anchor.diff:  
   [source,javascript]
   ----
  -var asciidoctor = require('asciidoctor.js')();
  +const asciidoctor = require('asciidoctor.js')();
   asciidoctor.Extensions.register(function () {
     this.block(function () {
  -    var self = this;
  +    const self = this;
       self.named('shout');
       self.onContext('paragraph');
       self.process(function (parent, reader) {
  -      var lines = reader.getLines().map(function (l) { return l.toUpperCase(); });
  +      const lines = reader.getLines().map(function (l) { return l.toUpperCase(); });
         return self.createBlock(parent, 'paragraph', lines);
       });
     });
  
positive.code: const registryB = asciidoctor.Extensions.create();
  
  registryA.block(function () {
    var self = this;
    self.named('shout');
    self.onContext('paragraph');
    self.process(function (parent, reader) {
  
negative.code: 
  [source,javascript]
  ----
  const content = '== Title';
  const doc = asciidoctor.load(content);
  console.log(doc.getAttribute('data-uri')); // prints undefined
  console.log(doc.getAttribute('data-uri', 'false')); // prints 'false'
  
  