Speedy

Incredibly fast ECMAScript & TypeScript toolchain optimized for development.

Motivation

Nobody should have to wait for build tools to be productive.

Purpose

The purpose of Speedy is to very quickly convert ECMAScript/TypeScript into something a web browser can execute.

Goals:

• Transpile fast. "Fast" is defined as "<= 3ms per un-minified file up to 1000 LOC" without a build cache
• Transpile JSX to ECMAScript
• Remove TypeScript annotations
• Conditionally support React Fast Refresh
• Rewrite CommonJS/SystemJS/UMD imports and exports to ESM
• Support most of tsconfig.json/jsconfig.json
• Support defines like in esbuild
• Support esbuild plugins
• Support importing CSS files from JavaScript
• Tree-shaking

Non-goals:

• Bundling for production
• Minification
• AST plugins
• Support Node.js
• CommonJS, UMD, IIFE
• ES6 to ES5
• Supporting non-recent versions of Chromium, Firefox, or Safari. (No IE)

How it works

Much of the code is a line-for-line port of esbuild to Zig. Thank you @evanw for building esbuild - a fantastic ECMAScript & CSS Bundler, and for inspiring this project.

Compatibility Table

Key:

Compatibility Table (more info)

Notes

Hot Module Reloading & React Fast Refresh

Speedy exposes a runtime API to support Hot Module Reloading and React Fast Refresh. React Fast Refresh depends on Hot Module Reloading to work, but you can turn either of them off. Speedy itself doesn't serve bundled files, it's up to the development server to provide that.

Code Splitting

Speedy supports code splitting the way browsers do natively: through ES Modules. This works great for local development files. It doesn't work great for node_modules or for production due to the sheer number of network requests. There are plans to make this better, stay tuned.

Support older browsers

To simplify the parser, Speedy doesn't support lowering features to non-current browsers. This means if you run a development build with Speedy with, for example, optional chaining, it won't work in Internet Explorer 11. If you want to support older browsers, use a different tool.

Implementation Notes

Deviations from other bundlers

Unused imports are removed by default, unless they're an import without an identifier. This is similar to what the TypeScript compiler does, but TypeScript only does it for TypeScript. This is on by default, but you can turn it off.

For example in this code snippet, forEach in unused:

import { forEach, map } from "lodash-es";

const foo = map(["bar", "baz"], (item) => {});

So it's never included.

import { map } from "lodash-es";

const foo = map(["bar", "baz"], (item) => {});

If

HMR & Fast Refresh implementation

This section only applies when Hot Module Reloading is enabled. When it's off, none of this part runs. React Fast Refresh depends on Hot Module Reloading.

What is hot module reloading?

HMR: "hot module reloading"

A lot of developers know what it does -- but what actually is it and how does it work? Essentially, it means when a source file changes, automatically reload the code without reloading the web page.

A big caveat here is JavaScript VMs don't expose an API to "unload" parts of the JavaScript context. In all HMR implementations, What really happens is this:

1. Load a new copy of the code that changed
2. Update references to the old code to point to the new code
3. Handle errors

The old code still lives there, in your browser's JavaScript VM until the page is refreshed. If any past references are kept (side effects!), undefined behavior happens. That's why, historically (by web standards), HMR has a reputation for being buggy.

Loading code is easy. The hard parts are updating references and handling errors.

There are two ways to update references:

• Update all module imports
• Update the exports

Either approach works.

How it's implemented in Speedy

TODO: doc