jQuery selector function in JavaScript

I've reproduced the jQuery selector function in JavaScript.

Here are the two rules that come with this function:

1. You may not use any JavaScript libraries.
2. document.querySelector/document.querySelectorAll may not be used.

var $ = function (selector) {
  var elements = [];
  // get starting index position of id / class
    var classPosition = selector.split('').indexOf(".");
    var idPosition = selector.split('').indexOf("#");
    // split selector into array
    var selectorArray = selector.split("");


    if ( classPosition === -1 && idPosition === -1){
    // simply set equal elements to DOM object with the specific tag name
      elements = document.getElementsByTagName(selector);

    }else if (classPosition > -1 && idPosition === -1 ){
            if (classPosition === 0)  {
                selectorArray.shift();
    // array elements returns DOM object with the desired class name
                elements = document.getElementsByClassName(selectorArray.join(""));
            }else {

    // array elements returns DOM object with the matching class

                var tagElem = selectorArray.slice(0,classPosition).join("");

                var className = selectorArray.slice(classPosition + 1,selectorArray.length).join("");

                var tagElemCollection = document.getElementsByTagName(tagElem);
                // iterate through DOM object check if class match match className
                for (var i = 0; i < tagElemCollection.length; i++) {
                    if (tagElemCollection[i].className != "") {
                        var eachClass = tagElemCollection[i].className.split(" ");

                        for (var e = 0;e < eachClass.length;e++) {
                            if (eachClass[e] === className ) {

                                elements.push(tagElemCollection[i]);
                            }

                        }

                    }
                }
            }

    }else if ( idPosition > -1 && classPosition === -1){
            if (idPosition === 0) {
                // array elements returns DOM object if an id is passed in
                selectorArray.shift();
                elements.push(document.getElementById(selectorArray.join("")));

            }else {

                var tagElem = selectorArray.slice(0,idPosition).join("");

                var idName = selectorArray.slice(idPosition + 1,selectorArray.length).join("");

                var tagElemCollection = document.getElementsByTagName(tagElem);
                // iterate through DOM object check if id match match idName (same logic as before)
                for (var i = 0; i < tagElemCollection.length; i++) {
                    if (tagElemCollection[i].id != "") {
                        var eachId = tagElemCollection[i].id.split(" ");
                        for (var e = 0;e < eachId.length;e++) {
                            if (eachId[e] === idName ) {

                                elements.push(tagElemCollection[i]);
                            }

                        }

                    }
                }
            }

    }else if (idPosition > -1 && classPosition > -1){
        // check if position of id is before position of class
            if (idPosition < classPosition) {
                var selectorIdTag = selectorArray.slice(idPosition,classPosition);
                selectorIdTag.shift();
        // push in elements the DOM object with id specified
                elements.push(document.getElementById(selectorIdTag.join("")));

            }else{
        // if class is before id loop through DOM object with that class and select
        // only the one that has also the id specified.
                var selectorClassTag = selectorArray.slice(classPosition,idPosition);
                var selectorIdTag = selectorArray.slice(idPosition, selectorArray.length);

                selectorClassTag.shift();
                selectorIdTag.shift();

                var ClassMatch = document.getElementsByClassName(selectorClassTag.join(""));

                for (var p=0; p < ClassMatch.length; p++){
                    if (ClassMatch[p].id === selectorIdTag.join("") ){
                        elements.push(ClassMatch[p]);
                    }
                }

            }
        }

  return elements;
}

Here, the function $ takes an argument selector which is a string representing the desired DOM element.

Example

Let's say I have an HTML page that looks like this:

<html>
  <head>
    <script src="Answer.js"></script>
  </head>
  <body onload="test$()">
    <div></div>
    <div id="some_id" class="some_class some_other_class"></div>
    <img id="some_other_id" class="some_class some_other_class"></img>
    <input type="text">
  </body>
</html>

Using the function $ just created, select the element div of class some_other_class:

$('div.some_other_class')

returns an array containing the desired element(s):

 [<div id=​"some_id" class=​"some_class some_other_class">​</div>​]

I'd like to know about the quality and performance of my code. I do feel like I'm doing too many iterations.