How to install only NPM dev dependencies

There are some situations where you wish to only install the devDependencies from the package.json. We encountered this recently where we set Playwright as a development dependency and wished to install the specified version in a CI scenario.

TL:DR;

You can install just the dev dependencies from your package.json with the following command (provinding you have node installed)

mv package.json _package.json && npm i $(node -e "let d=require('./_package.json').devDependencies,o='';Object.keys(d).forEach((e=>{o+=e+'@'+d[e]+' '})),console.log(o);") && mv _package.json package.json

It seems npm used to have an install --only=dev flag, however it seems this fell foul of a bug in version 6 and has never been fixed. NPM is also rather eager, in that if you specify an npm i <package> when you have package-lock.json, it will install everything listed in there, as well as the package you have specified (I was expecting it to install only the specified package).

What I needed was a short way of installing the devDependencies from my package.json (for speed & disk space savings).

Using the node CLI, I was able to construct a script which parsed the package.json and extracted the development dependencies and their versions:

let dependencies = require('./package.json').devDependencies,
	output  = '';
Object.keys(dependencies).forEach(key => {
	output += key + '@' + dependencies[key] + ' ';
})
console.log(output);

The problem was that piping this into a npm install also installed all the other dependencies, so before this runs you need to move the package.json file, run the script and then move the JSON file back.

If you use the code above and update the first line to read from _package.json, you could run the following (assumed you've saved the file as dev-deps-load.js)

mv package.json _package.json
node dev-deps-load.js

You can then copy the output and paste after an npm install.

This can simplify this further by minifying the JavaScript and passing that directly to the npm install command (without the need of an extra file). Note the addition of --no-save - this prevents your package-lock.json file from being updated.

mv package.json _package.json
npm install --no-save $(node -e "let d=require('./_package.json').devDependencies,o='';Object.keys(d).forEach((e=>{o+=e+'@'+d[e]+' '})),console.log(o);")
mv _package.json package.json

The above is what we now run in our CI pipelines when wanting to run Playwright tests against production. However, you could condense it to one line if desired:

mv package.json _package.json && npm i --no-save $(node -e "let d=require('./_package.json').devDependencies,o='';Object.keys(d).forEach((e=>{o+=e+'@'+d[e]+' '})),console.log(o);") && mv _package.json package.json