The best way to run npm install for nested folders?

Per @Scott's answer, the install|postinstall script is the simplest way as long as sub-directory names are known. This is how I run it for multiple sub dirs. For example, pretend we have api/, web/ and shared/ sub-projects in a monorepo root:

// In monorepo root package.json
{
...
 "scripts": {
    "postinstall": "(cd api && npm install); (cd web && npm install); (cd shared && npm install)"
  },
}

On Windows, replace the ; between the parentesis with &&.

// In monorepo root package.json
{
...
 "scripts": {
    "postinstall": "(cd api && npm install) && (cd web && npm install) && (cd shared && npm install)"
  },
}

I prefer using post-install, if you know the names of the nested subdir. In package.json:

"scripts": {
  "postinstall": "cd nested_dir && npm install",
  ...
}