NPM: Why is this package installed?

As you mention, npm ls shows packages and their dependencies:

> npm ls leveldown
appless@5.0.0 C:\Users\mikem\Code\appless
`-- @architect/architect@5.7.0
  `-- dynalite@2.2.0
    `-- UNMET OPTIONAL DEPENDENCY leveldown@4.0.2

If npm ls shows it at the top level, and it is not a dependency in the top level package.json, it's likely was previously required and is now no longer used.

Use npm prune to remove the unused package.

npm explain <package name> is what you're looking for. It explains why a package is in your node_modules folder by showing a "bottoms up" view. See docs here

There's a module called npm-why which identifies why a package has been installed.

Of course, if you're using yarn, you have a built-in command yarn why.

If you can't find a require or import, try looking at child package.jsons to see who else needs it.

(Note: find requires Linux/macOS, this won't work on Windows)

find . -name package.json -exec grep -l babelify /dev/null {} \;

./node_modules/browserify-zlib/package.json
./node_modules/cssnext/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/reporter/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/@cypress/browserify-preprocessor/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/async/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/babel-core/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/babelify/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/getos/node_modules/async/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/object-assign/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/node_modules/watchify/node_modules/browserify-zlib/package.json
./node_modules/cypress/dist/Cypress.app/Contents/Resources/app/packages/server/package.json
./node_modules/eslint/package.json
./node_modules/extract-text-webpack-plugin/node_modules/async/package.json
./node_modules/getos/node_modules/async/package.json
./node_modules/postcss-modules-extract-imports/package.json
./node_modules/postcss-modules-scope/package.json
./node_modules/webpack/node_modules/async/package.json

My one-liner, based on other answers: npm ls | grep -C 10 PACKAGE

Replace PACKAGE with the package you're looking for. This is simpler and faster than other suggestions. The shell is your friend, friends!

Breakdown/Explanation

• npm ls - As explained above, this prints the dependency tree representation of your app.
• | - The secret sauce of *nix shells. This sends the output to the next program, instead of printing it.
• grep [...] PACKAGE - Search for the string "PACKAGE" (This is actually regex, but that's irrelevant.)
• -C 10 - This tells grep to print 10 extra lines around the matching lines. Without this, grep would only print the lines were PACKAGE was found. Increasing this number gives you more context. If -C doesn't work on your system (less common linux versions), try using -B 10 -A 10 instead. It's a more verbose way of doing the same thing: "10 lines before, 10 lines after."