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Can i move my node_modules folder to another project?

I had some issues with a current iOS React Native project that i spent a long time working on. I've decided to start afresh. However i did spend a lot of time ensuring i got the correct versions of various react native modules and configuring these to work. These sit in my old 'node_modules' folder.
Can i copy these modules/folders (from the old 'node_modules') over to my new project? Do i need to update 'package.json' or link using 'react-native link xxxxx'? It it as simple as that in theory?

You don't need to copy over your node_modules directory. You can if you'd like, but it is not considered best practice. In any case, you shouldn't make any modifications to the files inside node_modules.
Preferably you need to only copy over your package.json file and optionally, your package-lock.json (or yarn.lock if you're using Yarn) file so that your project will be easily installable and upgradeable on other computers.
When you have a package.json or package-lock.json file, you can run npm install (or yarn install) to install the packages to your node_modules directory.
Copying your package-lock.json file over as well would ensure that the exact same versions of all of the packages that you have installed in your previous project will be installed in the new project as well. See this for more information on the package-lock.json file.
Unfortunately, I don't know much about react-native and linking react-native dependencies, but from this answer it seems that you would have to link any dependencies that contain native code again after you've run npm install.

No, I wouldn't recommend you to copy specific module from node_modules folder as once installed it has entries in .bin folder and files which you will miss while copying and it will be of no help in new project as they will be downloaded and installed again due to missing indexes.
Solutions :
You can use same package.json and package-lock in another project if you are sure that the dependencies version in it are exactly compatible or one you want, and install those dependencies in new project. The package-lock.json will ensure the version you choose.
Yes you can copy whole node_modules (have done it multiple times) from one project to another and use same package.json and package-lock (Will only save time in dependencies installation/download)
As you are starting fresh what i will recommend is create a new package.json and copy only those dependencies which are important for the project initialization, and as you progress add specific dependencies you need while developing. This will save you from huge garbage of unused dependencies which are hard to keep track of once the project inflates.

Maybe you can move it but you can't copy it, at least not with a naive cp -r! The node_modules/.bin directory ought to be full of symlinks to scripts in their respective directories in node_moudles, but after a cp -r these will be copies, which will now resolve relative module paths incorrectly.
For example, npm install typescript installs a executable script with the contents
#!/usr/bin/env node
require('../lib/tsc.js')
in two locations. Before a copy, that looks like this:
$ ls -l node_modules/.bin/tsc node_modules/typescript/bin/tsc
-rwxr-xr-x 45 node_modules/.bin/tsc
-rwxr-xr-x 45 node_modules/typescript/bin/tsc
and after a cp -r copy, like this:
$ ls -l node_modules/.bin/tsc node_modules/typescript/bin/tsc
lrwxr-xr-x 21 node_modules/.bin/tsc -> ../typescript/bin/tsc
-rwxr-xr-x 45 node_modules/typescript/bin/tsc
which makes the ../typescript/bin/tsc no longer exist, resulting in
$ tsc -b
...
Error: Cannot find module '../lib/tsc.js'
...

Installing gulp plugins locally vs globally

I am pretty new to Gulp and I've got a question: should I install gulp plugins (such as gulp-sass or gulp-imagemin) locally or globally? In examples around the web people mostly do it locally (with --save-dev option). As I understand, by doing so the modules are stored in the local node_modules folder, added in the local package.json as devDependencies and can be referenced to in the local gulpfile.js via require(). So, if I need to install the same modules for another project I work on, it may be accomplished by copying package.json to a new project's folder and typing in npm install in a command line tool (after getting to the project's folder). Fine.
But what if I, as a regular gulp user, don't have plans to upload and share my stuff on the npm space and not interested in maintaining devDependencies, can I in this case just install gulp plugins globally like npm install -g gulp-sass? Will they be found through a global path in my system? Is it an option on the whole, if I don't want to bother myself to copy package.json, run npm install every time I create a new project or have multiple copies of the same modules scattered around on my disk?

For plugins that you are using via gulp, what you have to do is install them locally to the project. Although you have to install gulp itself globally and locally, to run the file and then for the project to pick up the gulp based commands and functions.
The reason you should install the plugins via npm locally is so that it is specific to the project, for example if you then went to upload this project to your server or host on github then you would have to then go and have to globally install all of your packages again. If they are saved, they exist in your packages.json, this is so that when you go to run npm install to install all the packages for said project npm knows what to install.
If I can further clarify anything let me know.

A kind of "global install" for the typical case OP explained, could be to have a folder where you put your package.json with every dependencies you need for each of your projects, and run npm install from there.
And put here also your gulpfiles for each project (you could named them as you want), like gulpfile-project1.js, conf-project2.js... whatever.
Of course adjust your paths in those files accordingly.
And from this directory, run gulp using the -f flag:
gulp -f gulpfile-project1.js
So you end with this kind of scaffolding :
my_dev_directory
- project1
- dev
- main.less
- main.js
- www
- main.css
- main.js
- gulp
- node_modules
- package.json
- package-lock.json
- gulpfile-project1.js
In your gulpfiles, start to declare a const with the base path of your project to ease all path declarations, like:
const path_dev = '../project1/dev';
const path_www = '../project1/www';
This way you have only one node_modules directory installed on your disk and your projects directories contain only public files you want to upload, no dev files.
Of course one of the main drawback is that you share the same module version with all your projects, but it answered to OP's need and even if it's not recommended, not the "proper way to do", you are free to do as you want with your dev tools :)

Why is my es6 code not compiling after npm install?

