# Super.js

> A strict, type-safe superset of JavaScript that follows the ECMAScript standard. Sound types, null safety, sum types, no any — compiles to clean JS today, with native and WASM backends on the roadmap.

Super.js is a superset of JavaScript (not TypeScript) focused on type *safety*:
nullable types are spelled `T?` and a plain `T` can never be null; sum types
with exhaustive `match` make impossible states unrepresentable; `any` is
banned and the only escape hatch is the explicit `dynamic`. It compiles to
plain JavaScript with zero runtime overhead.

## Start here

- [Home](https://superjs.org/)
- [Interactive tour](https://superjs.org/tour): five runnable, in-browser lessons
- [Playground](https://superjs.org/playground): compile and run Super.js in the browser
- [Error code reference](https://superjs.org/errors): every SJS-P/E/W/L diagnostic

## Documentation

- [Introduction](https://superjs.org/docs/intro)
- [Language Reference](https://superjs.org/docs/language-reference)
- [Examples](https://superjs.org/docs/examples)
- [Type System](https://superjs.org/docs/type-system)
- [JavaScript & TypeScript Interop](https://superjs.org/docs/interop)
- [Tooling](https://superjs.org/docs/tooling)
- [CLI Reference](https://superjs.org/docs/cli)
- [Editor Setup](https://superjs.org/docs/editors)
- [Changelog](https://superjs.org/docs/changelog)
- [Roadmap](https://superjs.org/docs/roadmap)
- [Specification](https://superjs.org/docs/specification)
- [FAQ](https://superjs.org/docs/faq)
- [v1.0 RC Status](https://superjs.org/docs/roadmap/v1.0-rc-status)
- [Playground Worker Deploy](https://superjs.org/docs/ops/playground-deploy)
- [Maintenance & On-Call](https://superjs.org/docs/ops/maintenance)
- [VS Code Marketplace](https://superjs.org/docs/ops/vscode-marketplace)
- [RC maintainer gates](https://superjs.org/docs/ops/rc-maintainer-gates)
- [Launch drafts](https://superjs.org/docs/launch/index)
- [Language Tour](https://superjs.org/docs/tour/index)
- [01 — Hello world](https://superjs.org/docs/tour/01-hello-world)
- [02 — Variables and types](https://superjs.org/docs/tour/02-variables-and-types)
- [03 — Functions](https://superjs.org/docs/tour/03-functions)
- [04 — Control flow](https://superjs.org/docs/tour/04-control-flow)
- [05 — Null safety](https://superjs.org/docs/tour/05-null-safety)
- [06 — Pattern matching](https://superjs.org/docs/tour/06-pattern-matching)
- [07 — Sum types](https://superjs.org/docs/tour/07-sum-types)
- [08 — Object types](https://superjs.org/docs/tour/08-interfaces)
- [09 — Generics](https://superjs.org/docs/tour/09-generics)
- [10 — Classes](https://superjs.org/docs/tour/10-classes)
- [11 — Modules](https://superjs.org/docs/tour/11-modules)
- [12 — Async and await](https://superjs.org/docs/tour/12-async-await)
- [13 — JSX](https://superjs.org/docs/tour/13-jsx)
- [14 — Calling JS from SJS](https://superjs.org/docs/tour/14-calling-js-from-sjs)
- [15 — dynamic and Schema](https://superjs.org/docs/tour/15-dynamic-and-schema)
- [16 — Errors and Result](https://superjs.org/docs/tour/16-errors-and-result)
- [17 — Iterators and for...of](https://superjs.org/docs/tour/17-iterators-and-for-of)
- [18 — Serverless handlers](https://superjs.org/docs/tour/18-serverless-handlers)
- [19 — Tooling tour](https://superjs.org/docs/tour/19-tooling-tour)
- [20 — Migrating a TS file](https://superjs.org/docs/tour/20-migrating-a-ts-file)
- [README](https://superjs.org/docs/press/README)
- [sjs-vs-typescript-one-pager](https://superjs.org/docs/press/sjs-vs-typescript-one-pager)
- [Migration Guide](https://superjs.org/docs/migration/index)
- [Syntax Rewrites](https://superjs.org/docs/migration/01-syntax)
- [Idiom Changes](https://superjs.org/docs/migration/02-idioms)
