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Proposal Type Annotations

Name: Proposal Type Annotations
Author: tc39

ECMAScript proposal for type syntax that is erased - Stage 1

4,388 stars52 forksJavaScriptUpdated 8/27/2025100% free · open source

What it does

The proposal-type-annotations tool allows developers to add type annotations to their JavaScript code, which are then erased at runtime, providing a way to add type safety without runtime overhead.

When to use it

•When building a new JavaScript project and wanting to add type safety without using a separate type system like TypeScript
•When migrating an existing JavaScript project to use type annotations for better code maintainability and readability
•When experimenting with new JavaScript features and wanting to provide type information for better code completion and error detection

Ready-to-paste prompt

Try adding a type annotation to a variable, such as `let x: number = 5;`, and see how it is erased at runtime

Heads up: Note that this proposal is still at Stage 1 and not yet implemented in most JavaScript engines, so you may need to use a transpiler or a custom build process to experiment with type annotations

Saves to your device

Use with Claude

New

Skip the builder — one click puts this in Claude, Cursor, Antigravity and more.

✅ Light setup

Installs with a command or two; your AI agent can do it for you.

Try it instantly — no install

Claude Code

mkdir -p ~/.claude/skills/proposal-type-annotations && curl -fsSL https://workflowstacks.com/api/skills/proposal-type-annotations/claude-skill -o ~/.claude/skills/proposal-type-annotations/SKILL.md

Open in another AI app

Opens the app with this repo with the prompt ready to go — no copy-paste needed.

Connect the whole catalog (MCP)

claude mcp add --transport http workflowstacks https://workflowstacks.com/api/mcp

Adds a WorkflowStacks connector to Claude Code: search and load any skill here by chatting.

How Proposal Type Annotations works

Codeflow

Free to inspect

Proposal Type Annotations is a small JavaScript project (~700 lines across 3 code files). Setup is light: small project — see the README for how to run it. Last commit 13 months ago, no license file, no tests found.

Size

Small codebase

~700 lines · 3 code files · 6 min skim

Setup

Light setup

Small project — see the README for how to run it.

Runs on

JavaScript

No API keys detected

JavaScript 54%SCSS 24%HTML 22%

Where to start reading

1
README.md
Start here — what it does and how to install it
2
site/src/components.jsx
Inside site/src/ — the main logic begins here

What's in each folder

site/src/Core code — the actual logic7 files

syntax/Folder3 files

site/assets/Images & static assets12 files

.github/CI / automation (GitHub Actions)1 files

READMENo tests foundDocumentedCI checksNo licenseLast update 13 mo ago