Practicing recursion with getElementsByClassName I'm trying to get a good grasp on recursion by implementing document.getElementsByClassName. I've made something that works but I'd like to know if there is some blatant refactoring that should be done or perhaps some not-so-edge cases that might break it.

var getElementsByClassName = function(className){
  var result = [];
  function diveIn(motherNode, result, className){
    var maNodes = motherNode.childNodes
    for (var i = 0; i &lt; maNodes.length; i++){
        var classes = maNodes[i].classList || [];
        classes.forEach(function(classy){
            if (classy === className){
                result.push(maNodes[i]);
            }
        })
        if (maNodes[i].childNodes[0]){
            diveIn(maNodes[i], result, className);
        }
    }
  }
  diveIn(document, result, className);
  return result;
};
Apart from what cssGEEK pointed out, your code cannot work at all, since maNodes[i].classList doesn't return an Array but a DOMTokenList, which has no forEach() method.

So I'm surprised you said it works: it can happen only when no class is defined for an element (then your || [] makes classes to be an Array), but for each element having class(es) it fires an error "classes.forEach is not a function".

Here is a slighthly different code to achieve what you planned using the same way:

var getElementsByClassName = function(className){
  var result = [];
  function diveIn(motherNode, result, className){
    var maNodes = motherNode.childNodes
    for (var i = 0; i &lt; maNodes.length; i++){
        var classes = maNodes[i].classList;
        if (classes &amp;&amp; classes.contains(className)) {
            result.push(maNodes[i]);
        }
        if (maNodes[i].childNodes[0]){
            diveIn(maNodes[i], result, className);
        }
    }
  }
  diveIn(document, result, className);
  return result;
};


You can look at it working here.

But again don't forget that depending of how heavy is the HTML document it may encounter a max call stack size condition.