E: Unable to locate package npm

Encountered this in Ubuntu for Windows, try running first

sudo apt-get update
sudo apt-get upgrade

then

sudo apt-get install npm

I ran into the same issue on Debian 9.2, this is what I did to overcome it.

Installation

sudo apt install curl
curl -sL https://deb.nodesource.com/setup_6.x | sudo bash -
sudo apt-get install -y nodejs
sudo apt-get install -y npm

Check installed versions

node --version
npm --version

Originally sourced from "How to install Node.js LTS on Debian 9 stretch" http://linuxbsdos.com/2017/06/26/how-to-install-node-js-lts-on-debian-9-stretch/

For Debian Stretch (Debian version 9), nodejs does not include npm, and to install it as a separate package, you have to enable stretch-backports.

echo 'deb http://deb.debian.org/debian stretch-backports main' |
sudo tee /etc/apt/sources.list.d/stretch-backports.list
apt-get update -y
apt-get -t stretch-backports install -y npm

In Buster (Debian 10), npm is a regular package, so going forward, this should just work. But some of us will still be stuck partially on Stretch boxes for some time to come.

in my jenkins/jenkins docker sudo always generates error:

bash: sudo: command not found

I needed update repo list with:

curl -sL https://deb.nodesource.com/setup_10.x | apt-get update

then,

 apt-get install nodejs

All the command line results like this:

root@76e6f92724d1:/# curl -sL https://deb.nodesource.com/setup_10.x | apt-get update
Ign:1 http://deb.debian.org/debian stretch InRelease
Get:2 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Get:3 http://deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:4 http://deb.debian.org/debian stretch Release [118 kB]
Get:5 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [520 kB]
Get:6 http://deb.debian.org/debian stretch-updates/main amd64 Packages [27.9 kB]
Get:8 http://deb.debian.org/debian stretch Release.gpg [2410 B]
Get:9 http://deb.debian.org/debian stretch/main amd64 Packages [7083 kB]
Get:7 https://packagecloud.io/github/git-lfs/debian stretch InRelease [23.2 kB]
Get:10 https://packagecloud.io/github/git-lfs/debian stretch/main amd64 Packages [4675 B]
Fetched 7965 kB in 20s (393 kB/s)
Reading package lists... Done
root@76e6f92724d1:/#  apt-get install nodejs
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  libicu57 libuv1
The following NEW packages will be installed:
  libicu57 libuv1 nodejs
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 11.2 MB of archives.
After this operation, 45.2 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://deb.debian.org/debian stretch/main amd64 libicu57 amd64 57.1-6+deb9u3 [7705 kB]
Get:2 http://deb.debian.org/debian stretch/main amd64 libuv1 amd64 1.9.1-3 [84.4 kB]
Get:3 http://deb.debian.org/debian stretch/main amd64 nodejs amd64 4.8.2~dfsg-1 [3440 kB]
Fetched 11.2 MB in 26s (418 kB/s)
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package libicu57:amd64.
(Reading database ... 12488 files and directories currently installed.)
Preparing to unpack .../libicu57_57.1-6+deb9u3_amd64.deb ...
Unpacking libicu57:amd64 (57.1-6+deb9u3) ...
Selecting previously unselected package libuv1:amd64.
Preparing to unpack .../libuv1_1.9.1-3_amd64.deb ...
Unpacking libuv1:amd64 (1.9.1-3) ...
Selecting previously unselected package nodejs.
Preparing to unpack .../nodejs_4.8.2~dfsg-1_amd64.deb ...
Unpacking nodejs (4.8.2~dfsg-1) ...
Setting up libuv1:amd64 (1.9.1-3) ...
Setting up libicu57:amd64 (57.1-6+deb9u3) ...
Processing triggers for libc-bin (2.24-11+deb9u4) ...
Setting up nodejs (4.8.2~dfsg-1) ...
update-alternatives: using /usr/bin/nodejs to provide /usr/bin/js (js) in auto mode

Download the the repository key with:

curl -s https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add -

Then setup the repository:

sudo sh -c "echo deb https://deb.nodesource.com/node_8.x cosmic main \
                 > /etc/apt/sources.list.d/nodesource.list"
sudo apt-get update
sudo apt-get install nodejs

I had a similar issue and this is what worked for me.

Add the NodeSource package signing key:

curl -sSL https://deb.nodesource.com/gpgkey/nodesource.gpg.key | sudo apt-key add -
# wget can also be used:
# wget --quiet -O - https://deb.nodesource.com/gpgkey/nodesource.gpg.key | sudo apt-key add -

Add the desired NodeSource repository:

# Replace with the branch of Node.js or io.js you want to install: node_6.x, node_12.x, etc...
VERSION=node_12.x
# The below command will set this correctly, but if lsb_release isn't available, you can set it manually:
# - For Debian distributions: jessie, sid, etc...
# - For Ubuntu distributions: xenial, bionic, etc...
# - For Debian or Ubuntu derived distributions your best option is to use the codename corresponding to the upstream release your distribution is based off. This is an advanced scenario and unsupported if your distribution is not listed as supported per earlier in this README.
DISTRO="$(lsb_release -s -c)"
echo "deb https://deb.nodesource.com/$VERSION $DISTRO main" | sudo tee /etc/apt/sources.list.d/nodesource.list
echo "deb-src https://deb.nodesource.com/$VERSION $DISTRO main" | sudo tee -a /etc/apt/sources.list.d/nodesource.list

Update package lists and install Node.js:

sudo apt-get update
sudo apt-get install nodejs

If you have installed nodejs, then you also have npm. Npm comes with node.