July 19, 2020. 9 minutes read

Getting started with NodeJS and the WebAssembly System Interface

WASI, the WebAssembly System Interface, is a capability-oriented set of APIs designed to standardize the sandboxed execution of WebAssembly modules outside of browsers. Specifically, WASI aims to be the common layer that WebAssembly modules can use to interface with host runtimes, and get granular access to OS specific objects (such as files, environment variables, or sockets). For an introduction to the goals and architecture of WASI, head over to the Mozilla blog and read Lin Clark’s announcement post .

Node.js has recently added experimental support for WASI. This means that if you are running a recent version of Node.js, you can natively start a WASI instance and execute WebAssembly modules in a sandboxed environment, and granularly pass environment variables, arguments, or file handles to the process running in the module, and in this article we will briefly explore how to get started, and execute WASI modules written in Rust and AssemblyScript.

All examples in this article can be found in this repository on GitHub .

Let’s start by running the example in the official documentation . The module there is written in the WebAssembly Text Format (and is in fact the same example from the Wasmtime documentation ).

Mozilla Developer Network has an excellent set of articles dedicated to understanding the Wasm text format , as well as describing in detail how to convert between the text and binary formats using the WebAssembly binary toolkit .

    ;; Import the required fd_write WASI function which will write the given io vectors to stdout
    ;; The function signature for fd_write is:
    ;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
    (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))

    (memory 1)
    (export "memory" (memory 0))

    ;; Write 'hello world\n' to memory at an offset of 8 bytes
    ;; Note the trailing newline which is required for the text to appear
    (data (i32.const 8) "hello world\n")

    (func $main (export "_start")
        ;; Creating a new io vector within linear memory
        (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
        (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string

        (call $fd_write
            (i32.const 1) ;; file_descriptor - 1 for stdout
            (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
            (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
        drop ;; Discard the number of bytes written from the top of the stack

This hand-written module imports fd_write from wasi_snapshot_preview1, a function exposed by WASI used for writing to a provided file descriptor, which is used later to write "hello world" to standard output. We can convert it to its binary format using wat2wasm , from the WebAssembly binary toolkit, and we can now instantiate it in WASI:

"use strict";
const fs = require("fs");
const { WASI } = require("wasi");
const wasi = new WASI();
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };

(async () => {
  const wasm = await WebAssembly.compile(
  const instance = await WebAssembly.instantiate(wasm, importObject);


We are creating a new instance of WASI, and we pass the wasi_snapshot_preview1 import as the native Node implementation of the WASI libc. Then, using Node 14.5 and running with the experimental flags enabled, we can execute the file above and get the expected output:

$ node --version
$ node --experimental-wasi-unstable-preview1 --experimental-wasm-bigint wasi.js

hello world

V8 uses a custom WASI libc implementation based on libuv, and the fd_write system call is implemented by V8 , which exposes a small public API for interacting with WASI in JavaScript.

If you are interested in the stability of the WASI imports, check the different phases here .

But we don’t usually execute system calls just to open a file, or write something to the console - we use higher level programming languages that abstract that through their standard libraries. Let’s see how we could achieve the same thing in other programming languages.

Rust and WASI

To start, you need to install the Rust toolchain on your machine and create the default cargo new project:

fn main() {
    println!("Hello, world from Rust!");

Then add the wasm32-wasi compiler target and compile the project to WASI:

$ rustup target add wasm32-wasi
$ cargo build --target wasm32-wasi

If we change the path of the module to the one generated by the Rust compiler (target/wasm32-wasi/debug/<your-app-name>.wasm) and run again:

$ node --experimental-wasi-unstable-preview1 --experimental-wasm-bigint wasi.js
Hello, world from Rust!

In both examples, we granted the modules the ability to write to standard output by linking the functionality implemented by the runtime in wasi_snapshot_preview1. If we don’t provide the WASI import - not add the wasi_snapshot_preview1 import:

const importObject = {};

Then the module cannot print to standard output:

node --experimental-wasi-unstable-preview1 --experimental-wasm-bigint wasi.js
(node:30775) UnhandledPromiseRejectionWarning:
TypeError: WebAssembly.instantiate(): Import #0 module="wasi_snapshot_preview1" error: module is not an object or function
    at nodejs-wasi-examples/wasi.js:14:38

Ok, now let’s see how we can grant access to files and directories. Consider the following Rust program:

use std::fs;

fn main() {
    let contents =
        fs::read_to_string("/sandbox/file.txt").expect("Something went wrong reading the file");
    println!("Content: {}", contents);

