Error running npm run production

Following code is giving me error while running npm run production. I have rechecked everything, and code is working fine, but not able to go through uglifyjs.

// Variables
    let contactForm = $("#contact-form"),
        formResponse = $(".form-response"),
        submitButton = $("#submit");
        contactForm.validator().on("submit", function(e) {
        if(e.isDefaultPrevented()) {
            formResponse.text("Sorry, you didn't fill the form.").fadeIn(1000);
        } else {
            e.preventDefault();
            submitForm();
        }
    });
    // Submit Function
    function submitForm() {
        // Some Variables
        let name = $("#name").val(),
            mail = $("#mail").val(),
            message = $("#message").val();
        $.ajax({
            type: "POST",
            url: "contact.php",
            data: "name=" + name + "&mail=" + mail + "&message=" + message,
            beforeSend: function(text) {
                submitButton.html("<i class='fa fa-spinner fa-spin'></i> Sending...");
                formResponse.fadeOut(500).text("");
            },
            success: function(text) {
                if(text == "success") {
                    contactForm[0].reset();
                    formResponse.text("Thanks! Your message sent correctly.").fadeIn(1000);
                    submitButton.html("Send Message");
                } else {
                    formResponse.text(text).fadeIn(1000);
                }
            }
        });
    }

via manshu