How to use document.evaluate() and XPath to get a list of elements?

I found the following solution in the book I am currently reading. It says that the code is from the Prototype library.

function getElementsByXPath(xpath, parent)
{
    let results = [];
    let query = document.evaluate(xpath, parent || document,
        null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
    for (let i = 0, length = query.snapshotLength; i < length; ++i) {
        results.push(query.snapshotItem(i));
    }
    return results;
}

Use it like this:

let items = getElementsByXPath("//*"); // return all elements on the page