Very simple cookie handler, that has 2 methods: get cookie and set cookie. Based on functions from https://learn.javascript.ru/cookie (old, russian version of javascript.info)

cookies.js

const cookie = {
  get: name => {
    const matches = document.cookie.match(new RegExp(
      "(?:^|; )" + name.replace(/([\.$?*|{}\(\)\[\]\\\/\+^])/g, '\\$1') + "=([^;]*)"
    ));
    return matches ? decodeURIComponent(matches[1]) : undefined;
  },
  set: (name, value, options={}) => {
    
    const expires = options.expires;

    if (typeof expires == "number" && expires) {
      const d = new Date();
      d.setTime(d.getTime() + expires * 1000);
      expires = options.expires = d;
    }
    if (expires && expires.toUTCString) {
      options.expires = expires.toUTCString();
    }

    value = encodeURIComponent(value);

    let updatedCookie = name + "=" + value;

    for (let propName in options) {
      updatedCookie += "; " + propName;
      const propValue = options[propName];
      if (propValue !== true) {
        updatedCookie += "=" + propValue;
      }
    }

    document.cookie = updatedCookie;
  }
}