- [Library Ecosystem](https://superjs.org/docs/migration/03-library)
- [API Reference](https://superjs.org/docs/api/index)
- [std-async](https://superjs.org/docs/api/std-async)
- [std-collections](https://superjs.org/docs/api/std-collections)
- [std-core](https://superjs.org/docs/api/std-core)
- [std-fs](https://superjs.org/docs/api/std-fs)
- [std-json](https://superjs.org/docs/api/std-json)
- [std-math](https://superjs.org/docs/api/std-math)
- [std-path](https://superjs.org/docs/api/std-path)
- [std-process](https://superjs.org/docs/api/std-process)
- [std-schema](https://superjs.org/docs/api/std-schema)
- [std-string](https://superjs.org/docs/api/std-string)
- [std-time](https://superjs.org/docs/api/std-time)
- [Why SuperJS](https://superjs.org/docs/why/index)
- [Compatibility Matrix](https://superjs.org/docs/compat/index)
- [Performance Benchmarks](https://superjs.org/docs/perf/index)
- [LSP Memory Audit](https://superjs.org/docs/perf/lsp-memory-audit)
- [Error Codes](https://superjs.org/docs/error-codes/index)
- [SJS-E001 — Null or undefined assigned to non-nullable type](https://superjs.org/docs/error-codes/SJS-E001)
- [SJS-E002 — Type mismatch](https://superjs.org/docs/error-codes/SJS-E002)
- [SJS-E003 — Property access on possibly-null value](https://superjs.org/docs/error-codes/SJS-E003)
- [SJS-E004 — `any` is not a valid type in SJS](https://superjs.org/docs/error-codes/SJS-E004)
- [SJS-E005 — Intersection type `A & B` is not allowed](https://superjs.org/docs/error-codes/SJS-E005)
- [SJS-E006 — Mapped type is not allowed in SJS](https://superjs.org/docs/error-codes/SJS-E006)
- [SJS-E007 — Non-exhaustive match expression](https://superjs.org/docs/error-codes/SJS-E007)
- [SJS-E008 — Conditional type is not allowed in SJS](https://superjs.org/docs/error-codes/SJS-E008)
- [SJS-E009 — `infer` keyword is not allowed in SJS](https://superjs.org/docs/error-codes/SJS-E009)
- [SJS-E010 — TypeScript `enum` is not allowed in SJS](https://superjs.org/docs/error-codes/SJS-E010)
- [SJS-E011 — Non-null assertion `!` is not allowed](https://superjs.org/docs/error-codes/SJS-E011)
- [SJS-E012 — `namespace` is not allowed in SJS](https://superjs.org/docs/error-codes/SJS-E012)
- [SJS-E013 — `with` statement not allowed (SJS is always strict mode)](https://superjs.org/docs/error-codes/SJS-E013)
- [SJS-E014 — Private or protected member not accessible from this scope](https://superjs.org/docs/error-codes/SJS-E014)
- [SJS-E015 — Cannot narrow an access modifier on an overriding method or property](https://superjs.org/docs/error-codes/SJS-E015)
- [SJS-E016 — Cannot instantiate an abstract class directly with `new`](https://superjs.org/docs/error-codes/SJS-E016)
- [SJS-E017 — Circular import detected — module graph contains a cycle](https://superjs.org/docs/error-codes/SJS-E017)
- [SJS-E018 — Top-level `await` used outside an ES module context](https://superjs.org/docs/error-codes/SJS-E018)
- [SJS-E019 — Unknown JSX element type — identifier not in scope or not a valid component](https://superjs.org/docs/error-codes/SJS-E019)
- [SJS-E020 — Ambiguous variant constructor](https://superjs.org/docs/error-codes/SJS-E020)
- [SJS-L001 — Prefer `const` — `let` binding is never reassigned](https://superjs.org/docs/error-codes/SJS-L001)
- [SJS-L002 — Prefer `let` or `const` over `var`](https://superjs.org/docs/error-codes/SJS-L002)
- [SJS-L003 — Use `===` / `!==` — `==` / `!=` performs type coercion](https://superjs.org/docs/error-codes/SJS-L003)
- [SJS-L004 — Prefer `for…of` over `for…in` for array and iterable iteration](https://superjs.org/docs/error-codes/SJS-L004)
- [SJS-L005 — `debugger` statement found in committed code](https://superjs.org/docs/error-codes/SJS-L005)
- [SJS-L006 — `match` expression has no arms](https://superjs.org/docs/error-codes/SJS-L006)
