Transforming Modules

Note: The final code for this example can be found on Github: transforming-modules.

Irrespective of whether your JavaScript code runs on the client or the server, the statement shown below to transform a WASI module will be always needed until browsers land BigInt support in WebAssembly.

Setup Instructions

Please repeat the step-by-step instructions given in the Hello World example, but with the following changes:

  1. Call your project wasmer-js-transforming-wasi

  2. Download the Wasm module clocktimeget.wasm and store it in the static directory

JavaScript Coding

The coding seen below is very similar to the coding used for the previous Hello World example — but with one very important difference!

Inside function startWasiTask, we fetch the Wasm file contents and convert it to a Uint8Array as before, but then there is the additional line:

const loweredWasmBytes = await lowerI64Imports(wasmBytes)

The call to function lowerI64Imports performs the all-important transformation that allows a JavaScript BigInt to be transferred to a WebAssembly i64.

Now that the interface has been transformed, we can instantiate the WebAssembly module and invoke it as before.

import { WasmFs } from '@wasmer/wasmfs'
import { lowerI64Imports } from "@wasmer/wasm-transformer"
import { WASI } from '@wasmer/wasi'
import browserBindings from "@wasmer/wasi/lib/bindings/browser"
const wasmFilePath = '/clocktimeget.wasm' // Path to our WASI module
// Instantiate new WASI and WasmFs Instances
// Instantiating WasmFs is only needed when running in a browser.
// When running on the server, NodeJS's native FS module is assigned by default
const wasmFs = new WasmFs()
let wasi = new WASI({
// Arguments passed to the Wasm Module
// The first argument is usually the filepath to the executable WASI module
// we want to run.
args: [wasmFilePath],
// Environment variables that are accesible to the WASI module
env: {},
// Bindings used by the WASI instance (fs, path, etc...)
bindings: {
fs: wasmFs.fs
// Async Function to run our WASI module/instance
const startWasiTask =
async () => {
// Fetch our Wasm File
const response = await fetch(wasmFilePath)
const wasmBytes = new Uint8Array(await response.arrayBuffer())
// We must transform the WebAssembly module interface!
const loweredWasmBytes = await lowerI64Imports(wasmBytes)
// Instantiate the WebAssembly file
let wasmModule = await WebAssembly.compile(loweredWasmBytes);
let instance = await WebAssembly.instantiate(wasmModule, {
wasi.start(instance) // Start the transformed WASI instance
let stdout = await wasmFs.getStdOut() // Get the contents of stdout
document.write(`Standard Output: ${stdout}`) // Write stdout to the DOM
// Everything starts here

On the both the browser screen and the JavaScript console, you should see the text Done!.

Known Limitation

This example is somewhat contrived because the WebAssembly module has been hard-coded to return the text string Done! rather than the time value from clock_time_get.

Anything written to standard out should be a printable string followed by a carriage return character, not the raw i32 value returned from clock_time_get. Therefore, before being able to return the actual clock time, this WebAssembly module would additionally need to convert the raw i32 value to a printable string before then writing it to standard out.

Next, let's look at handling input and output via WASI.

If you want to run the examples from the docs codebase directly, you can also do:

git clone
npm run dev