getElementsByClassName.js

function getElementsByClassName(className) {
  var results = [];
  walkTheDOM(document.body, function (node) {
    var a, c = node.className, i;
    if (c) {
      a = c.split(' ');
      for (i = 0; i < a.length; i += 1) {
        if (a[i] ===  className) {
          results.push(node);
          break;
        }
      }
    }
  });
  return results;
}