getElementsByClassName

getElementsByClassName.js

var getElementsByClassName = function(className) {
var element = arguments[1] || document.body,
    results = [],
    has_class_name = [].indexOf.call(element.classList, className) !== -1,
    children_results = [].map.call(element.children, function(element) {
      return getElementsByClassName(className,element)[0];
    }).filter(Boolean);

if (has_class_name) {
  results.push(element);
} else {
  results = results.concat(children_results);
}

return results;
};