In the documentation it's stated: Use the prefix option together with the global option:
The prefix config defaults to the location where node is installed. On most systems, this is /usr/local. On windows, this is the exact location of the node.exe binary. On Unix systems, it's one level up, since node is typically installed at {prefix}/bin/node rather than {prefix}/node.exe.
When the global flag is set, npm installs things into this prefix. When it is not set, it uses the root of the current package, or the current working directory if not in a package already.
(Emphasis by them)
So in your root directory you could install with
npm install --prefix <path/to/prefix_folder> -g
and it will install the node_modules folder into the folder
<path/to/prefix_folder>/lib/node_modules
As of npm version 3.8.6, you can use
npm install --prefix ./install/here <package>
to install in the specified directory. NPM automatically creates node_modules folder even when a node_modules directory already exists in the higher up hierarchy.
You can also have a package.json in the current directory and then install it in the specified directory using --prefix option:
npm install --prefix ./install/here
As of npm 6.0.0, you can use
npm install --prefix ./install/here ./
to install the package.json in current directory to "./install/here" directory. There is one thing that I have noticed on Mac that it creates a symlink to parent folder inside the node_modules directory. But, it still works.
NOTE: NPM honours the path that you've specified through the --prefix option. It resolves as per npm documentation on folders, only when npm install is used without the --prefix option.
I am using a powershell build and couldn't get npm to run without changing the current directory.
Ended up using the start command and just specifying the working directory:
start "npm" -ArgumentList "install --warn" -wo $buildFolder
Source: Stackoverflow.com