JavaScript to implement retractable secondary menu

<!--Pull list-->
<style>
 
    li {
        /*Cancel the li dot*/
        list-style: none;
    }
 
    li span {
        /*Set the background image for the span tag (small plus and minus signs at the beginning, no-repeat, initial position 0 0)*/
        background: url(add.png) no-repeat left center;
        padding-left: 20px;
    }
 
    /*Starting style*/
    li ul{
        height: 0;
        /*Overflow hidden*/
        overflow: hidden;
        /*Add transition effect*/
        transition: all 0.5s;
    }
    /*Expand style*/
    .open{
        background-image: url(minus.png);
    }
    .open+ul{
        height: 70px;
    }
 
 
</style>
 
 
<ul class="tree">
    <li><span class="open">Attendance Management</span>
        <ul>
            <li>Daily Attendance</li>
            <li>Leave Application</li>
            <li>Overtime business trip</li>
        </ul>
    </li>
    <li><span>Information Center</span>
        <ul>
            <li>Daily Attendance</li>
            <li>Leave Application</li>
            <li>Overtime business trip</li>
        </ul>
    </li>
    <li><span>Collaborative Office</span>
        <ul>
            <li>Daily Attendance</li>
            <li>Leave Application</li>
            <li>Overtime business trip</li>
        </ul>
    </li>
</ul>
 
<script src="mJS.js"></script>

// Pull-out list // Implementation ideas:
 
var spans = document.querySelectorAll(".tree span");
 
for (var i = 0; i < spans.length; i++) {
    spans[i].onclick = function () {
        // console.log(this);
        // Find the element span to be modified // Modify, delete or add class -> open
        // Check if there is an open on itself, if so, delete it!
        // this -> points to the element that triggered the event if (this.className == "open") {
            // If there is, delete it!
            this.className = "";
        } else {
            // If not, add it and delete the others that are already opened!
            //Find the open (li of the open attribute)
            var openSpan = document.querySelector(".tree.open")
            if (openSpan !== null) {
                // If the collection is not empty, delete their open attribute openSpan.className = "";
            }
            // Set the open attribute for yourself this.className = "open";
        }
 
 
    }
}