Stability: 1 - Experimental

    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.

    1. 'use strict';
    2. const fs = require('fs');
    3. const { WASI } = require('wasi');
    4. const wasi = new WASI({
    5.   args: process.argv,
    6.   env: process.env,
    7.   preopens: {
    8.     '/sandbox': '/some/real/path/that/wasm/can/access'
    9.   }
    10. });
    11. const importObject = { wasi_snapshot_preview1: wasi.wasiImport };
    13. (async () => {
    14.   const wasm = await WebAssembly.compile(fs.readFileSync('./demo.wasm'));
    15.   const instance = await WebAssembly.instantiate(wasm, importObject);
    17.   wasi.start(instance);
    18. })();

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

    1. (module
    2.     ;; Import the required fd_write WASI function which will write the given io vectors to stdout
    3.     ;; The function signature for fd_write is:
    4.     ;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
    5.     (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
    7.     (memory 1)
    8.     (export "memory" (memory 0))
    10.     ;; Write 'hello world\n' to memory at an offset of 8 bytes
    11.     ;; Note the trailing newline which is required for the text to appear
    12.     (data (i32.const 8) "hello world\n")
    14.     (func $main (export "_start")
    15.         ;; Creating a new io vector within linear memory
    16.         (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
    17.         (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string
    19.         (call $fd_write
    20.             (i32.const 1) ;; file_descriptor - 1 for stdout
    21.             (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
    22.             (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
    23.             (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
    24.         )
    25.         drop ;; Discard the number of bytes written from the top of the stack
    26.     )
    27. )

    Use wabt to compile .wat to .wasm

    1. $ wat2wasm demo.wat

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