Hello Bun: How Sveld now deploys 2x faster on GitHub and Render

• Step-by-step how I migrated to Bun: The migration was simple. I call out minor gotchas to save you some Googling.
• My favorite part of Bun—devX improvements: Of course, Bun reduced the number of dependencies in Sveld. To my pleasant surprise, using Bun’s Glob and Shell APIs also helped make a build script much more readable. See the before and after code snippet.
• A breakdown of speed gains in Sveld’s CI/CD: After moving from Yarn and Vitest to Bun, Sveld’s CI/CD on GitHub Actions and Render sped up by 2x. Most of the time savings came from faster package installs. At the end, I share a detailed breakdown of the speed gains.

Quick intro to Bun

• Replace Node, npm, npx, Jest/Vitest, Parcel, ts-node, etc. with one tool.
• Stop futzing with CommonJS versus ESM.
• Install packages 15-30x faster.
• Simplify and speed up code with new Bun-only APIs.

import fs from "node:fs";

fs.readFileSync("file.txt", "utf8");

Sveld, before Bun

• Yarn: the package manager. When I created Sveld, Yarn was a popular choice.
• Vitest: the test runner. I originally chose Vitest because it’s faster than Jest, and it can compile and run TypeScript files without a separate dependency (e.g. ts-node-dev).
• TypeScript: Sveld is written in TypeScript. Sveld also uses the TypeScript compiler to generate TypeScript definitions.
• GitHub Actions: Sveld’s CI/CD pipeline consists of two GitHub Actions workflows.
• Render: The Sveld playground is hosted on Render.

• Bun: Bun replaced Yarn and Vitest.

Migration overview: four stages

1. Replace Yarn with Bun as the package manager.
2. Replace Vitest with Bun as the test runner.
3. Migrate the CI/CD workflow that publishes releases to npm.
4. Migrate the CI/CD workflow that deploys the demo site to Render.

Stage 1: Replace Yarn, the package manager

1. Update package.json.
2. Update the lockfile.

Updating package.json

- yarn build
+ bun run build

• Some keywords are reserved: Bun reserves some keywords, including bun install, bun build, and bun add. If you want to use these keywords as scripts in your package.json, you’ll need to invoke bun run <command>.
• File extensions are required: Bun requires you to specify the file extension when you execute a file.
• Runtime default differs for files versus packages: In the Bun CLI, Bun is used as the default runtime to execute a file. However, Node.js is used as the default runtime to execute a package. To use Bun as the runtime to execute a package, you need to pass an explicit --bun flag.

Updating the lockfile

rm yarn.lock && bun install

- yarn.lock
+ bun.lockb

Stage 2: Replace Vitest, the test runner

1. Uninstall Vite-related dependencies.
2. Convert vi APIs to jest APIs.
3. Install TypeScript definitions for Jest.

Uninstalling Vite-related dependencies.

bun uninstall vite vitest

Updating vi APIs to jest APIs

- vi.fn();
+ jest.fn();

- vi.spyOn(console, "error");
+ jest.spyOn(console, "error");

Installing TypeScript definitions for Jest.

# The -D flag installs the package as a development dependency.
bun install -D @types/jest

Stage 3: Migrate npm publish

1. Install Bun on the GitHub Actions runners.
2. Update the workflow file to use Bun instead of Yarn.

Installing Bun on the GitHub-hosted runners

steps:
  - uses: oven-sh/setup-bun@v1

- uses: oven-sh/setup-bun@v1
  with:
    # Subsequent workflow runs will use the cached version of Bun.
    bun-version: 1.1.3

Updating the workflow file to use Bun instead of Yarn

name: Install dependencies
- run: yarn install
+ run: bun install

name: Build the library
- run: yarn build
+ run: bun run build

name: Run unit tests
- run: yarn test
+ run: bun test

Stage 4: Migrate Render deploy

- cd playground; yarn install && yarn run build
+ cd playground; bun install && bun run build

Why TypeScript & npm stayed—and my Bun wishlist

• Type checking: Sveld needs the TypeScript compiler, tsc, to perform type checking and to generate TypeScript definitions. Bun can directly execute .ts and .tsx files, but can’t check or generate types.
• Bundling support for CJS: I was not able to use Bun's native bundling feature because Sveld is distributed in the CommonJS (CJS) format. Bun's bundler currently only supports ES modules (ESM).
• Publishing to npm with provenance: npm remains in my Github Actions workflow. When I publish Sveld to npm, I want to publish with provenance, and I can’t do this with Bun today.

