title: nested key with json.stringify replacer function
body: <p>I have the following code:</p>
  
  <pre><code>const result = JSON.stringify({
    drivingLicence: {
      documentNumber: '3333',
      countryOfIssue: 'UNITED_KINGDOM',
      regionOfIssue: 'UK'
    },
    mothersMaidenName: 'AAAAA',
    nationalIdentityCard: {},
    nationalInsuranceNumber: 'NW 26 52 66 A',
    passport: {}
  },(key, value) =&gt;{
    console.log(typeof key + ' ' + key);
    return value;
  });
  </code></pre>
  
  <p>I get this output:</p>
  
  <pre><code> console.log src/components/Submission/submissionTransformer.test.js:31
    string
  console.log src/components/Submission/submissionTransformer.test.js:31
    string drivingLicence
  console.log src/components/Submission/submissionTransformer.test.js:31
    string documentNumber
  console.log src/components/Submission/submissionTransformer.test.js:31
    string countryOfIssue
  console.log src/components/Submission/submissionTransformer.test.js:31
    string regionOfIssue
  console.log src/components/Submission/submissionTransformer.test.js:31
    string mothersMaidenName
  console.log src/components/Submission/submissionTransformer.test.js:31
    string nationalIdentityCard
  console.log src/components/Submission/submissionTransformer.test.js:31
    string nationalInsuranceNumber
  console.log src/components/Submission/submissionTransformer.test.js:31
    string passport
  </code></pre>
  
  <p>I do not understand where the first output is coming from.</p>
  
  <p>It says the key is an empty string.  Where is this coming from?</p>