Google Apps Script, How to import gs functions across other Google docs

Go to .gs file/project(Tools>Script editor..) which you want to use as a library.

Save file with the version at File>>Manager versions.

Copy project key from this menu location File>>Project properties>>Project key

Go to your second .gs file/project and add previous project key as a library at menul Resources>>Libraries…

“Add a library” -> click on Add button -> click on Save

Add identifier for this library once library is added with Resources>>Libraries… menu.

Now you can use functions from previous project/library with this identifier inside your local function.

Identifier.functionFromImportedLibrary();