JSON & Data Exchange

JSON Basics

JSON (JavaScript Object Notation) is the universal data exchange format. It uses double-quoted keys and supports strings, numbers, booleans, null, objects, and arrays:

const json = '{"name":"Alice","age":30,"hobbies":["JS","hiking"]}';
const obj = JSON.parse(json);
console.log(obj.name);       // "Alice"
console.log(obj.hobbies[1]); // "hiking"

const backToJson = JSON.stringify(obj);
console.log(backToJson);

JSON.stringify Options

const data = { name:"Alice", score:95, password:"secret" };

// Pretty-print with 2-space indent
console.log(JSON.stringify(data, null, 2));

// Replacer — exclude sensitive fields
const safe = JSON.stringify(data, ["name","score"]);
console.log(safe); // {"name":"Alice","score":95}

Reviver Function

Transform values as they are parsed:

const raw = '{"created":"2026-01-15","amount":"1299.99"}';

const parsed = JSON.parse(raw, (key, value) => {
  if (key === "created") return new Date(value);
  if (key === "amount")  return parseFloat(value);
  return value;
});

console.log(parsed.created instanceof Date); // true
console.log(parsed.amount + 1);             // 1300.99

Deep Clone with JSON

A quick (but imperfect) way to deep-copy plain objects:

const original = { a:1, b: { c: [1,2,3] } };
const clone = JSON.parse(JSON.stringify(original));
clone.b.c.push(4);

console.log(original.b.c); // [1,2,3] — unaffected
console.log(clone.b.c);    // [1,2,3,4]

// Note: loses undefined, functions, Date objects, etc.

structuredClone (Modern)

The modern way to deep-clone — handles Date, Map, Set, etc.:

const obj = { date: new Date(), scores: new Map([["Alice",95]]) };
const clone = structuredClone(obj);

clone.date.setFullYear(2000);
console.log(obj.date.getFullYear()); // 2026 — unaffected