zsh-node_modules-bin-update-path() {
  if [[ ! -x $(which npm) ]]; then
    # there is no npm
    return
  fi

  # figure out the path
  NPM_BIN_PATH=$(npm bin)

  if [[ ! -d $NPM_BIN_PATH ]]; then
    # there is no dir at bin path
    return
  fi

  if [[ -n $OLD_NPM_BIN_PATH && $NPM_BIN_PATH == $OLD_NPM_BIN_PATH ]]; then
    # bin path did not change
    return
  fi

  if [[ -n $OLD_NPM_BIN_PATH ]]; then
    # remove old bin path from $path
    path[$path[(i)$OLD_NPM_BIN_PATH]]=()
  fi

  # add new bin path to $path
  path=($NPM_BIN_PATH $path)
  export PATH

  # for next run
  export OLD_NPM_BIN_PATH=$NPM_BIN_PATH
}

precmd_functions+="zsh-node_modules-bin-update-path"