jarle/Bun JSON5

JSON5

Use Bun's built-in support for JSON5 files through both runtime APIs and bundler integration

In Bun, JSON5 is a first-class citizen alongside JSON, TOML, and YAML. You can:

• Parse and stringify JSON5 with Bun.JSON5.parse and Bun.JSON5.stringify
• import & require JSON5 files as modules at runtime (including hot reloading & watch mode support)
• import & require JSON5 files in frontend apps via Bun's bundler

---

Conformance

Bun's JSON5 parser passes 100% of the official JSON5 test suite. The parser is written in Zig for optimal performance. You can view our translated test suite to see every test case.

---

Runtime API

Bun.JSON5.parse()

Parse a JSON5 string into a JavaScript value.

import { JSON5 } from "bun";

const data = JSON5.parse(`{
  // JSON5 supports comments
  name: 'my-app',
  version: '1.0.0',
  debug: true,

  // trailing commas are allowed
  tags: ['web', 'api',],
}`);

console.log(data);
// {
//   name: "my-app",
//   version: "1.0.0",
//   debug: true,
//   tags: ["web", "api"]
// }

Supported JSON5 Features

JSON5 is a superset of JSON based on ECMAScript 5.1 syntax. It supports:

• Comments: single-line (//) and multi-line (/* */)
• Trailing commas: in objects and arrays
• Unquoted keys: valid ECMAScript 5.1 identifiers can be used as keys
• Single-quoted strings: in addition to double-quoted strings
• Multi-line strings: using backslash line continuations
• Hex numbers: 0xFF
• Leading & trailing decimal points: .5 and 5.
• Infinity and NaN: positive and negative
• Explicit plus sign: +42

const data = JSON5.parse(`{
  // Unquoted keys
  unquoted: 'keys work',

  // Single and double quotes
  single: 'single-quoted',
  double: "double-quoted",

  // Trailing commas
  trailing: 'comma',

  // Special numbers
  hex: 0xDEADbeef,
  half: .5,
  to: Infinity,
  nan: NaN,

  // Multi-line strings
  multiline: 'line 1 \
line 2',
}`);

Error Handling

Bun.JSON5.parse() throws a SyntaxError if the input is invalid JSON5:

try {
  JSON5.parse("{invalid}");
} catch (error) {
  console.error("Failed to parse JSON5:", error.message);
}

Bun.JSON5.stringify()

Stringify a JavaScript value to a JSON5 string.

import { JSON5 } from "bun";

const str = JSON5.stringify({ name: "my-app", version: "1.0.0" });
console.log(str);
// {name:'my-app',version:'1.0.0'}

Pretty Printing

Pass a space argument to format the output with indentation:

const pretty = JSON5.stringify(
  {
    name: "my-app",
    debug: true,
    tags: ["web", "api"],
  },
  null,
  2,
);

console.log(pretty);
// {
//   name: 'my-app',
//   debug: true,
//   tags: [
//     'web',
//     'api',
//   ],
// }

The space argument can be a number (number of spaces) or a string (used as the indent character):

// Tab indentation
JSON5.stringify(data, null, "\t");

Special Values

Unlike JSON.stringify, JSON5.stringify preserves special numeric values:

JSON5.stringify({ inf: Infinity, ninf: -Infinity, nan: NaN });
// {inf:Infinity,ninf:-Infinity,nan:NaN}

---

Module Import

ES Modules

You can import JSON5 files directly as ES modules:

{
  // Database configuration
  database: {
    host: "localhost",
    port: 5432,
    name: "myapp",
  },

  features: {
    auth: true,
    rateLimit: true,
    analytics: false,
  },
}

Default Import

import config from "./config.json5";

console.log(config.database.host); // "localhost"
console.log(config.features.auth); // true

Named Imports

You can destructure top-level properties as named imports:

import { database, features } from "./config.json5";

console.log(database.host); // "localhost"
console.log(features.rateLimit); // true

CommonJS

JSON5 files can also be required in CommonJS:

const config = require("./config.json5");
console.log(config.database.name); // "myapp"

// Destructuring also works
const { database, features } = require("./config.json5");

---

Hot Reloading with JSON5

When you run your application with bun --hot, changes to JSON5 files are automatically detected and reloaded:

{
  server: {
    port: 3000,
    host: "localhost",
  },
  features: {
    debug: true,
    verbose: false,
  },
}

import { server, features } from "./config.json5";

Bun.serve({
  port: server.port,
  hostname: server.host,
  fetch(req) {
    if (features.verbose) {
      console.log(`${req.method} ${req.url}`);
    }
    return new Response("Hello World");
  },
});

Run with hot reloading:

bun --hot server.ts

---

Bundler Integration

When you import JSON5 files and bundle with Bun, the JSON5 is parsed at build time and included as a JavaScript module:

bun build app.ts --outdir=dist

This means:

• Zero runtime JSON5 parsing overhead in production
• Smaller bundle sizes
• Tree-shaking support for unused properties (named imports)

Dynamic Imports

JSON5 files can be dynamically imported:

const config = await import("./config.json5");