The last blog post laid out the Rust and WebAssembly domain working group’s overall vision for Rust and WebAssembly. In this blog post, we will dive into the details of wasm-bindgen, the future we envision for it, and how you can help us build that future.

wasm-bindgen facilitates communication between JavaScript and Rust compiled to WebAssembly. It allows you to speak in terms of Rust structs, JavaScript classes, strings, etc… instead of only the integers and floats supported by WebAssembly’s raw calling convention. You only pay for what you use, so using alert won’t pull in unused bindings for requestAnimationFrame. Additionally, it is designed to support the upcoming “Host Bindings” proposal, which will eliminate the need for any kind of JavaScript shim functions between WebAssembly functions and native DOM functions. This promises to unlock even-faster-than-JavaScript DOM access, since DOM API calls can be validated at the time that the WebAssembly is compiled, and won’t need to be dynamically checked on every invocation.

wasm-bindgen is a Shared Foundation

We are building a shared foundation for an ecosystem of Rust crates that target JavaScript environments with wasm-bindgen. Sharing a foundation means sharing raw extern imports. Every library that uses the Web’s window.requestAnimationFrame function or ECMAScript’s Object.freeze function shouldn’t need to write the extern imports themselves.

Having bindings to all these APIs already in one place should make it easy for people to write really neat libraries like MPSC channels built on top of the postMessage API and other utility crates for the Web.

Sharing ECMAScript Global APIs

The global APIs that are available in every JavaScript environment, as defined by the ECMAScript standard, are being made available through the wasm_bindgen::js module. For example, the Object.freeze function is available as wasm_bindgen::js::Object::freeze.

But we aren’t done yet, and we need more help! Adding more of these global API bindings is a great way to start with wasm-bindgen and there is lots of work that can be done in concurrent pull requests from various contributors!

Check out the ECMAScript global APIs meta issue to help out!

Sharing Web APIs

ECMAScript’s global APIs aren’t the end of the story – we also need shared bindings for the Web’s DOM APIs, such as window.requestAnimationFrame, Node.prototype.appendChild, and WebSocket.

All of the Web’s API types, functions, and methods are specified with WebIDL, so we are working on a new WebIDL frontend to wasm-bindgen. When the WebIDL frontend is ready, we plan on taking the interface definitions for all the Web APIs from all their standards and automatically generating a big -sys style crate from them.

What do we mean by a new “frontend”? Recall the simplified description of wasm-bindgen’s architecture today from the last blog post. It is a procedural macro that parses #[wasm_bindgen] annotations that describe extern JavaScript imports and Rust things to export to JavaScript into an abstract syntax tree (AST). Then, it walks the AST and produces two results: Rust glue to use imported JavaScript things, JavaScript glue to use exported Rust things. We call the code responsible for parsing #[wasm_bindgen] declarations into the AST the “procedural macro frontend”.

wasm-bindgen's current architecture

With the WebIDL frontend, we are introducing a new way to create the AST: parsing WebIDL and turning its definitions into wasm-bindgen’s AST. When using the WebIDL frontend, there are no #[wasm_bindgen] annotations or procedural macros involved anymore, but the rest of the pipeline after the AST is constructed is the same.

wasm-bindgen's new architecture with a WebIDL frontend

Does this sound cool to you? Check out issues labeled “frontend:webidl” in the wasm-bindgen repository to help us build the WebIDL frontend! The WebIDL frontend’s code lives in wasm-bindgen/crates/webidl, and if you have any questions, don’t hesitate to ping fitzgen in #rust-wasm on or drop a comment in the relevant issue.

Special shout out to @ohanar, who has been doing a wonderful job adding support for converting various WebIDL constructs into wasm-bindgen’s AST! We need more folks like @ohanar willing to step up in the same way. 🙏