How to add swap file in Debian or Ubuntu

Swap is used to allow operating system run in fail-over mode, meaning that if it's running out of RAM, it will start swapping some memory pages from RAM into the swap file, on disk. The best way to create swap - is to mount it into partition. But if you don't have a swap partition, or don't have time to create it, you can also add swap using a regular file. Below I will show how to add swap file in Debian based Linux distros.