Demo
<iframe height='265' scrolling='no' title='Distance Converter' src='//codepen.io/fastfoodcoding/embed/wqeEMa/?height=265&theme-id=light&default-tab=result&embed-version=2' frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>See the Pen <a href='https://codepen.io/fastfoodcoding/pen/wqeEMa/'>Distance Converter</a> by fastfoodcoding (<a href='https://codepen.io/fastfoodcoding'>@fastfoodcoding</a>) on <a href='https://codepen.io'>CodePen</a>.
</iframe>
Code
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$(document).ready(function() {
$("#btn").click(function() {
console.log("after click");
var from = $("#cfrom").val();
console.log("selected from value is " + from);
var to = $("#cto").val();
console.log("selected to value is " + to);
var input = $("#value").val();
console.log("selected input value is " + input);
var output = -1;
if (from == to) {
alert("check your from & to values");
}
if (from == 'Meters' && to == 'Kilometers') {
output = (+input) / 1000;
} else if (from == 'Kilometers' && to == 'Meters') {
output = ((+input) * (1000))
} else if (from == 'Miles' && to == 'Kilometers') {
output = ((+input) * (1.6))
} else if (from == 'Kilometers' && to == 'Miles') {
output = ((+input) / (1.6))
} else if (from == 'Millimeters' && to == 'Centemeters') {
output = ((+input) / (10))
} else if (from == 'Centemeters' && to == 'Millimeters') {
output = ((+input) * (10))
} else if (from == 'Centemeters' && to == 'Meters') {
output = ((+input) / (100))
} else if (from == 'Meters' && to == 'Centemeters') {
output = ((+input) * (100))
}
// 2 kilometers = 2000 meters
output = input + " " + from + " = " + output + " " + to
console.log("output value is" + output);
$("#result").html(output);
});
});
</script>
</head>
<body>
Convert From :
<select id="cfrom">
<option selected="selected"> Meters </option>
<option> Kilometers </option>
<option> Inches </option>
<option> Miles </option>
<option> Millimeters </option>
<option> Centemeters </option>
</select><br><br> Convert To :
<select id="cto">
<option selected="selected"> Meters </option>
<option> Kilometers </option>
<option> Inches </option>
<option> Miles </option>
<option> Millimeters </option>
<option> Centemeters </option>
</select><br><br> Value:
<input type="text" name="number" id="value"></input><br><br>
<button id="btn"> Convert </button>
<p id="result"></p>
</body>
</html>