null-safe · sum types · zero runtime

JavaScript, Perfected.

A sound, null-safe superset of JavaScript — sum types, exhaustive match, no any. Compiles to plain JS with zero runtime overhead.

Try the Playground →Read the Docs

$npm i -g superjs

Features

Everything TypeScript should have been

Built from first principles. Every decision exists to make your code more correct.

🛡️

Null Safety

T? is the only nullable type. No null exceptions at runtime.

const x: string? = null
const y: string = x ?? "default"

∑

Sum Types

Algebraic data types with exhaustive match — no impossible states.

type Result<T,E> = Ok(T) | Err(E)
const r: Result<number,string> = Ok(42)

◈

Match Expressions

Exhaustive pattern matching. The compiler forces you to handle every case.

match r {
  Ok(v) => v * 2,
  Err(e) => 0
}

⚡

No any

any is banned. Use dynamic when you need an escape hatch — it's explicit.

// ❌  const x: any = fetch()
// ✅  const x: dynamic = fetch()

↝

Gradual Typing

Mix typed and untyped code freely. Migrate at your own pace.

function greet(name) {  // untyped
  return "Hello, " + name
}

Zero Runtime

Compiles to plain JS. No runtime library, no overhead, ships anywhere.

// sjs compile app.sjs --out app.js
// → plain JavaScript, no imports

Comparison

The same problem, solved correctly

Switch tabs — same program, three outcomes.

✅ The compiler forces you to handle it

// Super.js
function divide(a: number, b: number): Result<number, string> {
  if (b === 0) return Err("division by zero")
  return Ok(a / b)
}

match divide(10, 0) {
  Ok(v) => console.log(v),
  Err(e) => console.error(e)
}

Try it live

Edit the Super.js code on the left, hit Run, see compiled JavaScript on the right.

playground.sjs

Press Run to compile

Open full playground →

Quickstart

From zero to type-safe in 60 seconds

Install the compiler

npm install -g superjs

Write your first program

// hello.sjs
type Greeting = Formal(string) | Casual(string)

function greet(g: Greeting): string {
  return match g {
    Formal(name) => "Good day, " + name + ".",
    Casual(name) => "Hey " + name + "!"
  }
}

console.log(greet(Formal("World")))  // Good day, World.
console.log(greet(Casual("friend"))) // Hey friend!

Compile and run

sjs compile hello.sjs --out hello.js
node hello.js

Read the Docs →Try in Browser

Ecosystem

Works everywhere JavaScript does

🎨VS Code Extension

⚙️CLI Compiler

🛡️Type Checker

🔍Linter

✨Formatter

🟢Node.js

🌐Browser

🥟Bun

⚛️React

📦JSX / TSX