I prefer:

    for (let node of document.querySelectorAll("li"))
        node.classList.add('foo');

or:

    Array.from(document.querySelectorAll("li")).forEach(node => node.classList.add('foo'))

And if any particular pattern repeats in the code I'm writing I'll just create a helper function for that pattern (most of the time locally). Like:

    let qsa => s => Array.from(document.querySelectorAll(s));

There's nothing to test about that.