Parsing and Stringifying

JSON.parse() converts a JSON string into a JavaScript object. JSON.stringify() does the opposite — it converts a JavaScript object into a JSON string. These two functions are the core of JSON handling in the browser and Node.js. Every other language has equivalents with the same semantics.

{"name": "Alice", "age": 30, "active": true}

const str = '{"name":"Alice","age":30,"active":true}';
const obj = JSON.parse(str);

console.log(obj.name);   // "Alice"
console.log(obj.age);    // 30
console.log(obj.active); // true

If the input is not valid JSON, JSON.parse() throws a SyntaxError with a message indicating the position of the problem. Always wrap it in try/catch when parsing untrusted input:

try {
  const data = JSON.parse(untrustedString);
} catch (err) {
  console.error("Invalid JSON:", err.message);
}

const user = { name: "Alice", age: 30, active: true };

// Compact (minified)
const compact = JSON.stringify(user);
// '{"name":"Alice","age":30,"active":true}'

// Pretty-printed with 2-space indent
const pretty = JSON.stringify(user, null, 2);

The output of JSON.stringify(user, null, 2):

{
  "name": "Alice",
  "age": 30,
  "active": true
}

JSON.stringify() silently drops properties it cannot represent in JSON:

• undefined values are omitted
• Functions are omitted
• Symbol keys are omitted
• NaN and Infinity become null

const obj = {
  name: "Alice",
  fn: () => {},
  score: NaN,
  limit: Infinity
};

JSON.stringify(obj);
// '{"name":"Alice","score":null,"limit":null}'

The second argument to JSON.stringify() can be an array of keys to include, or a function that transforms values:

const user = { name: "Alice", password: "secret", age: 30 };

// Include only specific keys
JSON.stringify(user, ["name", "age"]);
// '{"name":"Alice","age":30}'

The second argument to JSON.parse() is a reviver function — useful for converting date strings back to Date objects:

const str = '{"name":"Alice","createdAt":"2024-01-15T00:00:00Z"}';

const obj = JSON.parse(str, (key, value) => {
  if (key === "createdAt") return new Date(value);
  return value;
});

console.log(obj.createdAt instanceof Date); // true

• Python: json.loads() / json.dumps()
• PHP: json_decode() / json_encode()
• Go: json.Unmarshal() / json.Marshal()
• Java: Jackson ObjectMapper.readValue() / writeValueAsString()
• Ruby: JSON.parse() / JSON.generate()

Paste a JSON string into the JSON Formatter to visualize what JSON.parse() would produce. Run it through the JSON Validator first if you suspect the string may be malformed — it will catch the issue before your code throws a runtime error.