The Package for using WASI easily from Node and the Browser


import { WASI } from "@wasmer/wasi"


new WASI(wasiConfigObject): WASI
Constructs a new WASI instance.
The wasiConfigObject is as follows:
const browserBindings = require("@wasmer/wasi/lib/bindings/browser");
// for Node:
// const nodeBindings = require("@wasmer/wasi/lib/bindings/node");
let myWASIInstance = new WASI({
// OPTIONAL: The pre-opened dirctories
preopenDirectories: {},
// OPTIONAL: The environment vars
env: {},
// OPTIONAL: The arguments provided
args: [],
// OPTIONAL: The environment bindings (fs, path),
// useful for using WASI in diferent environments
// such as Node.js, Browsers, ...
bindings: {
// hrtime: (time?: [number, number]) -> number
// exit: (code?: number) -> void
// kill: (pid: number, signal?: string | number) -> void
// randomFillSync: (buffer: Buffer, offset?: number, size?: number) -> Buffer
// isTTY: () -> bool
// fs: Filesystem (with similar API interface as Node 'fs' module)
// path: Path (with similar API Interface as Node 'path' module)
...browserBindings // Use `nodeBindings` for Node
This returns a WASI instance. Please see the Instance properties section to learn about the WASI instance.

Class Properties

Instance Properties


wasiInstance.memory: WebAssembly.Memory
WebAssembly.memory object, that is a view into the Wasm Module's linear memory.


wasiInstance.view: DataView
DataView object, that is a view into the Wasm Module's linear memory.


wasiInstance.FD_MAP: Map<number, File>
Javascript Map, where the key is the file descriptor, and the value is a Javascript File.


wasiInstance.exports: Exports
WASI API to be imported in the importObject on instantiation.


wasiInstance.bindings: WASIBindings
The bindings for common node like objects, such as fs for filesystem, these should work by default, but are applied depending on the platform. You can view the source code for your respective platform's bindings here.


wasiInstance.start(wasmInstance: WebAssembly.Instance): void
Function that takes in a WASI WebAssembly Instance and starts it.


wasiInstance.getImports(wasmModule: WebAssembly.Module): Exports
Function that returns the map of corresponding imports for the WASI module. It will throw an error in case the wasmModule is not a WASI Module, or it have an incompatible version.