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WASI JavaScript Bindings

This project aims to provide a generic framework for WASM programs built using WASI and run on the Web platform using JavaScript. It is meant to be portable across projects and avoids encoding project-specific logic in its runtime.

While the framework builds off of itself, it is not all-or-nothing. You‘re free to use some components if parts of the overall runtime model don’t align with your particular use case.

Table of Contents


[Wassh] is the project for building OpenSSH for WASM & WASI. As part of the development, we found that the WASI project lacked any real JS binding support. There is generated emscripten code, but that was built once a while ago, and hasn’t been updated since. Which means wassh will need to develop its own JS runtime.

As we fleshed things out, we recognized that a lot of the framework is not specific to wassh and could be reused by other WASM applications that want to run on the web. The market here is probably not significant (compared to e.g. emscripten), but it’s also probably not nothing, and we already need to do the majority of the work regardless. Most notably, the fact that WASM/WASI require all syscalls be handled synchronously while the JS world (and many of its APIs) can only be satisfied asynchronously.

Writing JS support code is basically writing an OS. The WASM code uses the WASI C library which makes syscalls to the JS world, and the JS world manages all the standard OS state (open files, etc...) while servicing the syscall requests.


As WASI is still under development, we often require current browser runtimes. No attempt is made to provide backwards compatibility or transpiling.

Framework Overview

These framework APIs are generally the ones we expect people to use. There are some more utility/internal APIs available if desired; see the API Reference for more details.

  • Process: Encapsulation of Programs & SyscallEntry's & SyscallHandlers.
    • Foreground: Programs that run synchronously in the current thread. All syscall handlers must be synchronous as well.
    • Background: Programs that run in dedicated background web workers. Syscall handlers may be synchronous or asynchronous.
  • Program: Encapsulation for the WASM instantiation & execution.
  • SyscallEntry: The initial entry point from the WASM world into JS. Takes care of reading/writing content to the WASM side and handing off to syscall handlers to implement things (using normal JS APIs).
    • WasiUnstable: Implementation of the WASI API.
  • SyscallHandler: The code that handles syscall requests using JS APIs. Does not speak to the WASM side at all -- everything is via standard JS.
    • DirectWasiUnstable: Handlers WASI API calls directly when possible (i.e. there is a general web platform implementation).
    • ProxyWasiUnstable: Dispatches WASI API calls via message passing and shared memory to a different thread so calls may be implemented asynchronously; does not provide any implementations itself.
  • Worker: Framework for implementing background web worker as Process.Background expects. Name bike-shedding TBD
  • WASI: Constants that match the WASI C library.

The SyscallHandler API is really how you bind your JS world to the WASM world. It is responsible for actually handling the syscalls via whatever unique state or paradigms used in your JS application.

For simple programs, it is expected that people will use Process.Foreground and SyscallEntry.WasiUnstable and SyscallHandler.DirectWasiUnstable APIs unmodified, perhaps with their own additional SyscallHandler class for things DirectWasiUnstable does not support.

For complicated programs, it is expected that people will use Process.Background SyscallEntry.WasiUnstable and SyscallHandler.ProxyWasiUnstable and SyscallHandler.DirectWasiUnstable APIs unmodified, and provide their own Worker and SyscallHandler implementations to round things out.


Check out [html/example.html].

Syscall Lifecycles

At a very high level:

WASM -> SyscallEntry -> SyscallHandler -> -> SyscallHandler -> SyscallEntry -> WASM


Most people will not need to implement their own SyscallEntry APIs at all. This framework provides complete coverage for the WASI API's.

SyscallEntry classes provide the initial entry points from the WASM world by implementing the WASI API. The function arguments are tightly coupled to that WASM world: constants reflect the WASI C library, pointers are absolute offsets into the program's memory to structures (which are also defined by the WASI C library). Their job is to unpack & translate the arguments to more natural JavaScript APIs before handing off execution to the SyscallHandler APIs.

They use a consistent naming convention like sys_<WASI syscall name>, so the fd_write WASI API syscall will be implemented here as sys_fd_write.

Once the WASI arguments have been unpacked, handle_<WASI syscall name> will be called in the SyscallHandler implementation. The APIs will often be simpler than the WASI API, so consult the API Reference section below.

In cases of errors, the errno value will always be returned directly. If cases of success, the handler will either return ESUCCESS (for simpler syscalls, or when outputs are via arguments), or return an object for more complicated outputs in which case ESUCCESS is always assumed.


SyscallHandler classes provide the actual implementation of syscalls. They use a consistent naming convention like handle_<WASI syscall name>, so the fd_write WASI API syscall will be implemented as handle_fd_write, although the function signature will be different. Consult the API Reference section below for the exact arguments.

There are two major variants: direct & proxied. Direct handlers must be synchronous as the WASM runtime does not support asynchronous calls. Proxied handlers may be asynchronous as they run in a different thread -- they may be marked async and/or return Promises that resolve to the right value (NB: in case of errors, return appropriate errno values instead of rejecting the promise).

API Reference

Replace with generated docs?

  • WasiView: DataView class with extensions for WASI C library structures.
  • SyscallLock: Utility class for managing shared memory/IPC between SyscallHandler.ProxyWasiUnstable and your syscall handler in another thread. Takes care of locking, passing return/error codes, and serializing objects.


See the common libapps for details.