https://starling-lang.org/docs/scripts/linenumber.js

1/*global document */
2(() => {
3    const source = document.getElementsByClassName('prettyprint source linenums');
4    let i = 0;
5    let lineNumber = 0;
6    let lineId;
7    let lines;
8    let totalLines;
9    let anchorHash;
10
11    if (source && source[0]) {
12        anchorHash = document.location.hash.substring(1);
13        lines = source[0].getElementsByTagName('li');
14        totalLines = lines.length;
15
16        for (; i < totalLines; i++) {
17            lineNumber++;
18            lineId = `line${lineNumber}`;
19            lines[i].id = lineId;
20            if (lineId === anchorHash) {
21                lines[i].className += ' selected';
22            }
23        }
24    }
25})();