add_noindex.js

// update existing robots tag if one is there 
const oldRobots = document.querySelector("[name='robots']");
if (oldRobots) {
    oldRobots.content = "noindex";
} else {
    // add one if none existed
    const newRobots = document.createElement('meta');
    newRobots.name = "robots";
    newRobots.content = "noindex";
    document.getElementsByTagName('head')[0].prepend(newRobots);
}