Validate JSON before using it

WHAT YOU WILL BUILD

A validator that separates parsing from type, range, and allowed-field checks.

JSON syntax only tells you whether a payload can be parsed. An application also needs a contract: required fields, types, ranges, and how extra fields are handled. A well-formed answer can still contain false facts.

FOLLOW ALONG

Work through the example.

1. 01

   Define the contract

   Accept exactly two fields: answer as a string up to 200 characters and days as an integer from 0 to 365. In this exercise, unknown values must follow a separate application path.

2. 02

   Run all four responses

   Only the first response passes. The others contain a string where an integer is expected, an unexpected execute field, or prose outside the JSON.

3. 03

   Add boundary cases

   Test null, an array, days=-1, days=366, and a 201-character answer. Add explicit expected failures for each input rather than relying on the four original cases.

4. 04

   Check meaning after structure

   Change a valid response to days=90. It passes this contract but contradicts the sample 30-day policy. Keep a separate evidence or business-rule check before using the value.

THE COMPLETE LAB

Run it locally.

Run this command from the folder containing your downloaded file:

node prompt-structured-output.mjs

// SaveMyToken local lab. Run with Node.js 22 or newer.
import assert from "node:assert/strict";

function validate(raw) {
  try {
    const value = JSON.parse(raw);
    return value !== null && typeof value === "object" && !Array.isArray(value)
      && Object.keys(value).length === 2
      && typeof value.answer === "string" && value.answer.length <= 200
      && Number.isInteger(value.days) && value.days >= 0 && value.days <= 365;
  } catch { return false; }
}
const inputs = [
  '{"answer":"Unused items only","days":30}',
  '{"answer":"Unused items only","days":"30"}',
  '{"answer":"Unused items only","days":30,"execute":"refund"}',
  'Here is your JSON: {"days":30}',
];
const results = inputs.map(validate);
assert.deepEqual(results, [true, false, false, false]);
console.log("Valid responses:", results.join(", "));

Expected output for the unchanged example

Valid responses: true, false, false, false

The assertions also check the baseline behavior. After changing an input, predict the result and update the relevant assertion.

EXPLAIN WHAT YOU LEARNED

Interview practice

Try answering aloud before opening the reference answer. These are original learning questions, not a record of any employer's interviews.