Creating and prepending a div with the Google Custom Search Javascript Callback

prepend('Some text', div) adds the text and div as separate children. To put the text inside the div, set div.textContent first:

var SearchResultsRenderedCallback = function() {
    let prependdiv = document.getElementsByClassName("gsc-table-result");
    for (let i = 0; i < prependdiv.length && i < 10; i++) {
        let div = document.createElement('div');
        div.className = 'test-div';
        div.textContent = 'Some text';
        prependdiv[i].prepend(div);
    }
};
window.gcse = window.gcse || {};
window.gcse.searchCallbacks = { web: { rendered: SearchResultsRenderedCallback } };