JSON.stringify(value, replacer?, space?)

• Converts a JavaScript object into a JSON string.
• Parameters:
  • value: The object or value to convert.
  • replacer (optional): A function or array to filter properties.
  • space (optional): Indentation level for formatting.

const obj = { name: "Alice", age: 25 };
console.log(JSON.stringify(obj));
// '{"name":"Alice","age":25}'

// Pretty-print with indentation
console.log(JSON.stringify(obj, null, 2));
/*
{
  "name": "Alice",
  "age": 25
}
*/

Filtering properties with replacer

• replacer can be a function or an array that filters which properties are included.

const user = { name: "Alice", age: 25, password: "secret" };

// Exclude `password`
console.log(JSON.stringify(user, ["name", "age"])); 
// '{"name":"Alice","age":25}'

console.log(
  JSON.stringify(user, (key, value) => key === "password" ? undefined : value)
);
// '{"name":"Alice","age":25}'

JSON.parse(jsonString, reviver?)

• Converts a JSON string into a JavaScript object.
• Parameters:
  • jsonString: The string to parse.
  • reviver (optional): A function to transform the result.

const jsonString = '{"name":"Alice","age":25}';
const obj = JSON.parse(jsonString);
console.log(obj.name); // "Alice"

Transforming values with reviver

• reviver allows modification of parsed values.

const data = '{"date":"2024-03-18T12:00:00.000Z"}';

const parsed = JSON.parse(data, (key, value) => 
  key === "date" ? new Date(value) : value
);

console.log(parsed.date instanceof Date); // true
console.log(parsed.date.getFullYear()); // 2024

Common Use Cases

Deep Copy an Object

JSON.stringify() + JSON.parse() is a simple way to create a deep copy:

const obj = { a: 1, b: { c: 2 } };
const deepCopy = JSON.parse(JSON.stringify(obj));

deepCopy.b.c = 42;
console.log(obj.b.c); // 2 (original object remains unchanged)

Does not preserve undefined, functions, Symbol, Map, Set, RegExp, or BigInt.

Handling Circular References