JSON.stringify()

What JSON.stringify() does

The method walks a JavaScript value and emits UTF‑16 JSON text. Only JSON‑representable data survives: for example Date becomes an ISO‑8601 string, functions and symbols are normally skipped, and BigInt throws unless you convert it yourself.

Syntax

JSON.stringify(value[, replacer[, space]])

By default, objects stringify to a compact single line with no indentation. Key order follows the object’s own enumerable string keys as enumerated by the engine (do not rely on key order across runtimes for protocol semantics).

const obj = { name: "John", age: 30, city: "New York" };
const jsonString = JSON.stringify(obj);
console.log(jsonString);

{"name":"John","age":30,"city":"New York"}

When the replacer returns undefined for a nested property, that property is omitted from the JSON string. Here we drop every string-valued field and keep numbers.

const obj = { name: "John", age: 30, city: "New York" };

function replacer(key, value) {
  if (typeof value === "string") {
    return undefined;
  }
  return value;
}

const jsonString = JSON.stringify(obj, replacer);
console.log(jsonString);

{"age":30}

Here every string property is dropped; numbers remain.

When replacer is an array, only listed keys are serialized (in the order they appear in the array).

const obj = { name: "John", age: 30, city: "New York" };
const jsonString = JSON.stringify(obj, ["name", "city"]);
console.log(jsonString);

{"name":"John","city":"New York"}

Pass a number (max effective 10) or a string as the third argument to insert indentation and line breaks. Use 0 or omit the argument for compact output in production logs and APIs.

const obj = { name: "John", age: 30, city: "New York" };
const jsonString = JSON.stringify(obj, null, 2);
console.log(jsonString);

{
  "name": "John",
  "age": 30,
  "city": "New York"
}

JSON is a tree, not a general graph. If an object appears twice on the same path through references, JSON.stringify throws TypeError (message text varies by engine).

const obj = {};
obj.self = obj;

try {
  JSON.stringify(obj);
} catch (error) {
  console.log(error.name, error.message);
}

TypeError Converting circular structure to JSON

Mitigations: design APIs around acyclic DTOs; use a custom replacer that replaces already‑seen objects with null or an id; use a small library such as flatted for lossless cycle encoding; or log a shallow copy for debugging.