A demo of substring with user selected start and end index

JavaScript / HTML/ CSS Code
Output
<!DOCTYPE html>
<html>
<head>


<style>
.substrdiv {
  background: #5AB65A;
  height: auto;
  width:300px;
  border-radius: 15px;
  padding:20px;
  font-size:16px;
  color:#fff;  
}
#substring {
    background: #D26900;
    font-size:20px;
}
.paracls {
    background: #D26900;
}
.btncls{
    background-color:#4B4FC5;
    border: #2e6da4;
  	font-family: Arial, Geneva, Arial, Helvetica,  sans-serif;
	font-size: 15px;
    color: #FFFFFF;
    letter-spacing: 1px;
    padding: 8px 12px;
    font-size: 14px;
    font-weight: normal;

    border-radius: 4px;
    line-height: 1.5;
    text-decoration:none;
} 
</style>
</head>

<body>
<div class="substrdiv">
<p><label>Select Start index: </label><select id="st_index">
    <option value="0">0</option>
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
    <option value="6">6</option>
</select></p>
<p><label>Select End index: </label><select id="end_index">
    <option value="11">11</option>
    <option value="12">12</option>
    <option value="13">13</option>
    <option value="14">14</option>
    <option value="15">15</option>
    <option value="16">16</option>
    <option value="20">20</option>
    <option value="25">25</option>
</select></p>

Main String: "The JavaScript substring tutorial!" 
<p><strong>After substring method:</strong> <span id="substring"></span></p>

</div>

<p><button class="btncls" onclick="getsubstring()">Execute substring method</button></p>
<script type="text/javascript">
function getsubstring(){
    var start_index = document.getElementById("st_index").value;;
    var end_index = document.getElementById("end_index").value;;    
    var source_string = ("The JavaScript substring tutorial!");
    var str_sub = source_string.substring(start_index,end_index);
    
    document.getElementById("substring").innerHTML = str_sub;
}
</script>
</body>
</html>
Select Start index:
Select End index:
Main String: "The JavaScript substring tutorial!"
After substring method: