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Wasm in JavaScript

Wasm can be used from within a Worker written in JavaScript or TypeScript by importing a Wasm module, and instantiating an instance of this module using WebAssembly.instantiate() ↗. This can be used to accelerate computationally intensive operations which do not involve significant I/O.

This guide demonstrates the basics of Wasm and JavaScript interoperability.

Simple Wasm Module

In this guide, you will use the WebAssembly Text Format to create a simple Wasm module to understand how imports and exports work. In practice, you would not write code in this format. You would instead use the programming language of your choice and compile directly to WebAssembly Binary Format (.wasm).

Review the following example module (;; denotes a comment):

;; src/simple.wat
(module
  ;; Import a function from JavaScript named `imported_func`
  ;; which takes a single i32 argument and assign to
  ;; variable $i
  (func $i (import "imports" "imported_func") (param i32))
  ;; Export a function named `exported_func` which takes a
  ;; single i32 argument and returns an i32
  (func (export "exported_func") (param $input i32) (result i32)
    ;; Invoke `imported_func` with $input as argument
    local.get $input
    call $i
    ;; Return $input
    local.get $input
    return
  )
)

Using wat2wasm ↗, convert the WAT format to WebAssembly Binary Format:

Terminal window
wat2wasm src/simple.wat -o src/simple.wasm

Bundling

Wrangler will bundle any Wasm module that ends in .wasm or .wasm?module, so that it is available at runtime within your Worker. This is done using a default bundling rule which can be customized in wrangler.toml. Refer to Wrangler Bundling for more information.

Use from JavaScript

After you have converted the WAT format to WebAssembly Binary Format, import and use the Wasm module in your existing JavaScript or TypeScript Worker:

import mod from "./simple.wasm";


// Define imports available to Wasm instance.
const importObject = {
  imports: {
    imported_func: (arg: number) => {
      console.log(`Hello from JavaScript: ${arg}`);
    },
  },
};


// Create instance of WebAssembly Module `mod`, supplying
// the expected imports in `importObject`. This should be
// done at the top level of the script to avoid instantiation on every request.
const instance = await WebAssembly.instantiate(mod, importObject);


export default {
  async fetch() {
    // Invoke the `exported_func` from our Wasm Instance with
    // an argument.
    const retval = instance.exports.exported_func(42);
    // Return the return value!
    return new Response(`Success: ${retval}`);
  },
};

When invoked, this Worker should log Hello from JavaScript: 42 and return Success: 42, demonstrating the ability to invoke Wasm methods with arguments from JavaScript and vice versa.

Next steps

In practice, you will likely compile a language of your choice (such as Rust) to WebAssembly binaries. Many languages provide a bindgen to simplify the interaction between JavaScript and Wasm. These tools may integrate with your JavaScript bundler, and provide an API other than the WebAssembly API for initializing and invoking your Wasm module. As an example, refer to the Rust wasm-bindgen documentation ↗.

Alternatively, to write your entire Worker in Rust, Workers provides many of the same Runtime APIs and bindings when using the workers-rs crate. For more information, refer to the Workers Rust guide.

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