Show HN: Coi – A language that compiles to WASM, beats React/Vue
by io_eric | 2026-01-20 19:01:53+00:00
I usually build web games in C++, but using Emscripten always felt like overkill for what I was doing. I don't need full POSIX emulation or a massive standard library just to render some stuff to a canvas and handle basic UI.

The main thing I wanted to solve was the JS/WASM interop bottleneck. Instead of using the standard glue code for every call, I moved everything to a Shared Memory architecture using Command and Event buffers.

The way it works is that I batch all the instructions in WASM and then just send a single "flush" signal to JS. The JS side then reads everything directly out of Shared Memory in one go. It’s way more efficient, I ran a benchmark rendering 10k rectangles on a canvas and the difference was huge: Emscripten hit around 40 FPS, while my setup hit 100 FPS.

But writing DOM logic in C++ is painful, so I built Coi. It’s a component-based language that statically analyzes changes at compile-time to enable O(1) reactivity. Unlike traditional frameworks, there is no Virtual DOM overhead; the compiler maps state changes directly to specific handles in the command buffer.

I recently benchmarked this against React and Vue on a 1,000-row table: Coi came out on top for row creation, row updating and element swapping because it avoids the "diffing" step entirely and minimizes bridge crossings. Its bundle size was also the smallest of the three.

One of the coolest things about the architecture is how the standard library works. If I want to support a new browser API (like Web Audio or a new Canvas feature), I just add the definition to my WebCC schema file. When I recompile the Coi compiler, the language automatically gains a new standard library function to access that API. There is zero manual wrapping involved.

I'm really proud of how it's coming along. It combines the performance of a custom WASM stack with a syntax that actually feels good to write (for me atleast :P). Plus, since the intermediate step is C++, I’m looking into making it work on the server side too, which would allow for sharing components across the whole stack.

Example (Coi Code):

component Counter(string label, mut int& value) {

def add(int i) : void {
value += i;
}

style {
.counter {
display: flex;
gap: 12px;
align-items: center;
}
button {
padding: 8px 16px;
cursor: pointer;
}
}

view {
<div class="counter">
<span>{label}: {value}</span>
<button onclick={add(1)}>+</button>
<button onclick={add(-1)}>-</button>
</div>
}

}

component App {
mut int score = 0;

style {
.app {
padding: 24px;
font-family: system-ui;
}
h1 {
color: #1a73e8;
}
.win {
color: #34a853;
font-weight: bold;
}
}

view {
<div class="app">
<h1>Score: {score}</h1>
<Counter label="Player" &value={score} />
<if score >= 10>
<p class="win">You win!</p>
</if>
</div>
}

}

app {
root = App;
title = "My Counter App";
description = "A simple counter built with Coi";
lang = "en";
}

Live Demo: https://io-eric.github.io/coi

Coi (The Language): https://github.com/io-eric/coi

WebCC: https://github.com/io-eric/webcc

I'd love to hear what you think. It's still far from finished, but as a side project I'm really excited about :)