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This repository consists of multiple packages:

Wasmer WASI

Isomorphic Javascript library for interacting with WASI Modules in Node.js, the Browser and Deno. The Javascript Package supports:

  • [X] WASI (with command args, envs and stdio)
  • [X] In-Memory filesystem (MemFS)


For instaling @wasmer/wasi run this command in your shell:

npm install --save @wasmer/wasi

And then import it in your server or client-side code with:

import { init, WASI } from '@wasmer/wasi';

Check the Node usage examples in https://github.com/wasmerio/wasmer-js/tree/main/examples/node


This package is published in Deno in the wasm package, you can import it directly with:

import { init, WASI } from 'https://deno.land/x/wasm/wasi.ts';

Check the Deno usage Examples in https://github.com/wasmerio/wasmer-js/tree/main/examples/deno


// This is needed to load the WASI library first (since is a Wasm module)
await init();

let wasi = new WASI({
  env: {
      // 'ENVVAR1': '1',
      // 'ENVVAR2': '2'
  args: [
      // 'command', 'arg1', 'arg2'

const moduleBytes = fetch("https://deno.land/x/wasm/tests/demo.wasm");
const module = await WebAssembly.compileStreaming(moduleBytes);
// Instantiate the WASI module
await wasi.instantiate(module, {});

// Run the start function
let exitCode = wasi.start();
let stdout = wasi.getStdoutString();

 // This should print "hello world (exit code: 0)"
console.log(`${stdout}(exit code: ${exitCode})`);

API Docs

Typescript API

export class WASI {
  constructor(config: any);
  readonly fs: MemFS;

  instantiate(module: any, imports: object): WebAssembly.Instance;
  // Start the WASI Instance, it returns the status code when calling the start
  // function
  start(instance: WebAssembly.Instance): number;
  // Get the stdout buffer
  // Note: this method flushes the stdout
  getStdoutBuffer(): Uint8Array;
  // Get the stdout data as a string
  // Note: this method flushes the stdout
  getStdoutString(): string;
  // Get the stderr buffer
  // Note: this method flushes the stderr
  getStderrBuffer(): Uint8Array;
  // Get the stderr data as a string
  // Note: this method flushes the stderr
  getStderrString(): string;
  // Set the stdin buffer
  setStdinBuffer(buf: Uint8Array): void;
  // Set the stdin data as a string
  setStdinString(input: string): void;

export class MemFS {
  readDir(path: string): Array<any>;
  createDir(path: string): void;
  removeDir(path: string): void;
  removeFile(path: string): void;
  rename(path: string, to: string): void;
  metadata(path: string): object;
  open(path: string, options: any): JSVirtualFile;

export class JSVirtualFile {
  lastAccessed(): BigInt;
  lastModified(): BigInt;
  createdTime(): BigInt;
  size(): BigInt;
  setLength(new_size: BigInt): void;
  read(): Uint8Array;
  readString(): string;
  write(buf: Uint8Array): number;
  writeString(buf: string): number;
  flush(): void;
  seek(position: number): number;


To build this library you will need to have installed in your system:

npm i
npm run build


Build the pkg and run the tests:

npm run build
npm run test

What is WebAssembly?

Quoting the WebAssembly site:

WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server applications.

About speed:

WebAssembly aims to execute at native speed by taking advantage of common hardware capabilities available on a wide range of platforms.

About safety:

WebAssembly describes a memory-safe, sandboxed execution environment […].


The entire project is under the MIT License. Please read the LICENSE file.



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