A script that can be sourced in shell scripts to enable nvm support

nvm-install

# shellcheck shell=sh
# https://gist.github.com/karfau/dcf98c6eefc2f2132c160f5c14d2112f

# needs to be sourced as part of your script
# 1. tries to configure nvm and run `nvm install`
# 2. checks if the node version is correct based on ./.nvmrc (`v` prefix not supported)
# if both doesn't work, exits with code 1 and some helpful messages

# Sometimes we prefer `nvm use` over `nvm install`
# you can basically put anything you want here, but the default is `install`
NVM_SETUP_COMMAND=${NVM_SETUP_COMMAND:-install}
NVM_DIR=${NVM_DIR:-$HOME}/.nvm}

# https://unix.stackexchange.com/a/184512/194420
# https://github.com/nvm-sh/nvm/issues/1290
if [ -f ${NVM_DIR}/nvm.sh ]; then
  echo "sourcing nvm from NVM_DIR:${NVM_DIR}"
  . ${NVM_DIR}/nvm.sh
elif command -v brew; then
  # https://docs.brew.sh/Manpage#--prefix-formula
  BREW_PREFIX=$(brew --prefix nvm)
  if [ -f "$BREW_PREFIX/nvm.sh" ]; then
    echo "sourcing nvm from brew ($BREW_PREFIX)"
    . $BREW_PREFIX/nvm.sh
  fi
fi

if command -v nvm ; then
  echo "NVM_SETUP_COMMAND is ${NVM_SETUP_COMMAND}"
  nvm ${NVM_SETUP_COMMAND}
else
  echo "WARN: not able to configure nvm"
fi

NODE_VERSION="v$(cat .nvmrc)"
which node
ACTIVE_VERSION=$(node --version)
GLOBAL_NPM=$(which npm || echo "not found on PATH")
# .nvmrc can contain only major or major.minor or full version
# so we replace active version with node version and anything afterwards
# if something is left, it's not a match
if [ "${ACTIVE_VERSION%%$NODE_VERSION*}" ] || [ ! -e "$GLOBAL_NPM" ]; then
  echo "expected node '$NODE_VERSION' and npm on path"
  echo "but was '$ACTIVE_VERSION' and npm:'$GLOBAL_NPM'"
  return 1
fi

Line 15 fails when the parent shell has set -u (error on unset variables)

I would suggest this:

@@ -9,6 +9,7 @@
 # Sometimes we prefer `nvm use` over `nvm install`
 # you can basically put anything you want here, but the default is `install`
 NVM_SETUP_COMMAND=${NVM_SETUP_COMMAND:-install}
+: ${NVM_DIR=$HOME/.nvm}
 
 # https://unix.stackexchange.com/a/184512/194420
 # https://github.com/nvm-sh/nvm/issues/1290

i.e. set a default. that default is implied by the last line of the nvm help message

Note:
to remove, delete, or uninstall nvm - just remove the $NVM_DIR folder (usually ~/.nvm)

Makes total sense, especially if nvm is installed via brew in which case NVM_DIR might not be set.
Update: changed it with a small tweak to keep the original value if it is present, in the same way it is done for NVM_INSTALL_COMMAND