This module, when it is used in another module, sends a module error whenever a global variable is written or read. Global variables are discouraged (see WT:LUA). It should not be used in modules that are invoked in entries, because it will bleed across to other modules, some of which have global variables, causing module errors. It can be used as a tool while previewing pages with the module.
To use it, type
require("Module:no globals"). The module does not return a value, so you do not have to assign a variable to it.