JS JSON

JavaScript JSON: Serializing and Parsing Data Safely

JSON (JavaScript Object Notation) is a lightweight, language-independent text format for storing and exchanging structured data across APIs. JavaScript gives you two built-in helpers: JSON.stringify() to turn objects into JSON text and JSON.parse() to turn JSON text back into JavaScript values.

Valid JSON Syntax Rules vs. JS Objects

JSON looks like a JavaScript object literal, yet JSON syntax is much stricter:

• Double quotes are required: Every key and string value must use "double quotes", never 'single quotes'.
• No trailing commas or comments: { "a": 1, } or // comments will fail JSON.parse().
• Unsupported types are dropped or converted: JSON.stringify() strips undefined properties and functions, turns NaN and Infinity into null, and throws on BigInt or circular references.

Practical Serialization and Error-Safe Parsing

Pass null, 2 to JSON.stringify() for readable two-space indentation when logging or saving config files, and always wrap JSON.parse() in try...catch.

const payload = {
  id: 42,
  tags: ["js", "json"],
  createdAt: new Date("2026-11-12T10:00:00Z"),
  helper: () => "ignored", // Stripped by JSON.stringify
};

const jsonText = JSON.stringify(payload, null, 2);
console.log("Serialized JSON:
" + jsonText);

function parseSafely(raw) {
  try {
    return JSON.parse(raw);
  } catch (err) {
    console.error("Invalid JSON:", err.message);
    return null;
  }
}

console.log("Parsed object:", parseSafely(jsonText));

Common Mistakes to Avoid

• Calling JSON.parse() without try...catch: Malformed localStorage strings or broken API text will throw a SyntaxError and crash your UI.
• Expecting Date strings to revive automatically: JSON.parse() returns ISO date strings as plain strings, not Date instances; convert them back with new Date(obj.createdAt).
• Using JSON.parse(JSON.stringify(obj)) for cloning everything: It silently drops undefined, functions, Map, Set, and breaks on BigInt. Prefer structuredClone(obj) for deep cloning.

Frequently Asked Questions

What is a reviver function in JSON.parse()?

The optional second parameter (key, value) => ... transforms parsed values as they load—ideal for turning ISO timestamp strings back into Date objects.

Can JSON.stringify() filter which keys get serialized?

Yes, pass an array of allowed property names as the second argument, such as JSON.stringify(user, ["id", "name"]), to exclude sensitive fields like passwordHash.

Why does JSON.stringify({ x: undefined }) return "{}"?

JSON has no undefined type, so JSON.stringify() omits object properties whose value is undefined (and converts undefined inside arrays to null).