In witch way updating a global package will affect a local package?

Hi Carlos. I've just covered this course and actually used NPM in a project, so although you posed these great questions 3 months ago, I'd like to take a stab at them.

ins't the local package a kind of a different package because it has its own rules in package.json?

Yes. The entire (local) installation resides within a single umbrella folder so packages are not intended to be accessed from outside.

So, OK, it still is the global package but controlled with restrictions

No. Think of them as 2 separate installations. Each are controlled by their own package.json file.

In which way changing jQuery globally to the last version will change a jQuery package already installed locally that have rules set by package.json that don't allow an update to a major version(for instance)?

The package.json responsible for the local installation solely dictates what will happen.

If this happens, if changing global packages can affect already existing projects with package.json rules, does a change in the global npm package can affect dramatically already existing projects?

As explained above, for as long as an app relies solely on the local installation of a given package, any change made to the global installation will have no effect. So are there times when one would want an app to rely on a globally installed package? Well apparently there are. More about that below**.

Shouldn't we be REALLY CAREFUL updating npm whenever it goes up a major version?

Absolutely. Local or global, a major version will bring many benefits no doubt but some new features will not be backward compatible, meaning for example that older browsers may no longer work reliably with your app.

So now I imagine you are left wondering when your app should ever rely on a globally installed package? Well you are not alone. If you google, you will find many feeling the same way. Here is a blog by 'Bart' who used to work on NPM support...

http://www.bartread.com/2014/02/17/whats-difference-locally-globally-installing-npm-packages/

To quote his 'takeaways'...

Global packages are for anything that you need to access from the shell. By contrast local packages are for using within your apps.

always try to use local packages unless there’s a very good reason not to! (I.e., you need to use the package from the command line.)

**Bart also gives an example of why you might want to associate multiple apps to a single globally installed package called 'Express', in order to use its 'Command Line' functionality. He explains how this can be done by using a feature called 'link'. The advantage here being that when you want to update 'Express' you just do it globally once, and all the apps get the benefit.