anchor.diff:  const registryB = asciidoctor.Extensions.create();
   
   registryA.block(function () {
  -  var self = this;
  +  const self = this;
     self.named('shout');
     self.onContext('paragraph');
     self.process(function (parent, reader) {
  
positive.code: 
  [source,javascript]
  ----
  var asciidoctor = require('asciidoctor.js')();
  asciidoctor.Extensions.register(function () {
    this.block(function () {
      var self = this;
      self.named('shout');
      self.onContext('paragraph');
      self.process(function (parent, reader) {
        var lines = reader.getLines().map(function (l) { return l.toUpperCase(); });
        return self.createBlock(parent, 'paragraph', lines);
      });
    });
  
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'
  
  