input validation between 2 numbers javascript
===

I am currently using google maps and attempting to use input validation to require the user to use a number between 0 and 20 to set as a zoom which will then zoom in on my location.
 Below is the code i am using. The first if statement works perfectly for any number over 20 but the second does not work when i use numbers like 0 and -1 for instance. I may be missing a javascript rule. This is also my first time using input validation with Google maps.

Any suggestions to fix this?

    function inputIsValid() {

                           console.log("Inside inputIsValid function");

                           //Check for Above 20
                           if (document.getElementById("txtZoom").value  > 20) {
                               alert("Please insertAmount between 0 and 20");
                               document.getElementById("txtZoom").focus();
                               return false;

                               //Check for Number below 0
                               if (document.getElementById("txtZoom").value < 0) {
                                   alert("Please insertAmount between 0 and 20");
                                   document.getElementById("txtZoom").focus();
                                   return false;
                               }
                           }
                       }