For brew users, OSX:

To remove:

brew uninstall node; 
# or `brew uninstall --force node` which removes all versions
brew cleanup;
rm -f /usr/local/bin/npm /usr/local/lib/dtrace/node.d;
rm -rf ~/.npm;

To install:

brew install node;
which node # => /usr/local/bin/node
export NODE_PATH='/usr/local/lib/node_modules' # <--- add this ~/.bashrc

You can run brew info node for more details regarding your node installs.

consider using NVM instead of brew

NVM (node version manager) is a portable solution for managing multiple versions of node

> nvm uninstall v4.1.0
> nvm install v8.1.2
> nvm use v8.1.2
> nvm list
         v4.2.0
         v5.8.0
        v6.11.0
->       v8.1.2
         system

you can use this with AVN to automatically switch versions as you hop between different projects with different node dependencies.

I know this post is a little dated but just wanted to share the commands that worked for me in Terminal when removing Node.js.

lsbom -f -l -s -pf /var/db/receipts/org.nodejs.pkg.bom | while read f; do  sudo rm /usr/local/${f}; done
 
sudo rm -rf /usr/local/lib/node /usr/local/lib/node_modules /var/db/receipts/org.nodejs.*

UPDATE: 23 SEP 2016

If you're afraid of running these commands...

Thanks to jguix for this quick tutorial.

First, create an intermediate file:

lsbom -f -l -s -pf /var/db/receipts/org.nodejs.node.pkg.bom >> ~/filelist.txt

Manually review your file (located in your Home folder)

 ~/filelist.txt

Then delete the files:

cat ~/filelist.txt | while read f; do sudo rm /usr/local/${f}; done

sudo rm -rf /usr/local/lib/node /usr/local/lib/node_modules /var/db/receipts/org.nodejs.*

For 10.10.5 and above

Thanks Lenar Hoyt

Gist Comment Source: gistcomment-1572198

Original Gist: TonyMtz/d75101d9bdf764c890ef

lsbom -f -l -s -pf /var/db/receipts/org.nodejs.node.pkg.bom | while read f; do sudo rm /usr/local/${f}; done

sudo rm -rf /usr/local/lib/node /usr/local/lib/node_modules /var/db/receipts/org.nodejs.*

On Mavericks I install it from the node pkg (from nodejs site) and I uninstall it so I can re-install using brew. I only run 4 commands in the terminal:

1. sudo rm -rf /usr/local/lib/node_modules/npm/
2. brew uninstall node
3. brew doctor
4. brew cleanup --prune-prefix

If there is still a node installation, repeat step 2. After all is ok, I install using brew install node

Run following commands to remove node completely from system in MACOS

sudo rm -rf ~/.npm ~/.nvm ~/node_modules ~/.node-gyp ~/.npmrc ~/.node_repl_history
sudo rm -rf /usr/local/bin/npm /usr/local/bin/node-debug /usr/local/bin/node /usr/local/bin/node-gyp
sudo rm -rf /usr/local/share/man/man1/node* /usr/local/share/man/man1/npm*
sudo rm -rf /usr/local/include/node /usr/local/include/node_modules
sudo rm -rf /usr/local/lib/node /usr/local/lib/node_modules /usr/local/lib/dtrace/node.d
sudo rm -rf /opt/local/include/node /opt/local/bin/node /opt/local/lib/node
sudo rm -rf /usr/local/share/doc/node
sudo rm -rf /usr/local/share/systemtap/tapset/node.stp

brew uninstall node
brew doctor
brew cleanup --prune-prefix

After this I will suggest to use following command to install node using nvm (check for latest version)

curl -o- | bash

from

Why nvm?
this is a good question, there will be projects requiring different versions of node,
i.e. A requires node version 12 while B requires node version 14. This version management of node is provided by nvm only.

I have summarized the existing answers and made sure Node.js is COMPLETELY ERASED along with NPM.

Lines to copy to terminal:

brew uninstall node;
which node;
sudo rm -rf /usr/local/bin/node;
sudo rm -rf /usr/local/lib/node_modules/npm/;
brew doctor;
brew cleanup --prune-prefix;

1. First:

   lsbom -f -l -s -pf /var/db/receipts/org.nodejs.pkg.bom | while read f; do  sudo rm /usr/local/${f}; done
   
   sudo rm -rf /usr/local/lib/node /usr/local/lib/node_modules /var/db/receipts/org.nodejs.*
   

2. To recap, the best way (I've found) to completely uninstall node + npm is to do the following:

   go to /usr/local/lib and delete any node and node_modules

   cd /usr/local/lib
   
   sudo rm -rf node*
   

3. go to /usr/local/include and delete any node and node_modules directory

   cd /usr/local/include
   
   sudo rm -rf node*
   

4. if you installed with brew install node, then run brew uninstall node in your terminal

   brew uninstall node
   

5. check your Home directory for any "local" or "lib" or "include" folders, and delete any "node" or "node_modules" from there

   go to /usr/local/bin and delete any node executable

   cd /usr/local/bin
   
   sudo rm -rf /usr/local/bin/npm
   
   ls -las
   

6. You may need to do the additional instructions as well:

   sudo rm -rf /usr/local/share/man/man1/node.1
   
   sudo rm -rf /usr/local/lib/dtrace/node.d
   
   sudo rm -rf ~/.npm
   

Source: tonyMtz

downgrade node to 0.10.36

  sudo npm cache clean -f
  sudo npm install -g n
  sudo n 0.10.36

upgrade node to stable v

  sudo npm cache clean -f
  sudo npm install -g n
  sudo n stable

I'm not sure if it's because I had an old version (4.4.5), or if it's because I used the official installer, but most of the files referenced in other answers didn't exist on my system. I only had to remove the following:

~/.node-gyp
~/.node_repl_history
/usr/local/bin/node
/usr/local/bin/npm
/usr/local/include/node
/usr/local/lib/dtrace/node.d
/usr/local/lib/node_modules
/usr/local/share/doc/node
/usr/local/share/man/man1/node.1
/usr/local/share/systemtap/tapset/node.stp

I decided to keep ~/.npm because I was planning on reinstalling Node with Homebrew.

Complete uninstall Nodejs on macOS Big Sur version 11.2.3 (20D91)