The NodeJS buildpack detects if your build is Node by looking for a package.json file. If found, the build will execute the following NPM (or Yarn) commands.

npm install
npm build

If no Procfile is found, a web process will be started with npm start .

You can customize the node version using the engines field of your package.json. For example

"engines": {
"node": ">=0.10.3 <15"