Ant: A Custom-Engine JavaScript Runtime for the Edge

For years, the JavaScript runtime wars have been fought on a massive scale. Node.js remains the enterprise default, Deno offers a secure-by-default alternative, and Bun delivers blistering speed. Yet all three share a common trait: they are heavy. Weighing in between 60 MB and 120 MB, these runtimes rely on monolithic engines like Google's V8 or Apple's JavaScriptCore.

This footprint is fine for a long-running server, but it is a bottleneck for edge computing, serverless cold starts, and embedded systems.

Enter Ant, a lightweight JavaScript runtime built from scratch in C and Zig. Created by developer theMackabu, Ant does not wrap an existing browser engine. Instead, it runs on a custom virtual machine called Silver VM, paired with a lightweight JIT compiler. The result is a runtime that boots in single-digit milliseconds and compiles down to a fraction of the size of its competitors.

Under the Hood of Silver VM

Most modern JavaScript runtimes are essentially wrappers around browser engines. While this guarantees high spec compliance, it pulls in millions of lines of legacy code designed for web browsers. Ant takes a different path. Written primarily in C (about 74%) and Zig (6%), its engine is hand-built.

Silver VM uses a custom bytecode virtual machine with a 64-bit NaN-boxing value representation. NaN-boxing is a technique where double-precision floating-point numbers (which have unused bit patterns when representing "Not a Number") are used to store pointers and other data types. This keeps memory overhead incredibly low.

For execution speed, Ant does not rely on a massive JIT infrastructure like V8's TurboFan. Instead, its JIT compiler uses a fork of MIR (Medium Intermediate Representation), a lightweight compiler backend designed for fast compilation and low memory usage. This architecture allows Ant to achieve near-compiled performance without the massive binary bloat.

When compiled with size optimization (-Os), the Ant binary shrinks to just 4.1 MB. Even a standard build is only about 8.5 MB.

The Cold Start Advantage

In serverless environments, cold start latency is the metric that matters most. To isolate module resolution and initialization overhead, the creator benchmarked how quickly each runtime could import the Hono framework, register routes, and exit.

The benchmark was executed on an Apple M5 Pro (18 cores, 64 GB RAM) running macOS.

xychart-beta
    title "Cold Start Latency (ms) - Hono Import & Exit"
    x-axis [Ant, Bun, Deno, Node.js]
    y-axis "Latency (ms)" 0 --> 35
    bar [5.5, 10.6, 24.8, 28.7]

Ant initialized and exited in 5.5 ms. Bun followed at 10.6 ms, while Deno and Node.js lagged behind at 24.8 ms and 28.7 ms respectively. For high-frequency, short-lived edge functions, cutting startup latency by half compared to Bun and by nearly 80% compared to Node.js is a significant win.

Spec Conformance and the Compatibility Trade-off

Building a JavaScript engine from scratch is notoriously difficult because the ECMAScript specification is vast and full of edge cases. Ant targets the WinterTC Minimum Common API specification, the standard for server-side JavaScript interoperability managed by Ecma TC55.

Its current compatibility profile reveals a pragmatic, albeit incomplete, implementation:

• compat-table: 100% pass rate across ES1 through ES5, ES6, ES2016+, and ESNext.
• test262: Approximately 64% pass rate on the official ECMAScript conformance suite.

The 64% pass rate on test262 is the project's biggest caveat. While Ant is highly optimized for real-world APIs and common web frameworks like Hono, it is not yet a drop-in replacement for general-purpose Node.js applications. Complex libraries that rely on obscure language features or deep Node.js internal APIs will fail.

The Developer Angle: Where Does Ant Fit?

If you are building a standard React SSR application or a massive enterprise backend, you should stick to Node.js or Bun. Ant is not ready for those workloads, and its ecosystem is still in its infancy.

However, Ant is worth evaluating for three specific scenarios:

1. Ultra-Low-Latency Edge Functions: If you are running lightweight HTTP APIs on edge networks where you are billed per millisecond of execution time, Ant's 5.5 ms cold start and tiny memory footprint can directly reduce cloud spend.
2. Embedded and IoT Systems: Running JavaScript on resource-constrained hardware has historically required specialized runtimes like JerryScript or XS. Ant offers a modern alternative with JIT compilation and TypeScript support out of the box.
3. Fast CLI Tools: Command-line utilities written in Node.js often feel sluggish because of the startup delay. Ant's instant boot time makes it an excellent candidate for building snappy developer tools.

Adopting Ant also comes with some practical friction. The most amusing, yet annoying, hurdle is its name. "Ant" is already a legendary Apache project (the Java-based build tool from the early 2000s) and it completely dominates the ant namespace on npm. If you are searching for documentation, troubleshooting errors, or managing packages, you will have to carefully filter out legacy Java build tool results.

A Promising Alternative

Ant is a highly impressive technical achievement. It proves that the JavaScript runtime ecosystem does not have to be a monoculture of massive, browser-derived engines. By focusing on a custom VM and a lightweight JIT, it carves out a compelling niche where size and startup speed are the only metrics that matter.

It is still too early to migrate production workloads to Ant, but the project is moving quickly. If the maintainers can push the test262 conformance score past 80% while keeping the binary under 10 MB, they will have a formidable contender for the future of edge computing.