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6.2.2 • Public • Published

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🍣 A Rollup which allows importing and bundling WebAssembly modules.

WebAssembly Modules are imported asynchronous as base64 strings. Small modules can be imported synchronously.


This plugin requires an LTS Node version (v14.0.0+) and Rollup v1.20.0+.


Using npm:

npm install @rollup/plugin-wasm --save-dev


Create a rollup.config.js configuration file and import the plugin:

import { wasm } from '@rollup/plugin-wasm';

export default {
  input: 'src/index.js',
  output: {
    dir: 'output',
    format: 'cjs'
  plugins: [wasm()]

Then call rollup either via the CLI or the API.



Type: String | Array[...String]
Default: null

A picomatch pattern, or array of patterns, which specifies the files in the build the plugin should ignore. By default no files are ignored.


Type: String | Array[...String]
Default: null

A picomatch pattern, or array of patterns, which specifies the files in the build the plugin should operate on. By default all wasm files are targeted.


Type: Array[...String]
Default: null

Specifies an array of strings that each represent a WebAssembly file to load synchronously. See Synchronous Modules for a functional example.


Type: Number
Default: 14336 (14kb)

The maximum file size for inline files. If a file exceeds this limit, it will be copied to the destination folder and loaded from a separate file at runtime. If maxFileSize is set to 0 all files will be copied.

Files specified in sync to load synchronously are always inlined, regardless of size.


Type: String
Default: '[hash][extname]'

This option can be used to rename the emitted Wasm files. It accepts the following string replacements:

  • [hash] - The hash value of the file's contents
  • [name] - The name of the imported file (without its file extension)
  • [extname] - The extension of the imported file (including the leading .)


Type: String
Default: (empty string)

A string which will be added in front of filenames when they are not inlined but are copied.


Type: "auto" | "browser" | "node"
Default: "auto"

Configures what code is emitted to instantiate the Wasm (both inline and separate):

  • "auto" will determine the environment at runtime and invoke the correct methods accordingly
  • "auto-inline" always inlines the Wasm and will decode it according to the environment
  • "browser" omits emitting code that requires node.js builtin modules that may play havoc on downstream bundlers
  • "node" omits emitting code that requires fetch


This plugin looks for import statements where the file specifier ends with .wasm, such as:

import wasm from './example.wasm';

The WebAssembly is inlined as a base64 encoded string. At runtime the string is decoded and a module is returned.

Note: The base64 string that represents the WebAssembly within the bundle will be ~33% larger than the original file.

When bundled, wasm is a function you can use to instantiate the functionality inside of the WebAssembly module. This function returns a promise, so you can instantiate the module like this:

import wasm from './example.wasm';

wasm().then(({ instance }) => {
  // use instance

// or use top-level await
const { instance } = await wasm();

The promise returns an object with an instance property that is a WebAssembly.Module object that you can use to interact with the WebAssembly module.

For example, given the following simple C file:

int main() {
  return 42;

Compile the file using emscripten, or the online WasmFiddle tool. Then import and instantiate the resulting file:

import sample from './sample.wasm';

sample().then(({ instance }) => {

Passing Imports and Memory

If you'd like to pass any imports or memory into your WebAssembly module, you can do so by passing those as arguments to the WebAssembly loader function, like this:

import sample from './sample.wasm';

const memory = new WebAssembly.Memory({ initial: 10, maximum: 100 });

const options = {
  js: {
    mem: memory
  imports: {
    log: (arg) => console.log(arg)

sample(options).then(({ instance }) => {

Because this example passes in imports, the instance becomes an instance of WebAssembly.Instance and gives the WebAssembly code access to the imports and memory from JavaScript.

__Note: Just passing in memory will result in a WebAssembly.Module as instance; it's only imports that triggers the creation of WebAssembly.Instance.

Using with wasm-bindgen and wasm-pack

If you are writing Rust code and using wasm-bindgen or wasm-pack, you'll need to use a different process for instantiating your WebAssembly modules. Because WebAssembly files generated by these tools require specific imports, you cannot provide these yourself.

The best setting to use with wasm-bindgen or wasm-pack is --target web. This will generate your WebAssembly files (such as sample_bg.wasm) with a JavaScript wrapper (such as sample.js). The JavaScript wrapper contains all of the instantiation code you'll need.

You'll need to import both the JavaScript file and the WebAssembly file into your project. Then, run the WebAssembly loading function and pass that into the init() function from the JavaScript file. Here's an example:

import init, { main } from '../build/sample.js';
import sample from '../build/sample_bg.wasm';

  .then({ instance } => init(instance))
  .then(() => main());

// or using top-level await

await init(await sample());

Unlike with the emscripten example, you'll need to import the methods you want to use directly from the JavaScript file rather than accessing them on the WebAssembly.Instance.

Synchronous Modules

JavaScript runtimes allow small modules (< 4KB) to be compiled synchronously. If you'd like to specify some files to be compiled synchronously, you can do so in your rollup.config.js file.

import { wasm } from '@rollup/plugin-wasm';

export default {
  input: 'src/index.js',
  output: {
    dir: 'output',
    format: 'cjs'
  plugins: [
      sync: ['web/sample.wasm', 'web/foobar.wasm']

Synchronous modules return a synchronous loader function that returns either a WebAssembly.Module or WebAssembly.Instance directly (not a promise). So you can use the module directly, like this:

import sample from './sample.wasm';

const instance = sample({ ...imports });





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