wp-content/themes/twentyfifteen/functions.php:414JavaScript Detection.
function twentyfifteen_javascript_detection() { echo "<script>(function(html){html.className = html.className.replace(/\bno-js\b/,'js')})(document.documentElement);</script>\n";}Introduced in Twenty Fifteen 1.1. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-content/themes/twentyfifteen/functions.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.