WynJS is a JavaScript runtime written entirely in Wyn — 2,700 lines that pass 217 tests covering ES6 classes, closures, template literals, try/catch, destructuring, and more.

Why build a JS runtime?

Dogfooding. The best way to find compiler bugs is to write something complex in your own language. WynJS found over 20 bugs in the Wyn compiler — from enum match codegen to closure capture to array sort corruption.

What it supports

• Variables, functions, closures, arrow functions
• ES6 classes with constructors, methods, inheritance
• Arrays with map, filter, reduce, push, pop, join
• Objects with dot access and bracket notation
• Template literals with ${expression} interpolation
• try/catch/throw with Error objects
• for, for-of, for-in, while, do-while
• switch/case with break and fallthrough
• Ternary operator, spread operator, destructuring
• Math, JSON, console, typeof, instanceof

Architecture

WynJS uses a tagged-value system where every JS value is a string with a type prefix. Objects are stored in a HashMap-based registry. Functions are stored as source strings and re-parsed on each call — simple but effective.

// The core eval loop
fn eval_node(src: string, env: int) -> string {
    var type = node_type(src)
    match type {
        "num" => return src
        "str" => return src
        "call" => return eval_call(src, env)
        "bin" => return eval_binary(src, env)
        _ => return "undefined"
    }
}

Results

• 217/217 tests passing
• 2,700 lines of Wyn
• 314KB compiled binary
• Compiles in ~1.2 seconds

The full source is at sample-apps/wynjs.