If you don’t set anything else, the cookie will expire when the browser is closed. To prevent so, add an expiration date, expressed in the UTC format (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 expressed in number of 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: