src/doc/book/2018-edition/ferris.js

var ferrisTypes = [
  {
    attr: 'does_not_compile',
    title: 'This code does not compile!'
  },
  {
    attr: 'panics',
    title: 'This code panics!'
  },
  {
    attr: 'unsafe',
    title: 'This code block contains unsafe code.'
  },
  {
    attr: 'not_desired_behavior',
    title: 'This code does not produce the desired behavior.'
  }
]

document.addEventListener('DOMContentLoaded', () => {
  for (var ferrisType of ferrisTypes) {
    attachFerrises(ferrisType)
  }
})

function attachFerrises (type) {
  var elements = document.getElementsByClassName(type.attr)

  for (var codeBlock of elements) {
    var lines = codeBlock.textContent.split(/\r|\r\n|\n/).length - 1;

    if (lines >= 4) {
      attachFerris(codeBlock, type)
    }
  }
}

function attachFerris (element, type) {
  var a = document.createElement('a')
  a.setAttribute('href', 'ch00-00-introduction.html#ferris')
  a.setAttribute('target', '_blank')

  var img = document.createElement('img')
  img.setAttribute('src', 'img/ferris/' + type.attr + '.svg')
  img.setAttribute('title', type.title)
  img.className = 'ferris'

  a.appendChild(img)

  element.parentElement.insertBefore(a, element)
}