaid9r"
  }
  </code></pre>
  
  <p>My goal is to create a javascript function <code>getField(json, fieldName)</code> that receives a json and a string representing the field name and returns the value represented by <code>fieldName</code>.</p>
  
  <pre><code>getField(jsonExample, "name") = "foo"
  getField(jsonExample, "number") = 13
  </code></pre>
  
  <p>How can I write such function?</p>