Better readability with Bun’s Glob and Shell APIs

const fs = require("fs");
const path = require("path");
const { exec: child_process_exec } = require("child_process");
const { promisify } = require("util");
const exec = promisify(child_process_exec);
const { name } = require("../package.json");

const execCwd = async (dir, ...args) => await exec(`yarn --cwd ${dir} ${args}`);

await exec("yarn link");

const dirs = fs
  .readdirSync("tests/e2e")
  .map((file) => path.join("tests/e2e", file))
  .filter((file) => fs.lstatSync(file).isDirectory());

for await (const dir of dirs) {
  const typesDir = path.join(dir, "types");

  if (fs.existsSync(typesDir)) {
    fs.rmSync(typesDir, { recursive: true, force: true });
  }

  await execCwd(dir, `link "${name}"`);
  await execCwd(dir, "install");

  const build = await execCwd(dir, "build");
  console.log(build.stdout + "\n");

  const svelteCheck = await execCwd(dir, "svelte-check");
  console.log(svelteCheck.stdout + "\n");
}

import { Glob, $ } from "bun";
import { name } from "../package.json";

// Create a symlink to the library.
await $`bun link`;

// Get a list of directories in the e2e test directory.
const dirs = new Glob("*").scanSync({
  cwd: "tests/e2e",
  onlyFiles: false,
  absolute: true,
});

// Iterate over each directory and run the following commands.
for await (const dir of dirs) {
  // Remove previously generated TypeScript definitions.
  await $`cd ${dir} && rm -rf types`;

  // Link the library to the e2e test directory.
  await $`cd ${dir} && bun link ${name}`;

  // Install the test directory dependencies and build the library.
  await $`cd ${dir} && bun install`;
  await $`cd ${dir} && bun run build`;
}

2x speed gains with Bun across build and test

• Project dependencies were installed 2-20x faster on GitHub, depending on OS platform.
• Unit tests ran 1.6-2.2x faster on GitHub.

Observed: Package install runs 2-20x faster on GitHub Actions

• Without cache: If node_modules (the cache) is not present, the package manager will resolve and install dependencies from scratch.
• With cache: If node_modules is present, the GitHub runner will download the cache. Then the package manager will check to see if there are any new dependencies and download any needed packages.

Observed: Package install runs 16x faster locally

# Run on a 2019 Macbook Pro (Intel Core i9)

hyperfine --warmup 3 'yarn install' --runs 10
Benchmark 1: yarn install
  Time (mean ± σ):     300.2 ms ±   7.7 ms    [User: 304.8 ms, System: 58.6 ms]
  Range (min … max):   293.4 ms … 316.1 ms    10 runs

hyperfine --warmup 3 'bun install' --runs 10
Benchmark 1: bun install
  Time (mean ± σ):      18.8 ms ±   0.4 ms    [User: 17.4 ms, System: 10.0 ms]
  Range (min … max):    18.0 ms …  19.1 ms    10 runs

Observed: Unit tests run 1.6-2x faster on GitHub Actions, 2x faster locally

# Run on a 2019 Macbook Pro (Intel Core i9)

hyperfine --warmup 3 'yarn test run' --runs 10
Benchmark 1: yarn test run
  Time (mean ± σ):      2.343 s ±  0.153 s    [User: 6.899 s, System: 1.153 s]
  Range (min … max):    2.194 s …  2.589 s    10 runs

hyperfine --warmup 3 'bun test' --runs 10
Benchmark 1: bun test
  Time (mean ± σ):      1.059 s ±  0.038 s    [User: 1.653 s, System: 0.225 s]
  Range (min … max):    1.020 s …  1.161 s    10 runs

2x faster Render deploys with Bun

Try Bun, on Render

• Build an existing Render service with Bun: Already have a Node service deployed on Render? Try replacing your npm install or yarn install command with bun install.
• Deploy a new Render service: If you don't already have a Node service, you can deploy a web server with Bun on Render in 5 minutes. For example, ElysiaJS is a new web framework designed to use Bun, and it’s now very easy to get it running on Render. Check out our ElysiaJS guide to get started.
• Migrate an entire project: If you have an existing Node project, I hope this guide can help you migrate it.