Implement document.getElementsByClassName from scratch.

getElementsByClassName.js

var getElementsByClassName = function(className) {
  return getNodesWithClassName(document.childNodes, className);
};

var getNodesWithClassName = function(nodes, className) {
  if (nodes.length === 0) return [];
  return (hasClassOf(nodes[0], className) ? [nodes[0]] : [])
          .concat(getNodesWithClassName(nodes[0].childNodes, className))
          .concat(getNodesWithClassName([...nodes].slice(1), className));
};

var hasClassOf = function(element, className) {
  return element.classList && element.classList.contains(className);
};