Explain npm scripts code

What this npm scripts sample shows

npm run

Each entry under scripts is a name and a shell command. npm run build runs the build line, and npm run on its own listed every script. start and test are special enough to have shortcuts: npm start and npm test work without run, while npm run dev needs it.

pre and post

A script named pre plus another script's name runs first. npm run build printed clean: removed dist/ from prebuild, then build: wrote dist/app.js. Each step is announced with a > line, such as > shop@1.2.0 prebuild.

local tools

npm puts the project's own node_modules/.bin first on the PATH while a script runs. After npm install added ESLint 9.39.5, typing eslint --version in the terminal gave eslint: command not found, but npm run lint found it and, for a file with one stray variable, reported 'unused' is assigned a value but never used.

chaining and exit codes

&& runs the next command only if the one before it succeeded. With a lint error, npm run check stopped after ESLint with exit code 1 and npm test never ran; with the error fixed, lint passed and the test ran, exit code 0. That exit code is what CI checks.

Good to know

• Run with npm 11.10.1 on Node 25.7.0. Before npm install, npm run lint stopped with eslint: command not found and exit code 127: listing a tool in devDependencies isn't enough until it's installed.
• npm passes the package's fields to scripts as environment variables: npm run hello printed shop v1.2.0 from $npm_package_name and $npm_package_version.
• npm run nope stopped with npm error Missing script: "nope" and pointed to npm run for the list.

Watch out: Extra arguments for the script go after --. npm run hello -- --loud ran echo ... --loud and printed shop v1.2.0 --loud, but npm test --watch without the -- gave the flag to npm itself: npm warn Unknown cli config "--watch", and node --test ran without it.