How do I get the value of text input field using JavaScript?

I am working on a search with JavaScript. I would use a form, but it messes up something else on my page. I have this input text field:

<input name="searchTxt" type="text" maxlength="512" id="searchTxt" class="searchField"/>

And this is my JavaScript code:

<script type="text/javascript">
  function searchURL(){
    window.location = "http://www.myurl.com/search/" + (input text value);
  }
</script>

How do I get the value from the text field into JavaScript?