Вставка скрипта в начало body из js

(function (d){
    let b = d.querySelector('body');
    let target = document;
    let observer = null;

    const config = {
        attributes: true,
        childList: true,
        subtree: true
    };

    const observerCallback = function(mutationsList, observer) {
        b = d.querySelector("body");
        for (let mutation of mutationsList) {
            console.log(mutation);
            if (b && mutation.target === b && !d.querySelector("body > script[src=https://relap.io/v7/relap.js]"))
            {
                addScript();
                observer.disconnect();
            }
        }
    };

    const addScript = function() {
        let s = d.createElement("script");
        s.setAttribute('async', 'async');
        s.setAttribute('type', 'text/javascript');
        s.setAttribute('src', 'https://relap.io/v7/relap.js');
        s.setAttribute('data-relap-token', '45vjvDi8uv8CO87L');
        b.insertBefore(s, d.querySelector('body > :first-child'));
    };

    if(b)
    {
        addScript();
    }
    else
    {
        observer = new MutationObserver(observerCallback);
        observer.observe(target, config);
    }
})(document);