npm でError: Cannot find module 'are-we-there-yet' というエラーが出た．

$ npm

internal/modules/cjs/loader.js:796
    throw err;
    ^

Error: Cannot find module 'are-we-there-yet'
Require stack:
- /usr/local/lib/node_modules/npm/node_modules/npmlog/log.js
- /usr/local/lib/node_modules/npm/bin/npm-cli.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:793:17)
    at Function.Module._load (internal/modules/cjs/loader.js:686:27)
    at Module.require (internal/modules/cjs/loader.js:848:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at Object.<anonymous> (/usr/local/lib/node_modules/npm/node_modules/npmlog/log.js:2:16)
    at Module._compile (internal/modules/cjs/loader.js:955:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:991:10)
    at Module.load (internal/modules/cjs/loader.js:811:32)
    at Function.Module._load (internal/modules/cjs/loader.js:723:14)
    at Module.require (internal/modules/cjs/loader.js:848:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/usr/local/lib/node_modules/npm/node_modules/npmlog/log.js',
    '/usr/local/lib/node_modules/npm/bin/npm-cli.js'
  ]
}

「node.js をアップデートしたら、npm でエラー」を読む限り，npm のみが壊れているようなので，npm をすばやく葬り去って

cd /usr/local/lib/node_modules

rm -rf npm

公式のgithub「npm(1) -- a JavaScript package manager」に行って，

curl -L https://www.npmjs.com/install.sh | sh

ドキュメント通りnpm を入れ直して解決！

$ npm

Usage: npm <command>

where <command> is one of:
    access, adduser, audit, bin, bugs, c, cache, ci, cit,
    clean-install, clean-install-test, completion, config,
    create, ddp, dedupe, deprecate, dist-tag, docs, doctor,
    edit, explore, fund, get, help, help-search, hook, i, init,
    install, install-ci-test, install-test, it, link, list, ln,
    login, logout, ls, org, outdated, owner, pack, ping, prefix,
    profile, prune, publish, rb, rebuild, repo, restart, root,
    run, run-script, s, se, search, set, shrinkwrap, star,
    stars, start, stop, t, team, test, token, tst, un,
    uninstall, unpublish, unstar, up, update, v, version, view,
    whoami

npm <command> -h  quick help on <command>
npm -l            display full usage info
npm help <term>   search for help on <term>
npm help npm      involved overview

Specify configs in the ini-formatted file:
    /Users/kohei/.npmrc
or on the command line via: npm <command> --key value
Config info can be viewed via: npm help config

npm@6.13.6 /Users/whoami/.nodebrew/node/v12.14.1/lib/node_modules/npm