JSON

JavaScript · Reference cheat sheet

📋 Overview

JSON is a text data format. JSON.parse and JSON.stringify convert between strings and plain data (objects, arrays, numbers, strings, booleans, null). Functions, undefined, Symbols, and many class instances need custom handling.

🔧 Core concepts

• JSON.parse(text, reviver?): string → value; throws SyntaxError on bad input.
• JSON.stringify(value, replacer?, space?): value → string; omits undefined in objects.
• Reviver / replacer: transform keys during parse/stringify.
• toJSON(): objects can customize serialization.
• Safe parse: wrap in try/catch; validate shape after parse.

const obj = JSON.parse('{"a":1}');
const text = JSON.stringify(obj, null, 2);

💡 Examples

// Pretty print
JSON.stringify({ user: "Ada", ok: true }, null, 2);

// Replacer allowlist
JSON.stringify(user, ["id", "name"]);

// Reviver for dates
const parsed = JSON.parse(payload, (key, value) => {
  if (key.endsWith("At") && typeof value === "string") {
    const d = new Date(value);
    if (!Number.isNaN(d.getTime())) return d;
  }
  return value;
});

// toJSON
const money = {
  amount: 10,
  currency: "USD",
  toJSON() {
    return `${this.amount} ${this.currency}`;
  },
};
JSON.stringify({ price: money }); // {"price":"10 USD"}

// Map ↔ JSON
function mapToJson(map) {
  return JSON.stringify([...map.entries()]);
}
function jsonToMap(text) {
  return new Map(JSON.parse(text));
}

// Deep clone via JSON (limitations!)
const clone = JSON.parse(JSON.stringify(data));

function safeParse(text, fallback = null) {
  try {
    return JSON.parse(text);
  } catch {
    return fallback;
  }
}

⚠️ Pitfalls

• JSON.stringify drops undefined, functions, and Symbols in objects; turns them to null in arrays for undefined/functions.
• NaN / Infinity become null.
• Date becomes ISO string; Map/Set become \{\} unless converted.
• Circular structures throw TypeError.
• JSON clone loses prototypes, methods, and undefined keys.

🔗 Related

• objects.md — plain objects
• arrays.md — JSON arrays
• fetch.md — JSON HTTP bodies
• date.md — Date ↔ ISO
• map.md — serializing Maps
• error.md — SyntaxError on parse
• strings.md — text handling