Node.js v15.0.0-v8-canary20200627d5b32a9a63 Documentation

WebAssembly System Interface (WASI)#

Stability: 1 - Experimental

Source Code: lib/wasi.js

The WASI API provides an implementation of the WebAssembly System Interface specification. WASI gives sandboxed WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.

'use strict';
const fs = require('fs');
const { WASI } = require('wasi');
const wasi = new WASI({
  args: process.argv,
  env: process.env,
  preopens: {
    '/sandbox': '/some/real/path/that/wasm/can/access'
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };

(async () => {
  const wasm = await WebAssembly.compile(fs.readFileSync('./demo.wasm'));
  const instance = await WebAssembly.instantiate(wasm, importObject);


To run the above example, create a new WebAssembly text format file named demo.wat:

    ;; 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.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
        ( (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

Use wabt to compile .wat to .wasm

$ wat2wasm demo.wat

The --experimental-wasi-unstable-preview1 and --experimental-wasm-bigint CLI arguments are needed for this example to run.

Class: WASI#

The WASI class provides the WASI system call API and additional convenience methods for working with WASI-based applications. Each WASI instance represents a distinct sandbox environment. For security purposes, each WASI instance must have its command line arguments, environment variables, and sandbox directory structure configured explicitly.

new WASI([options])#

  • options <Object>

    • args <Array> An array of strings that the WebAssembly application will see as command line arguments. The first argument is the virtual path to the WASI command itself. Default: [].
    • env <Object> An object similar to process.env that the WebAssembly application will see as its environment. Default: {}.
    • preopens <Object> This object represents the WebAssembly application's sandbox directory structure. The string keys of preopens are treated as directories within the sandbox. The corresponding values in preopens are the real paths to those directories on the host machine.
    • returnOnExit <boolean> By default, WASI applications terminate the Node.js process via the __wasi_proc_exit() function. Setting this option to true causes wasi.start() to return the exit code rather than terminate the process. Default: false.
    • stdin <integer> The file descriptor used as standard input in the WebAssembly application. Default: 0.
    • stdout <integer> The file descriptor used as standard output in the WebAssembly application. Default: 1.
    • stderr <integer> The file descriptor used as standard error in the WebAssembly application. Default: 2.


Attempt to begin execution of instance as a WASI command by invoking its _start() export. If instance does not contain a _start() export, or if instance contains an _initialize() export, then an exception is thrown.

start() requires that instance exports a WebAssembly.Memory named memory. If instance does not have a memory export an exception is thrown.

If start() is called more than once, an exception is thrown.


wasiImport is an object that implements the WASI system call API. This object should be passed as the wasi_snapshot_preview1 import during the instantiation of a WebAssembly.Instance.