Explain JSON code

What this JSON sample shows

objects and arrays

The whole file is one JSON object: keys in double quotes, a colon, then a value, with commas between pairs. A value can be another object, like scripts and dependencies here, so each one groups its own named entries.

scripts

npm start runs the start line and npm test runs test. npm also runs a script called pre plus the name first, so npm test printed data/sales.csv found from pretest before the test ran. npm pkg get dependencies.dayjs reads one field back without opening the file.

version ranges

^5.5.0 allows any 5.x.x from 5.5.0 up, ~1.11.10 only 1.11.x patches, and 3.3.3 that exact version. On 5 Nov npm installed csv-parse 5.6.0 although 7.0.3 was out, dayjs 1.11.23, and prettier 3.3.3 while 3.9.10 was the latest.

strict syntax

JSON is stricter than JavaScript: no comments, no comma after the last item and only double quotes. One stray comma after "3.3.3" made every npm command stop with npm error code EJSONPARSE and Note: package.json must be actual JSON, not just JavaScript.

Good to know

• Run with npm 11.10.1 on Node 25.7.0: npm install added the 3 packages, npm start printed 2026-10 200.50 and 2026-11 42.25, and npm test passed 1 test.
• "type": "module" tells Node the .js files use import. With that line removed, Node 25 still ran the tool but printed a MODULE_TYPELESS_PACKAGE_JSON warning first, because it had to read each file twice to find out.
• engines is a request, not a lock. With "node": ">=26" on Node 25, npm install printed npm warn EBADENGINE Unsupported engine and installed everything anyway, exit code 0.

Watch out: devDependencies are for working on the project, not for using it. With import prettier from "prettier" added to bin/report.js, the tool ran after a full npm install but failed after npm install --omit=dev, which skips devDependencies, with Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'prettier'.