How do I pretty-print JSON in JavaScript?

Use JSON.stringify(data, null, 2) where the third argument is the number of spaces to use for indentation. This produces human-readable JSON with each property on its own line, indented by 2 spaces per level.

What does JSON.stringify's second argument (replacer) do?

The replacer can be a function or an array. A function receives each key-value pair and can transform or omit values. An array of strings specifies which keys to include in the output, filtering out everything else.

How do I format JSON already stored as a string?

Parse the string first, then re-stringify with indentation: JSON.stringify(JSON.parse(jsonString), null, 2). This round-trips the JSON and applies pretty-printing.

How to Format JSON in JavaScript – JSON.stringify with Indent

Formatting JSON in JavaScript comes down to one function: JSON.stringify(). Its third argument controls indentation and transforms compact JSON into human-readable output. This article covers everything from the basic pretty-print call to advanced replacer patterns.

The basic pretty-print

const data = { name: "Alice", age: 30, roles: ["admin", "editor"] };

// Compact (default)
JSON.stringify(data);
// '{"name":"Alice","age":30,"roles":["admin","editor"]}'

// Pretty-printed with 2-space indent
JSON.stringify(data, null, 2);
// '{
//   "name": "Alice",
//   "age": 30,
//   "roles": [
//     "admin",
//     "editor"
//   ]
// }'

The three arguments are: the value to serialize, the replacer (pass null to skip), and the space argument. The space argument can be a number (spaces per level) or a string (used as the indent character, e.g., "\t" for tabs).

Using a tab indent

JSON.stringify(data, null, "\t");
// Uses tab characters instead of spaces

Reformatting an existing JSON string

If you have a minified JSON string and want to pretty-print it, parse it first:

const minified = '{"name":"Alice","age":30}';
const pretty = JSON.stringify(JSON.parse(minified), null, 2);
console.log(pretty);

This is the same operation that the online JSON Formatter performs in the browser.

The replacer: filtering and transforming keys

The second argument to JSON.stringify() lets you control what gets included in the output.

Array replacer — whitelist specific keys

const user = { id: 1, name: "Alice", password: "secret", age: 30 };

JSON.stringify(user, ["id", "name", "age"], 2);
// Only includes "id", "name", and "age" — "password" is excluded

Function replacer — transform or omit values

JSON.stringify(user, (key, value) => {
    if (key === "password") return undefined; // omit
    if (typeof value === "number") return value * 2; // transform
    return value;
}, 2);

Returning undefined from the replacer omits the property from the output.

Logging formatted JSON to the console

A common use case is debugging — logging a complex object in a readable way:

console.log(JSON.stringify(response.data, null, 2));

Note that console.log(obj) already provides interactive tree inspection in browser DevTools. The stringify approach is more useful when you want to copy the output, write it to a file, or compare it as a plain string.

Writing formatted JSON to a file in Node.js

import fs from 'fs';

const data = { name: "config", version: 1 };
fs.writeFileSync('config.json', JSON.stringify(data, null, 2));

Always use an indent of 2 or 4 for JSON files committed to version control — it makes diffs readable.

Handling special values

A few JavaScript values are not supported in JSON and are handled specially by stringify:

undefined — object properties are dropped; array elements become null
NaN and Infinity — become null
Circular references — throw TypeError

For more on these edge cases, see undefined Is Not Valid in JSON and JSON NaN and Infinity Are Not Valid.

Quick online formatting

For one-off formatting without writing code, paste your JSON into the JSON Formatter. It pretty-prints the structure with syntax highlighting and collapses nested nodes for navigation.