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WebAssembly tools for JavaScript

Universal WebAssembly tools for JavaScript.


This project aims to make a common and future-proof solution for WebAssembly modules support suitable for various JavaScript runtimes, frameworks, and build Tools following WebAssembly/ES Module Integration proposal from WebAssembly Community Group as much as possible while also trying to keep compatibility with current ecosystem libraries.

Bindings API

When importing a .wasm module, unwasm resolves, reads, and then parses the module during the build process to get the information about imports and exports and even tries to automatically resolve imports and generate appropriate code bindings for the bundler.

If the target environment supports top level await and also the wasm module requires no imports object (or they are auto resolvable), unwasm generates bindings to allow importing wasm module like any other ESM import.

If the target environment lacks support for top-level await or the wasm module requires an imports object or lazy plugin option is set to true, unwasm will export a wrapped Proxy object which can be called as a function to evaluate the module with custom imports object lazily. This way we still have a simple syntax as close as possible to ESM modules and also we can lazily initialize modules.

Example: Using static import

import { sum } from "unwasm/examples/sum.wasm";

Example: Using dynamic import

const { sum } = await import("unwasm/examples/sum.wasm");

If your WebAssembly module requires an import object (unwasm can automatically infer them), the usage syntax would be slightly different as we need to initiate the module with an import object first.

Example: Using dynamic import with imports object

const { rand } = await import("unwasm/examples/rand.wasm").then((r) =>
    env: {
      seed: () => () => Math.random() * Date.now(),

Example: Using static import with imports object

import initRand, { rand } from "unwasm/examples/rand.wasm";

await initRand({
  env: {
    seed: () => () => Math.random() * Date.now(),
When using static import syntax, and before initializing the module, the named exports will be wrapped into a function by proxy that waits for the module initialization and if called before init, will immediately try to call init without imports and return a Promise that calls a function after init.

Module compatibility

There are situations where libraries require a WebAssembly.Module instance to initialize WebAssembly.Instance themselves. In order to maximize compatibility, unwasm allows a specific import suffix ?module to import .wasm files as a Module directly.

import _sumMod from "unwasm/examples/sum.wasm?module";
const { sum } = await WebAssembly.instantiate(_sumMod).then((i) => i.exports);
Open an issue to us! We would love to help those libraries to migrate!


Unwasm needs to transform the .wasm imports to the compatible bindings. Currently, the only method is using a rollup plugin. In the future, more usage methods will be introduced.


First, install the unwasm npm package.

# ✨ Auto-detect
npx nypm install unwasm

# npm
npm install unwasm

# yarn
yarn add unwasm

# pnpm
pnpm install unwasm

# bun
bun install unwasm

Builder Plugins

// rollup.config.js
import { rollup as unwasm } from "unwasm/plugin";

export default {
  plugins: [
      /* options */

Plugin Options

  • esmImport: Direct import the wasm file instead of bundling, required in Cloudflare Workers and works with environments that allow natively importing a .wasm module (default is false)
  • lazy: Import .wasm files using a lazily evaluated proxy for compatibility with runtimes without top-level await support (default is false)


unwasm provides useful build tools to operate on .wasm modules directly.

Note: unwasm/tools subpath export is not meant or optimized for production runtime. Only rely on it for development and build time.


Parses wasm binary format with useful information using webassemblyjs/wasm-parser.

import { readFile } from "node:fs/promises";
import { parseWasm } from "unwasm/tools";

const source = await readFile(new URL("./examples/sum.wasm", import.meta.url));
const parsed = parseWasm(source);
console.log(JSON.stringify(parsed, undefined, 2));

Example parsed result:

  "modules": [
      "exports": [
          "id": 5,
          "name": "rand",
          "type": "Func"
          "id": 0,
          "name": "memory",
          "type": "Memory"
      "imports": [
          "module": "env",
          "name": "seed",
          "params": [],
          "returnType": "f64"

Auto Imports

unwasm can automatically infer the imports object and bundle them using imports maps (read more: MDN, Node.js and WICG).

To hint to the bundler how to resolve imports needed by the .wasm file, you need to define them in a parent package.json file.


  "exports": {
    "./rand.wasm": "./rand.wasm"
  "imports": {
    "env": "./env.mjs"

Note: The imports can also be prefixed with # like #env if you like to respect Node.js conventions.


Local development
  • Clone this repository
  • Install the latest LTS version of Node.js
  • Enable Corepack using corepack enable
  • Install dependencies using pnpm install
  • Run tests using pnpm dev or pnpm test


Published under the MIT license. Made by @pi0 and community 💛