Practicing recursion with getElementsByClassName

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 < maNodes.length; i++){
        var classes = maNodes[i].classList;
        if (classes && 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.