JavaScript function that return with different types (Array vs NodeList)

I wrote a JavaScript function that will return a "list" of elements that has Id that start with some value:

function getElementsWithIdPrefix(prefix){
    if (document.querySelectorAll){
        return document.querySelectorAll('*[id^="' + prefix + '"]');
    } else {
        // none modern browsers support
        var elements = document.getElementsByTagName('*');
        var relevantElements = [];
        for (var i = 0; i < elements.length; i++){
            if (element.id && element.id.indexOf(prefix) !== -1){
                relevantElements.push(element);
            }
        }
        return relevantElements;
    }
}

As you can see my function will return different types depends on browser support for document.querySelectorAll and I have two question regarding this:

1. Is this bad? I mean - In JavaScript typing system does it consider as a code smell or bad practice?
2. How can I create Node objects for each Element by my self and construct a new NodeList containing these elements to return?