var myBtn = document.getElementById('my-button');

var tipElem = document.getElementById("tip");

var totalElem = document.getElementById("total");

​

myBtn.addEventListener('click', calc);

​

function calc(e) {

  e.preventDefault();

  var input = document.getElementById('bill');

  var bill = parseFloat(input.value);

  var tip = bill * 0.15;

  var total = bill + tip;

  tipElem.textContent = "$"+(tip).toFixed(2);

  totalElem.textContent = "$"+(total).toFixed(2);  

}