three.js/utils/docs/template/static/scripts/linenumber.js

/*global document */
( () => {

	const source = document.getElementsByClassName( 'prettyprint source linenums' );
	let i = 0;
	let lineNumber = 0;
	let lineId;
	let lines;
	let totalLines;
	let anchorHash;

	if ( source && source[ 0 ] ) {

		anchorHash = document.location.hash.substring( 1 );
		lines = source[ 0 ].getElementsByTagName( 'li' );
		totalLines = lines.length;

		for ( ; i < totalLines; i ++ ) {

			lineNumber ++;
			lineId = `line${lineNumber}`;
			lines[ i ].id = lineId;
			if ( lineId === anchorHash ) {

				lines[ i ].className += ' selected';

			}

		}

	}

} )();