Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / node_modules / underscore.string / helper / adjacent.js

var makeString = require('./makeString');

module.exports = function adjacent(str, direction) {
  str = makeString(str);
  if (str.length === 0) {
    return '';
  }
  return str.slice(0, -1) + String.fromCharCode(str.charCodeAt(str.length - 1) + direction);
};