// Get the container element

var btnContainer = document.getElementById("myDIV");

​

// Get all buttons with class="btn" inside the container

var btns = btnContainer.getElementsByClassName("btn");

​

// Loop through the buttons and add the active class to the current/clicked button

for (var i = 0; i < btns.length; i++) {

  btns[i].addEventListener("click", function() {

    var current = document.getElementsByClassName("active");

    current[0].className = current[0].className.replace(" active", "");

    this.className += " active";

  });

}