How to use JavaScript form submition with other function validation

If validateForm(...) and validateForm1() return a boolean (true means that no validation error occurred), then you try to do that :

function returnTrue() {
            if(validateForm1() && validateForm2 && validateForm3()) {
  document.form1.submit();
                return true;
            } else {
                document.getElementById('validation').innerHTML = 'Validation failed!';
                return false;
            }
        }

<form action=""  method="post"  name="form1" enctype="multipart/form-data" onSubmit="return returnTrue()">
First name: 
<input type="text" name="FirstName" value="Mickey"><br>
Last name: 
<input type="text" name="LastName" value="Mouse"><br>
<input type="text" name="Submit" value="Submit">

</form>