How do you run a js file using npm scripts?

You should use npm run-script build or npm build <project_folder>. More info here: https://docs.npmjs.com/cli/build.

{ "scripts" :
  { "build": "node build.js"}
}

npm run build OR npm run-script build

{
  "name": "build",
  "version": "1.0.0",
  "scripts": {
    "start": "node build.js"
  }
}

npm start

NB: you were missing the { brackets } and the node command

folder structure is fine:

+ build
  - package.json
  - build.js

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