I have created a npm module with es6. You can see it on github here.
There are some scripts in the package.json file which should ensure that the es6 modules get built. It works when running npm publish and npm install when in the context (directory) of the npm module. Running npm install in another project (ie including the module as a dependency) does not however build the es6 modules. There don't seem to be any errors in the npm-debug.log file either.
I'd really like to understand why.

It was caused by the lack of a .npmignore file. As there wasn't one, npm was using the .gitignore file instead. This filters out the dist/ folder which I don't want in source control. This commit fixed the issue

Your script is missing an install (or postinstall) script.

If you are building the code with a script on the prepublish hook, then you don't need it to be built again when others install it. It should ship with the built ES6 code.
In fact, you probably want to add your src directory to your .npmignore file, so that it doesn't ship with any JS that can't be used directly.

Install node extensions without commands

I am doing an internship in a company.
I need to create a node server.
I installed node on the computer (Windows) and I should install some plugins like:
- nodejs-webpack
- colors
- uglify
Normally I need to enter a command like : npm install "theModule"
But the software can not access the internet (due to company restrictions) and support service can not authorize the software (or do not want).
Can I install modules in any other way ? (download from Google and slide archives in the correct folder for example).
If the answer is no, do you know how can i get around this security?

You need a private npm repository.
Check out this answer:
can you host a private repository for your organization to use with npm?

I found it !
Just for exemple, we will install 'nodejs-websocket' :
1) You just have to download it here.
2) Put files into your Node's directory (for me it's "C:\Program Files\nodejs\node_modules\npm\node_modules")
3) in your .js file just add this line : var ws = require("C:/Program Files/nodejs/node_modules/npm/node_modules/nodejs-websocket/")
Done ! Thanks for all :D

I added this as a comment on your own answer, but I figured I should add a real answer with a better explanation.
Normally when you run npm install package-name npm installs the package to a node_modules directory in the directory you are in at that moment. So if your app was located at C:\code\my-app then you would cd into that directory and run npm install package-name. This would create a node_modules directory at C:\code\my-app\node_modules if it didn't already exist. Then it would install package-name into that directory at C:\code\my-app\node_modules\package-name.
As long as the module is in the node_modules directory for your app, you can require the module in your code without entering a big long file path.
var ws = require('nodejs-websocket');
The place you manually installed your module is the global node_modules directory. It's where npm would install a module if you did npm install -g package-name. The global node_modules directory is added to your system path when you install npm. In other words, any modules you install in there would be accessible from the command line like any other command. For example:
npm install -g bower
That would install the "bower" package to the global npm module directory. Bower would then be accessible as a command line tool for you to use. For example:
bower install angularjs
The global directory is more for tools like that and not really for modules that you intend to use in your code. Technically you can require a module from anywhere by including the full path in the require call like you did, but the standard practice would be to place it in the node_modules directory in the root of your application, and then require it with just its name and not a full path.
Edit: Here's another tip that you might like to take advantage of as well.
When you normally install a module with npm install package-name, your application usually has a package.json file at the root of it. If it does, you can do npm install package-name --save and npm will add the package-name module to a list in your app's package.json file. Once a module is listed in your app's package.json file it's called a "dependency" of your app because it basically says your app depends on package-name.
Normally, when you have dependencies listed in package.json, you can completely delete your app's node_modules directory and then simply run npm install from within your app's root directory and npm will automatically install all dependencies it finds listed in your app's package.json file. Since your corporate firewall won't allow this automatic downloading of modules, you won't get that benefit. However it is still good practice to follow the same conventions.
A trick you can do to create your package.json file is to manually install your dependencies into your app's node_modules directory. Once you have the modules your app needs, you can instruct npm to create a package.json file for your app by simply running npm init. It will walk you through a few little prompts to fill out your package.json file with details about your app. It will then peek inside your node_modules directory to see if you've already installed any modules before having a package.json file. If it finds any, it will automatically add them to the dependencies field in the package.json file it creates :D
With a proper package.json in place you'll always have a nice tidy list of what dependencies your application needs, even if your node_modules directory gets deleted. Normally people add their app's node_modules directory to their .gitignore file, only checking in the package.json file. This way they don't store their dependencies in source control but they can still be easily installed on new machines that clone it by simply running npm install from inside the app's directory. In your case though you may want to just add node_modules to your source control since you can't let npm install dependencies automatically.

npm install on modules not in node_modules/

I'm building a project and I want to split it up into different modules. I know I should put modules into node_modules/ but I want to commit the modules to the main version control for the time being (I'm using .gitignore to ignore the node_modules/ directory at the moment).
My current project setup:
services/
services/service1/package.json
services/service1/index.js
services/service2/package.json
services/service2/index.js
node_modules/*
app.js
package.json
My problem is it works with relative requires but the dependences in the services/*/package.json aren't loaded. Only the main package.json are loaded.
Is this a good approach? Is there a better one? What command can I run to install all the dependences? This is more of a problem for my CI/CD setup.
Thanks.

i found similar feature request here https://github.com/npm/npm/issues/4017 and seems you can't do it
one approach i see is to create soft links from node_modules to services you want and maintain them somehow
maybe you can use https://www.npmjs.org/doc/misc/npm-scripts.html postinstall script for this?