JavaScript Object Notation: Parse JSON and Inspect the Result

The task

Convert a fictional route record from text, increase its stop count, and serialize the update. Then test a trailing comma and an undefined field. The record is ordinary data, with no dates, methods or circular references.

Runnable Node 22 example

const assert = require('node:assert/strict');

const text = '{"route":"harbor","stops":3}';
const route = JSON.parse(text);
route.stops += 1;
const encoded = JSON.stringify(route);
assert.equal(typeof text, 'string');
assert.equal(route.stops, 4);
assert.equal(encoded, '{"route":"harbor","stops":4}');
assert.throws(() => JSON.parse('{"stops":3,}'), SyntaxError);
assert.equal(JSON.stringify({note: undefined, stops: 4}), '{"stops":4}');
assert.equal(JSON.stringify([undefined]), '[null]');
console.log(encoded);

Expected output and explanation

The input string stays text; parse creates the object that the program updates. Stringify produces a new string reflecting four stops. The assertions demonstrate the distinction rather than relying on the fact that both forms can look similar in a console.

The invalid-input assertion catches the trailing comma. JSON property names and string values need double quotes; JavaScript object-literal conveniences are not necessarily valid JSON. The final checks expose a lossy case: an undefined object property is omitted, while an undefined array position becomes null.

Pitfall to check

Do not parse untrusted text without planning for failure. A successful parse also does not prove the result has the fields or types your application requires. Validate the parsed record separately; this fixture assumes the supplied route schema.