revision:
The substring() method extracts characters, between two indices (positions), from a string, and returns the substring. The method extracts characters from start to end (exclusive) and does not change the original string.
If "start" is greater than "end", arguments are swapped: (4, 1) = (1, 4). "Start" or "end" values less than 0, are treated as 0.
string.substring(start, end)
Parameters:
start : required. The start position. First character is at index 0.
end : optional. End position (up to, but not including). If omitted: the rest of the string.
<p>substring() extracts a part of a string:</p>
<p id="demo"></p>
<script>
let text = "Hello world!";
let result = text.substring(1, 4);
document.getElementById("demo").innerHTML = result;
</script>
<div>
<p id="sub-1"></p>
<p id="sub-2"></p>
<p id="sub-3"></p>
<p id="sub-4"></p>
<p id="sub-5"></p>
<p id="sub-6"></p>
<p id="sub-7"></p>
</div>
<script>
let text = "Hello world!";
document.getElementById("sub-1").innerHTML = " text : " + text;
let result = text.substring(1, 4);
document.getElementById("sub-2").innerHTML = "substring : " + result;
let result1 = text.substring(2);
document.getElementById("sub-3").innerHTML = "substring : " + result1;
let result2 = text.substring(4, 1);
document.getElementById("sub-4").innerHTML = "substr : " + result2;
let result3 = text.substring(-3);
document.getElementById("sub-5").innerHTML = "substr : " + result3;
let result4 = text.substring(0, 1);
document.getElementById("sub-6").innerHTML = "substr : " + result4;
let result5 = text.substring(text.length -1);
document.getElementById("sub-7").innerHTML = "substr : " + result5;
</script>