You have your utils.js with the classic document.querySelector shortcut:

function q(selector) {
    return document.querySelector(selector);
}

Lets say that the "assertion" is that this should be only called when we know the selector will always match a single element (for the other cases we simply call other functions)

We leave this "efficient" version, and we replicate the an error checking version of it in a new file utils.dev.js:

function q(selector) {
    const res = document.querySelectorAll(selector);
    if (res.length == 1) return res[0];
    throw `Found ${res.length} matches for '${selector}'`;
}

And then in the "build system" we specify one of the files based on the release mode.

done_