How can you share code between files?

This depends on the JavaScript environment.

On the client (browser environment), as long as the variables/functions are declared in the global scope (window), all scripts can refer to them. Alternatively, adopt the Asynchronous Module Definition (AMD) via RequireJS for a more modular approach.

On the server (Node.js), the common way has been to use CommonJS. Each file is treated as a module and it can export variables and functions by attaching them to the module.exports object.

ES2015 defines a module syntax which aims to replace both AMD and CommonJS. This will eventually be supported in both browser and Node environments.