JS understanding form validation return value

Hello,
To avoid submitting a form the JavaScript-function (see below) must return false.
Why doesn't it work like that? (see below) The return value must be put into the anonymous function. Thanks!

 document.getElementById("myform").onsubmit = function() {
                  validate();
              };
         
             function validate () {
          
                 var element = document.getElementById("error_div");...

Code (JavaScript):

JS understanding form validation return value

from JavaScript https://ift.tt/3bZHcLm
via IFTTT