windmill/system_prompts/languages/bun.md

TypeScript (Bun)

Bun runtime with full npm ecosystem and fastest execution. Bun is the default and preferred TypeScript runtime — choose it for any TypeScript script unless there is a major reason to use Deno for that specific use-case.

Structure

Export a single async function called main:

export async function main(param1: string, param2: number) {
  // Your code here
  return { result: param1, count: param2 };
}

Do not call the main function. Libraries are installed automatically.

Resource Types

On Windmill, credentials and configuration are stored in resources and passed as parameters to main.

Use the RT namespace for resource types:

export async function main(stripe: RT.Stripe) {
  // stripe contains API key and config from the resource
}

Only use resource types if you need them to satisfy the instructions. Always use the RT namespace.

Before using a resource type, check the rt.d.ts file in the project root to see all available resource types and their fields. This file is generated by wmill resource-type generate-namespace.

Imports

import Stripe from "stripe";
import { someFunction } from "some-package";

Prefer //native when the runtime allows it

If a script only needs fetch and the JavaScript standard library — including when it uses windmill-client — prefer making it a native script: add //native as the first line and write it with the write-script-bunnative skill. Native scripts run on a lightweight V8 isolate, start faster, and parallelize heavily. windmill-client works on the native worker (its calls go over fetch), so needing the Windmill client is not a reason to avoid //native. Use the regular bun language only when the code (or a dependency) needs Node/Bun runtime APIs — node:* modules, the filesystem, child processes, or native addons.

Windmill Client

Import the windmill client for platform interactions:

import * as wmill from "windmill-client";

Prefer windmill-client over raw fetch for anything that talks to Windmill — reading resources/variables/states, running scripts and flows, S3 object operations, etc. It handles auth, the workspace, and the base URL for you, so you don't hand-roll URLs or tokens. Reserve fetch for calling external HTTP APIs that aren't Windmill.

The full windmill-client API reference (every exported function and its signature) is included in this skill below — consult it for the exact method to use instead of guessing or falling back to fetch.

Preprocessor Scripts

For preprocessor scripts, the function should be named preprocessor and receives an event parameter:

type Event = {
  kind:
    | "webhook"
    | "http"
    | "websocket"
    | "kafka"
    | "email"
    | "nats"
    | "postgres"
    | "sqs"
    | "mqtt"
    | "gcp";
  body: any;
  headers: Record<string, string>;
  query: Record<string, string>;
};

export async function preprocessor(event: Event) {
  return {
    param1: event.body.field1,
    param2: event.query.id,
  };
}

S3 Object Operations

Windmill provides built-in support for S3-compatible storage operations. The wmill.S3Object type covers both the s3://storage/key URI form (s3:///key for the workspace default storage) and the { s3, storage? } record form — always use it instead of redefining your own.

Receiving an S3Object as a script parameter

import * as wmill from "windmill-client";

export async function main(file: wmill.S3Object) {
  const content = await wmill.loadS3File(file);
  // ...
}

S3 operations

import * as wmill from "windmill-client";

// Load file content from S3
const content: Uint8Array = await wmill.loadS3File(s3object);

// Load file as stream
const blob: Blob = await wmill.loadS3FileStream(s3object);

// Write file to S3
const result: wmill.S3Object = await wmill.writeS3File(
  s3object, // Target path (or undefined to auto-generate)
  fileContent, // string or Blob
  s3ResourcePath // Optional: specific S3 resource to use
);