docs-contributing-architecture.md

Architecture of Javy

This document is intended to provide an overview of the architecture of javy.

Crates

flowchart TD
  javy-cli --> javy-codegen --> wasm
  subgraph wasm[plugin.wasm]
  javy-plugin --> javy-plugin-api
  javy-plugin-api --> javy
  javy --> rquickjs
  end

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We anticipate most changes will be to the javy-cli, javy-codegen, and javy crates.

javy

The entrypoint for working with Javy as a library for third parties. This crate is intended to compile to wasm32-wasip1 and provide ergonomic APIs for configuring a QuickJS-based runtime. If there is a configuration option for QuickJS that would be helpful, this is the place to add it.

Important concepts

• javy::Runtime - a configurable QuickJS runtime.
• javy::Config - a configuration for the runtime.

Example

This is a contrived example of how to make a change. If I want to add a configuration to set a global variable called javy_rocks to true, I would do the following:

In crates/javy/src/config.rs:

  /// A configuration for [`Runtime`](crate::Runtime).
  #[derive(Debug)]
  pub struct Config {
+     pub(crate) javy_rocks: bool,
  }

  impl Default for Config {
      /// Creates a [`Config`] with default values.
      fn default() -> Self {
          Self {
+             javy_rocks: false,
          }
      }
  }

  impl Config {
+     /// Sets `globalThis.javy_rocks` to `true`.
+     pub fn javy_rocks(&mut self) -> &mut Self {
+         self.javy_rocks = true;
+         self
+     }
  }

In crates/javy/src/apis/javy_rocks.rs:

pub struct JavyRocks;

impl Instrinsic for JavyRocks {
    unsafe fn add_intrinsic(ctx: NonNull<qjs::JSContext>) {
        register(Ctx::from_raw(ctx)).expect("registering Javy Rocks to succeed")
    }
}

fn register<'js>(this: Ctx<'js>) -> Result<()> {
    let globals = this.globals();
    globals.set("javy_rocks", true);
}

In crates/javy/src/runtime.rs:

+        if cfg.javy_rocks {
+            unsafe {
+                JavyRocks::add_intrinsic(ctx.as_raw())
+            }
+        }

Define a new property on the Config, in runtime's build_from_config add the intrinsic if the config property is enabled, and use context to define the configuration.

When to add a cargo feature

You should consider gating your feature by a Cargo feature when your feature would materially increase the size of the produced Wasm module.

These are guidelines and we're willing to discuss if a feature needs to be gated by a Cargo feature on a case-by-case basis.

javy-cli

The CLI that drives the javy-codegen crate to compile JS to Wasm. This isn't intended to be a CLI that accommodates all uses for all users but rather to provide a useful base of functionality.

When to add a cargo feature

You should gate your feature with a cargo feature if your feature/change:

• Is not commonly going to be used and it would complicate the CLI options to include enabling it. For example, printing the .wat of a dynamic module is not something users would want 99.9% of the time and including it as an option on the CLI would make the --help output harder for most users to understand.

• You want to have integration tests in the javy-cli crate that should only run when the javy-plugin crate is built with a non-default configuration (that is, with different cargo features enabled).

javy-codegen

A Rust crate for compiling JS to Wasm.

javy-plugin

Gets compiled to plugin.wasm for use by the CLI and in environments for running dynamically linked modules. This is the default plugin for Javy. This isn't intended to be used as a code library by third parties. Defines a an initialize_runtime function that uses a configuration structure to allow the CLI to set various JS runtime configuration options.

When to add a cargo feature

You should gate your feature with a cargo feature if you want to support building a Wasm module with a very unusual configuration of the runtime.

javy-plugin-api

Used by Javy plugins to provide common implementations for exports and custom sections the plugin is expected to expose. This drives the APIs exposed by the javy crate. For example, this crate adds a Wasm function export for invoke to the plugin which is used for actually running the JavaScript.

npm packages

javy

A JS library providing ergonomic helpers around the lower level APIs for I/O exposed by the javy crate.