Problem with Javascript, php and Contact Form [duplicate]

Hi i have some functions in JavaScript for a contact form, but Dreamweaver cs6 is showing syntax error. Can someone please help me? The first and second (let) syntax give an error as well as the final class user.

var fields = {};
document.addEventListener("DOMContentLoaded", function() {
fields.name = document.getElementById('name');
fields.email = document.getElementById('email');
fields.message = document.getElementById('message');
})

function isNotEmpty(value) {
if (value == null || typeof value == 'undefined' ) return false;
return (value.length > 0);
}

function isEmail(email) {
let regex = /^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/;
return regex.test(String(email).toLowerCase());
}

function fieldValidation(field, validationFunction) {
if (field == null) return false;

let isFieldValid = validationFunction(field.value)
if (!isFieldValid) {
field.className = 'placeholderRed';
} else {
field.className = '';
}

return isFieldValid;
} 

function isValid() {
var valid = true;

valid &= fieldValidation(fields.name, isNotEmpty);
valid &= fieldValidation(fields.email, isNotEmpty);
valid &= fieldValidation(fields.message, isNotEmpty);
valid &= fieldValidation(fields.email, isEmail);
return valid;
}

class User {
constructor(name, email, message) {
this.name = name;
this.email = email;
this.message = message;
 }
}