Many people have already mentioned that document.cookie gets you all the cookies (except http-only ones).
I'll just add a snippet to keep up with the times.
document.cookie.split(';').reduce((cookies, cookie) => {
const [ name, value ] = cookie.split('=').map(c => c.trim());
cookies[name] = value;
return cookies;
}, {});
The snippet will return an object with cookie names as the keys with cookie values as the values.
Slightly different syntax:
document.cookie.split(';').reduce((cookies, cookie) => {
const [ name, value ] = cookie.split('=').map(c => c.trim());
return { ...cookies, [name]: value };
}, {});