A gist to validate form inputs

formControl.js

$(document).ready(function() {
    let f1err = 0,
	f2err = 0,
	f3err = 0,
	date = new Date();    

    $("#sendBTN").click(function() {
        if (($("#username").val() == "") || ($("#phone").val() == "")) {
            f1err = 1;
            if ($("#username").val() == "") {
                $(".username").addClass("has-error");
                $(".namesurname_txt").css("display","block");
            } else {
                $(".phone").addClass("has-error");
                $(".phone_txt").css("display","block");
            }
        } else {
            f1err = 0;
            if ($("#username").val() != "") {
                $(".username").removeClass("has-error");
                $(".namesurname_txt").css("display","none");
            } else {
                $(".phone").removeClass("has-error");
                $(".phone_txt").css("display","none");
            }
                        
        }    
        if (($("#byear").val() == "") || ((date.getFullYear() - $("#byear").val()) < 18)) {
            f3err = 1;
            $(".byear").addClass("has-error");
            $(".byear_txt").css("display","block");                   
        } else {
            f3err = 0;
            $(".byear").removeClass("has-error");
            $(".byear_txt").css("display","none");                                
        }
        if (f1err == 0 && f2err == 0 && f3err == 0) {
            window.location = "confirmation.html?username=" + $("#username").val() + "&phone=" + $("#phone").val();
            return true;
        } else
            return false;
    });
    
    if ($("#username").exists()) {
        let usernametxt = $.urlParam('username').replace("%20", " ");            
        $("#username").html(usernametxt);
    }
    if ($("#phone").exists()) {
        let phonetxt = $.urlParam2('phone').replace("%20", " ");            
        $("#phone").html(phonetxt);
    }
    
});

$.urlParam = function(name) {
	let results = new RegExp('[\?&]' + name + '=([^&#]*)').exec(window.location.href);
	return results[1] || 0;
}
$.urlParam2 = function(name) {
	var results = new RegExp('[\?&]' + name + '=([^&#]*)').exec(window.location.href);
	return results[1] || 0;
}
jQuery.fn.exists = function() {
    return this.length > 0;
}