bmi2.html

<h3>BMI</h3>
 
<label for="height">Height(cm)</label>
<input id="height"></input>
<br/>
 
<label for="weight">Weight(kg)</label>
<input id="weight"></input>
<br/>
 
<button id="calc">Submit</button>
 
<p id="report"></p>

<script>
var calc = document.getElementById('calc');
var report = document.getElementById('report');
var inputHeight = document.getElementById('height');
var inputWeight = document.getElementById('weight');

function calc_bmi() {
  var height = parseFloat(inputHeight.value);
  var weight = parseFloat(inputWeight.value);
  var val = weight / (height * height / 10000);
  return val.toFixed(2);
}

calc.addEventListener('click', function() {
  report.innerHTML = calc_bmi();
});
</script>