A blazing fast JSON formatting library that pretty-prints JSON like strings

JSON.stringify(JSON.parse(str), null, 2) is fast — but it’s also lossy and strict:

• ❌ Breaks on BigInt: 12345678901234567890n, precision is lost.
• ⚙️ Loses numeric precision: 1.2300 becomes 1.23, zeroes are dropped.
• 🚫 Fails on imperfect JSON: Minor syntax issues in “JSON-like” strings can crash it.

fast-json-format aims to pretty-print without losing data or precision, while staying lightweight and forgiving.
It preserves BigInt literals, decimal formatting, and handles malformed input gracefully

• 🔧 Handles invalid/malformed JSON gracefully
• 📦 Works with BigInt literals
• 🎨 Custom indentation support
• 🪶 Lightweight - single file, zero dependencies
• ✅ Thoroughly tested

npm install fast-json-format

const fastJsonFormat = require('fast-json-format');

const minified = '{"name":"John","age":30,"city":"New York"}';
const formatted = fastJsonFormat(minified);

console.log(formatted);
// {
//   "name": "John",
//   "age": 30,
//   "city": "New York"
// }

// Use 4 spaces
const formatted = fastJsonFormat(jsonString, '    ');

Run benchmarks yourself:

npm run benchmark

JSON.stringify is inherently faster (as it’s native and C++-optimized) Performance improvements are welcome :)

Size         │ fast-json-format     │ jsonc-parser         │ json-bigint          │ lossless-json        │ JSON.stringify
─────────────┼──────────────────────┼──────────────────────┼──────────────────────┼──────────────────────┼─────────────────────
100 KB       │ 1839 ops/sec         │ 1265 ops/sec         │ 1053 ops/sec         │ 886 ops/sec          │ 3025 ops/sec
1 MB         │ 178 ops/sec          │ 125 ops/sec          │ 98 ops/sec           │ 61 ops/sec           │ 296 ops/sec
5 MB         │ 28 ops/sec           │ 21 ops/sec           │ 18 ops/sec           │ 9 ops/sec            │ 58 ops/sec
10 MB        │ 15 ops/sec           │ 11 ops/sec           │ 9 ops/sec            │ 4 ops/sec            │ 30 ops/sec          

npm test

MIT License - Copyright (c) Bruno Software Inc.

Issues and pull requests are welcome on the project repository.