Bun Macros

Bun Macros

Bun macros run JavaScript at bundle time and inline the results into the output.

Quick Start

// src/config.ts (macro file)
export function getVersion() {
  return "1.0.0";
}

export function getBuildTime() {
  return new Date().toISOString();
}

// src/index.ts (consumer)
import { getVersion, getBuildTime } from "./config" with { type: "macro" };

// At bundle time, these become:
const version = "1.0.0";
const buildTime = "2024-01-15T12:00:00.000Z";

Macro Syntax

// Import with macro attribute
import { fn } from "./macro-file" with { type: "macro" };

// Call the macro (evaluated at build time)
const result = fn();

Common Use Cases

Environment Inlining

// macros/env.ts
export function env(key: string): string {
  return process.env[key] ?? "";
}

// src/index.ts
import { env } from "./macros/env" with { type: "macro" };

const apiUrl = env("API_URL");
// Becomes: const apiUrl = "https://api.example.com";

Git Information

// macros/git.ts
export function gitCommit(): string {
  return Bun.spawnSync(["git", "rev-parse", "HEAD"])
    .stdout.toString().trim();
}