If you do not set anything else, the cookie expires when the browser is closed. To prevent that, add an expiration date in UTC format (e.g. Mon, 26 Mar 2018 17:04:05 UTC):

document.cookie = 'name=Flavio; expires=Mon, 26 Mar 2018 17:04:05 UTC'

A simple JavaScript snippet to set a cookie that expires in 24 hours is:

const date = new Date()
date.setHours(date.getHours() + 24)
document.cookie = 'name=Flavio; expires=' + date.toUTCString()

Alternatively, you can use the max-age parameter to set an expiration in seconds:

document.cookie = 'name=Flavio; max-age=3600' // expires in 60 minutes
document.cookie = 'name=Flavio; max-age=31536000' // expires in 1 year

Lessons in this unit:

0:	Introduction
1:	Setting cookies
2:	▶︎ Setting a cookie expiration date
3:	Setting a cookie path
4:	Setting a cookie domain
5:	Cookies security
6:	Updating a cookie
7:	Deleting a cookie
8:	Accessing the value of a cookie
9:	Checking if a cookie exists
10:	Inspecting cookies
11:	Cookie not being set in Safari