YAML – API | Bun Docs

In Bun, YAML is a first-class citizen alongside JSON and TOML.

Bun provides built-in support for YAML files through both runtime APIs and bundler integration. You can

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

Conformance

Bun's YAML parser currently passes over 90% of the official YAML test suite. While we're actively working on reaching 100% conformance, the current implementation covers the vast majority of real-world use cases. The parser is written in Zig for optimal performance and is continuously being improved.

Runtime API

`Bun.YAML.parse()`

Parse a YAML string into a JavaScript object.

import { YAML } from "bun";
const text = `
name: John Doe
age: 30
email: john@example.com
hobbies:
  - reading
  - coding
  - hiking
`;

const data = YAML.parse(text);
console.log(data);
// {
//   name: "John Doe",
//   age: 30,
//   email: "john@example.com",
//   hobbies: ["reading", "coding", "hiking"]
// }

Multi-document YAML

When parsing YAML with multiple documents (separated by ---), Bun.YAML.parse() returns an array:

const multiDoc = `
---
name: Document 1
---
name: Document 2
---
name: Document 3
`;

const docs = Bun.YAML.parse(multiDoc);
console.log(docs);
// [
//   { name: "Document 1" },
//   { name: "Document 2" },
//   { name: "Document 3" }
// ]

Supported YAML Features

Bun's YAML parser supports the full YAML 1.2 specification, including:

Scalars: strings, numbers, booleans, null values
Collections: sequences (arrays) and mappings (objects)
Anchors and Aliases: reusable nodes with & and *
Tags: type hints like !!str, !!int, !!float, !!bool, !!null
Multi-line strings: literal (|) and folded (>) scalars
Comments: using #
Directives: %YAML and %TAG

const yaml = `
# Employee record
employee: &emp
  name: Jane Smith
  department: Engineering
  skills:
    - JavaScript
    - TypeScript
    - React

manager: *emp  # Reference to employee

config: !!str 123  # Explicit string type

description: |
  This is a multi-line
  literal string that preserves
  line breaks and spacing.

summary: >
  This is a folded string
  that joins lines with spaces
  unless there are blank lines.
`;

const data = Bun.YAML.parse(yaml);

Error Handling

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

try {
  Bun.YAML.parse("invalid: yaml: content:");
} catch (error) {
  console.error("Failed to parse YAML:", error.message);
}

Module Import

ES Modules

You can import YAML files directly as ES modules. The YAML content is parsed and made available as both default and named exports:

config.yaml
database:
  host: localhost
  port: 5432
  name: myapp

redis:
  host: localhost
  port: 6379

features:
  auth: true
  rateLimit: true
  analytics: false

Default Import

app.ts
import config from "./config.yaml";

console.log(config.database.host); // "localhost"
console.log(config.redis.port); // 6379

Named Imports

You can destructure top-level YAML properties as named imports:

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

console.log(database.host); // "localhost"
console.log(redis.port); // 6379
console.log(features.auth); // true

Or combine both:

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

// Use the full config object
console.log(config);

// Or use specific parts
if (features.rateLimit) {
  setupRateLimiting(database);
}

CommonJS

YAML files can also be required in CommonJS:

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

// Destructuring also works
const { database, redis } = require("./config.yaml");
console.log(database.port); // 5432

Hot Reloading with YAML

One of the most powerful features of Bun's YAML support is hot reloading. When you run your application with bun --hot, changes to YAML files are automatically detected and reloaded without closing connections

Configuration Hot Reloading

config.yaml
server:
  port: 3000
  host: localhost

features:
  debug: true
  verbose: false

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

console.log(`Starting server on ${server.host}:${server.port}`);

if (features.debug) {
  console.log("Debug mode enabled");
}

// Your server code here
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

Now when you modify config.yaml, the changes are immediately reflected in your running application. This is perfect for:

Adjusting configuration during development
Testing different settings without restarts
Live debugging with configuration changes
Feature flag toggling

