How to make javascript function external which is having PHP code inside?

I want to make my all javascript functions exteal. But problem is most of them are having PHP code inside. For Example-

function resend_otp_registration()
                    {
                        var a = 'sample';
                        $.ajax({
                                type:'POST',
                                url:'<?php echo base_url()?>admin/customer/resend_otp_registration',
                                data:
                                {
                                    a:a,
                                },
                                success:function(data)
                                {
                                    if(data==1)
                                    {
                                        alertify.alert("OTP resent on your mobile number.");
                                    }
                                },
                            });
                    }

How can I put this type of JS function in exteal JS file?