It tries to read the content of the file from /sandbox/file.txt - but by default, the WASI process does not have access to the file system:

thread 'main' panicked at
 'Something went wrong reading the file: Custom { kind: Other, error: "failed to find a preopened file descriptor through which \"/sandbox/file.txt\" could be opened" }', src/main.rs:6:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
(node:33051) UnhandledPromiseRejectionWarning: RuntimeError: unreachable
    at __rust_start_panic (<anonymous>:wasm-function[218]:0x9f29)
    at rust_panic (<anonymous>:wasm-function[212]:0x99a2)

We have to explicitly provide a directory mapping for the directories we want to allow access to - in this case the current directory.

const wasi = new WASI({
  preopens: {
    "/sandbox": process.cwd(),

And now it can read the file:

node --experimental-wasi-unstable-preview1 --experimental-wasm-bigint wasi.js
This is a file on disk mounted inside the sandboxed process at /sandbox.
It can be read, modified, or even deleted by the process running inside the module.

This is a very simple scenario, where the module always tries to open the file from /sandbox/file.txt - but this can also work by passing the file path as an argument to the WASI process, and the sandbox always make sure the process actually has access to the specified file.

AssemblyScript and WASI

AssemblyScript is a strict variant of TypeScript which is natively compiled to WebAssembly. (as opposed to TypeScript, which is transpiled to JavaScript). This means that users can easily create modules by writing a familiar language. Note, however, that most likely you will not be able to simply compile your favorite TypeScript library to WebAssembly, as it probably makes use of either browser or NodeJS features, which will not be available in the WebAssembly runtime without the necessary module imports. For a list of the basic functionality and limitations of AssemblyScript, check out the official documentation .

AssemblyScript doesn’t have native WASI support just yet - this is why we are going to use a library that offers the API layer for WASI syscalls - as-wasi . Basically, this creates a few high-level AssemblyScript objects to interact with WASI (for getting environment variables, or writing to files), and implements the translation layer that compiles to WebAssembly, allowing us to write AssemblyScript modules that can be then run in any WASI runtime.

Let’s see how a small AssemblyScript program that prints its arguments and the environment variables looks like.

First, we need the two dependencies - AssemblyScript itself, and as-wasi, and a small script that calls the AssemblyScript compiler:

    "scripts": {
        "asbuild": "asc index.ts -b build/as-wasi-example.wasm --use abort=wasi_abort"
    "dependencies": {
        "as-wasi": "0.0.1",
        "assemblyscript": "^0.14.0"

The --use abort=wasi_abort flag is used to explicitly bind the AssemblyScript abort function to the one provided by the runtime. There are ways to avoid having to use it , but for the purpose of this example, it is easier to just use it.

0.0.1 is not the latest version for as-wasi - however, there is a bug in newer versions that results in an allocation error when trying to use environment variables and command line arguments. A fix for the bug has already been merged, but until there is a new release, we can use version 0.0.1.

We read both environment variables and command line arguments and print them to standard output. Notice that the objects used are imported from as-wasi, and are different from the native NodeJS objects:

import { Console, Environ, CommandLine } from "as-wasi";

export function _start(): void {
  let env = new Environ();
  let all_vars = env.all();
  all_vars.forEach(function (val) {
    Console.log(val.key + "=" + val.value);

  let cmd = new CommandLine();
  Console.log("args: " + cmd.all().toString());

Now we can install the dependencies and build the module:

$ npm install
$ npm run-script asbuild

Running this example is similar - notice, however, the different arguments and environment variables:

const wasi = new WASI({
  args: ["arg1", "arg2"],
  env: {
    abc: "def",
    foo: "bar",
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };

(async () => {
  const wasm = await WebAssembly.compile(
  const instance = await WebAssembly.instantiate(wasm, importObject);


The args, env, and preopen objects from the WASI options can be used to granularly control the environment of the process running in the sandbox, and in this case, we are passing a set of new environment variables and arguments (although a subset of the real process.env and process.args can be passed instead, as needed):

$ node --experimental-wasi-unstable-preview1 --experimental-wasm-bigint wasi.js
Args are: arg1,arg2

Another programming language with native support for WASI is Zig - you can watch a live coding session by Jakub Konka exemplifying how to write Zig that compiles to WASI.


In this article we took a first look at running WebAssembly modules using the early native support in NodeJS for WASI, and explored writing WASI modules in Rust and AssemblyScript. While WASI support is still early in most environments, it is rapidly becoming a great way of running sandboxed and portable code across runtimes and operating systems.

Radu M

© Radu M 2020