Configuration Management

Environment-Based Configuration

YAML excels at managing configuration across different environments:

config.yaml
defaults: &defaults
  timeout: 5000
  retries: 3
  cache:
    enabled: true
    ttl: 3600

development:
  <<: *defaults
  api:
    url: http://localhost:4000
    key: dev_key_12345
  logging:
    level: debug
    pretty: true

staging:
  <<: *defaults
  api:
    url: https://staging-api.example.com
    key: ${STAGING_API_KEY}
  logging:
    level: info
    pretty: false

production:
  <<: *defaults
  api:
    url: https://api.example.com
    key: ${PROD_API_KEY}
  cache:
    enabled: true
    ttl: 86400
  logging:
    level: error
    pretty: false

app.ts
import configs from "./config.yaml";

const env = process.env.NODE_ENV || "development";
const config = configs[env];

// Environment variables in YAML values can be interpolated
function interpolateEnvVars(obj: any): any {
  if (typeof obj === "string") {
    return obj.replace(/\${(\w+)}/g, (_, key) => process.env[key] || "");
  }
  if (typeof obj === "object") {
    for (const key in obj) {
      obj[key] = interpolateEnvVars(obj[key]);
    }
  }
  return obj;
}

export default interpolateEnvVars(config);

Feature Flags Configuration

features.yaml
features:
  newDashboard:
    enabled: true
    rolloutPercentage: 50
    allowedUsers:
      - admin@example.com
      - beta@example.com

  experimentalAPI:
    enabled: false
    endpoints:
      - /api/v2/experimental
      - /api/v2/beta

  darkMode:
    enabled: true
    default: auto # auto, light, dark

feature-flags.ts
import { features } from "./features.yaml";

export function isFeatureEnabled(
  featureName: string,
  userEmail?: string,
): boolean {
  const feature = features[featureName];

  if (!feature?.enabled) {
    return false;
  }

  // Check rollout percentage
  if (feature.rolloutPercentage < 100) {
    const hash = hashCode(userEmail || "anonymous");
    if (hash % 100 >= feature.rolloutPercentage) {
      return false;
    }
  }

  // Check allowed users
  if (feature.allowedUsers && userEmail) {
    return feature.allowedUsers.includes(userEmail);
  }

  return true;
}

// Use with hot reloading to toggle features in real-time
if (isFeatureEnabled("newDashboard", user.email)) {
  renderNewDashboard();
} else {
  renderLegacyDashboard();
}

Database Configuration

database.yaml
connections:
  primary:
    type: postgres
    host: ${DB_HOST:-localhost}
    port: ${DB_PORT:-5432}
    database: ${DB_NAME:-myapp}
    username: ${DB_USER:-postgres}
    password: ${DB_PASS}
    pool:
      min: 2
      max: 10
      idleTimeout: 30000

  cache:
    type: redis
    host: ${REDIS_HOST:-localhost}
    port: ${REDIS_PORT:-6379}
    password: ${REDIS_PASS}
    db: 0

  analytics:
    type: clickhouse
    host: ${ANALYTICS_HOST:-localhost}
    port: 8123
    database: analytics

migrations:
  autoRun: ${AUTO_MIGRATE:-false}
  directory: ./migrations

seeds:
  enabled: ${SEED_DB:-false}
  directory: ./seeds

db.ts
import { connections, migrations } from "./database.yaml";
import { createConnection } from "./database-driver";

// Parse environment variables with defaults
function parseConfig(config: any) {
  return JSON.parse(
    JSON.stringify(config).replace(
      /\${([^:-]+)(?::([^}]+))?}/g,
      (_, key, defaultValue) => process.env[key] || defaultValue || "",
    ),
  );
}

const dbConfig = parseConfig(connections);

export const db = await createConnection(dbConfig.primary);
export const cache = await createConnection(dbConfig.cache);
export const analytics = await createConnection(dbConfig.analytics);

// Auto-run migrations if configured
if (parseConfig(migrations).autoRun === "true") {
  await runMigrations(db, migrations.directory);
}

Bundler Integration

When you import YAML files in your application and bundle it with Bun, the YAML is parsed at build time and included as a JavaScript module:

bun build app.ts --outdir=dist

This means:

Zero runtime YAML parsing overhead in production
Smaller bundle sizes
Tree-shaking support for unused configuration (named imports)

Dynamic Imports

YAML files can be dynamically imported, useful for loading configuration on demand:

Load
const env = process.env.NODE_ENV || "development";
const config = await import(`./configs/${env}.yaml`);

// Load user-specific settings
async function loadUserSettings(userId: string) {
  try {
    const settings = await import(`./users/${userId}/settings.yaml`);
    return settings.default;
  } catch {
    return await import("./users/default-settings.yaml");
  }
}