cookie.es6

const getCookieByName = (name) => document.cookie.split(';')
  .map(x => x.split('='))
  .filter(x => x[0].trim() === name.trim())
  .map(x => decodeURIComponent(x[1]))[0]

cookie.js

function getCookieByName(cname) {
    var name = cname + "=";
    var ca = document.cookie.split(';');
    for (var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1);
        if (c.indexOf(name) != -1) return c.substring(name.length, c.length);
    }
    return "";
}