- [SJS-L007 — Redundant match arm](https://superjs.org/docs/error-codes/SJS-L007)
- [SJS-L008 — Prefer an arrow function callback](https://superjs.org/docs/error-codes/SJS-L008)
- [SJS-L009 — Unused import](https://superjs.org/docs/error-codes/SJS-L009)
- [SJS-L010 — Import out of order](https://superjs.org/docs/error-codes/SJS-L010)
- [SJS-L011 — BiDi control character rejected](https://superjs.org/docs/error-codes/SJS-L011)
- [SJS-L012 — Unused top-level declaration](https://superjs.org/docs/error-codes/SJS-L012)
- [SJS-L013 — Explicit `dynamic` type annotation](https://superjs.org/docs/error-codes/SJS-L013)
- [SJS-L014 — Variable shadowing](https://superjs.org/docs/error-codes/SJS-L014)
- [SJS-L015 — Floating promise](https://superjs.org/docs/error-codes/SJS-L015)
- [SJS-L016 — Unhandled `Result`](https://superjs.org/docs/error-codes/SJS-L016)
- [SJS-L017 — Prefer `Result` over `throw`](https://superjs.org/docs/error-codes/SJS-L017)
- [SJS-L018 — Mixed spaces and tabs in indentation](https://superjs.org/docs/error-codes/SJS-L018)
- [SJS-P001 — Unexpected token](https://superjs.org/docs/error-codes/SJS-P001)
- [SJS-P002 — Unexpected end of file](https://superjs.org/docs/error-codes/SJS-P002)
- [SJS-P003 — Invalid syntax in type annotation](https://superjs.org/docs/error-codes/SJS-P003)
- [SJS-P004 — Invalid sum type declaration](https://superjs.org/docs/error-codes/SJS-P004)
- [SJS-P005 — Invalid match expression](https://superjs.org/docs/error-codes/SJS-P005)
- [SJS-P099 — Too many parse errors; recovery abandoned](https://superjs.org/docs/error-codes/SJS-P099)
- [SJS-W001 — Implicit `dynamic` — unannotated position in `--strict` mode](https://superjs.org/docs/error-codes/SJS-W001)
- [SJS-W002 — `dynamic` value assigned to a typed position without a narrowing check](https://superjs.org/docs/error-codes/SJS-W002)
- [SJS-W003 — Unreachable `match` arm — earlier arm already covers this variant](https://superjs.org/docs/error-codes/SJS-W003)
- [SJS-W004 — Reserved or future SJS keyword used as an identifier](https://superjs.org/docs/error-codes/SJS-W004)
- [SJS-W005 — Explicit `public` modifier is redundant — class members are public by default](https://superjs.org/docs/error-codes/SJS-W005)
- [SJS-W006 — Excess property on a fresh object literal assigned to a typed position](https://superjs.org/docs/error-codes/SJS-W006)
- [SJS-W007 — Missing `key` prop on JSX element in a list or iterator context](https://superjs.org/docs/error-codes/SJS-W007)
- [SJS-W008 — Implicit switch fallthrough between non-empty case clauses](https://superjs.org/docs/error-codes/SJS-W008)
- [SJS-W009 — Unreachable code following a terminator statement](https://superjs.org/docs/error-codes/SJS-W009)
- [SJS-W010 — `catch` binding not typed as `Error` or `unknown` — untyped catch may hide bugs](https://superjs.org/docs/error-codes/SJS-W010)
- [SJS-W012 — BiDi control character in source file](https://superjs.org/docs/error-codes/SJS-W012)
- [Language Specification](https://superjs.org/docs/spec/index)
- [Standard Library](https://superjs.org/docs/stdlib/index)
- [Beta Program](https://superjs.org/docs/beta/index)
- [Dynamic Usage Survey (C5)](https://superjs.org/docs/beta/dynamic-usage-survey)
- [Bug Bash](https://superjs.org/docs/beta/bug-bash)

## Machine-readable

- [llms-full.txt](https://superjs.org/llms-full.txt): all documentation inlined as plain text
- [Blog RSS](https://superjs.org/feed.xml)
- [Sitemap](https://superjs.org/sitemap.xml)

## Blog

- [Preparing for SuperJS v1.0 RC](https://superjs.org/blog/2026-06-24-preparing-for-v1-rc)
- [What's coming in Super.js 0.2.0](https://superjs.org/blog/2024-01-15-0-2-0-released)
- [Welcome to Super.js](https://superjs.org/blog/2024-01-01-welcome)