code: function language(node) {
    var className = node.properties.className || []
    var length = className.length
    var index = -1
    var value
  
    while (++index < length) {
      value = className[index]
  
      if (value === 'no-highlight' || value === 'nohighlight') {
        return false
      }
  
      if (value.slice(0, 5) === 'lang-') {
        return value.slice(5)
      }
  
      if (value.slice(0, 9) === 'language-') {
        return value.slice(9)
      }
    }
  
    return null
  }
docs: Get the programming language of `node`.