Using .text() to retrieve only text not nested in child tags

If I have html like this:

<li id="listItem">
    This is some text
    <span id="firstSpan">First span text</span>
    <span id="secondSpan">Second span text</span>
</li>

I’m trying to use .text() to retrieve just the string “This is some text”, but if I were to say $('#list-item').text(), I get “This is some textFirst span textSecond span text”.

Is there a way to get (and possibly remove, via something like .text("")) just the free text within a tag, and not the text within its child tags?

The HTML was not written by me, so this is what I have to work with. I know that it would be simple to just wrap the text in tags when writing the html, but again, the html is pre-written.