For Windows users, run npm commands from the Command Prompt (cmd.exe), not Node.Js (node.exe). So your "normal shell" is cmd.exe. (I agree this message can be confusing for a Windows, Node newbie.)

By the way, the Node.js Command Prompt is actually just an easy shortcut to cmd.exe.

Below is an example screenshot for installing grunt from cmd.exe:

Don't run npm install xxx in the REPL - the REPL is meant for testing out Node.js code, not running executables (well, npm).

Just run it like the second time you tried it. The fix to that can be found here.

Add npm to your path (environment variable on windows not sur on linux) if you are using a portable version, if you are using an installed version of node, it should add npm to your path automatically on windows, not sure on linux, npm start will run the start script from your project package.json file. try going into your npm executable folder (generally in node installation folder), n do a ./npm i -g npm

Update

Regarding William Entriken's comment below this answer, there is a better way of installing Node.js natively in Ubuntu as a snap package.

Node.js is available as a snap package in all currently supported versions of Ubuntu. Specific to Node.js, developers can choose from one or more of the currently supported releases and get regular automatic updates directly from NodeSource. Node.js versions 6, 8, 9, 10, 11, 13, 14, 15, 16, 17 and 18 are currently available, with the Snap Store being updated within hours or minutes of a Node.js release.

Node can be installed with a single command, for example:

sudo snap install node --classic --channel 9/stable 

The node snap can be accessed by the command node, for example:

$ node -v  
v9.9.0

An up-to-date version of npm will installed as part of the node snap. npm should be run outside of the node repl, in your normal shell. After installing the node snap run the following command to enable npm update checking:

sudo chown -R $USER:$(id -gn $USER) /home/your-username/.config

Replace your-username in the above command with your own username. Then run npm -v to check if the version of npm is up-to-date. As an example I checked that npm was up-to-date, checked the version of an already installed package named yarn with the command npm list yarn and then updated the existing yarn package to the latest version with the command npm update yarn

Users can switch between versions of Node.js at any time without needing to involve additional tools like nvm (Node Version Manager), for example:

sudo snap refresh node --channel=8/stable

Users can test bleeding-edge versions of Node.js that can be installed from the latest edge channel by switching with:

sudo snap switch node --edge

This approach is only recommended for those users who are willing to participate in testing and bug reporting upstream.

Original answer

To install the latest LTS versions of Node.js (nodejs) and Package manager for Node.js (npm) in all currently supported versions of Ubuntu, open the terminal and run the following commands:

sudo apt-get remove nodejs npm ## remove existing nodejs and npm packages
sudo apt-get install curl  
curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -
sudo apt-get install -y nodejs  

If you are copying these three commands, don't neglect the hyphen at the end of the second command.

The nodejs package contains the nodejs binary as well as npm, so you don't need to install npm separately. However, in order for some npm packages to work (such as those that require building from source), you will need to install the build-essential package:

sudo apt-get install build-essential  

LTS Plan

New semver-major releases of Node.js are cut from master every six months. New even-numbered versions (e.g. v6, v8, v10, etc.) are cut in April. New odd-numbered versions (e.g. v5, v7, v9) are cut in October.

When a new odd-numbered major release is cut, the previous even-numbered major version transitions to the Long Term Support plan.

Every major version covered by the LTS plan will be actively maintained for a period of 18 months from the date it enters LTS coverage. Following those 18 months of active support, the major version will transition into "maintenance" mode for 12 additional months.

non-LTS installation

Current to March, 2017 the commands to install the latest non-LTS version of Node.js (v8 at the time this was posted) are as follows:

sudo apt-get remove nodejs npm ## remove existing nodejs and npm packages
sudo apt-get install curl  
curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
sudo apt-get install -y nodejs  

Node.js LTS schedule