No ES Modules

Explained a bit more in the internal design section one of the key foundational principles of wasm-bindgen is ES modules. It supports working without ES modules, however! Not all JS tooling and browsers are ready for ES modules by default, so it can sometimes be helpful to quickly get up and running without them to kick the tires and see how wasm-bindgen works.

Let's start out with our hello-world example from previous chapters, and you can also follow along in the repository.

# #![allow(unused_variables)]
#fn main() {
extern "C" {
    fn alert(msg: &str);

pub fn greet(name: &str) -> String {
    alert(&format!("Hello, {}!", name);

Like usual, we first compile this to wasm:

$ cargo build --target wasm32-unknown-unknown

Next, to avoid using ES modules, pass the --no-modules option to the wasm-bindgen command:

$ wasm-bindgen target/wasm32-unknown-unknown/debug/hello.wasm --no-modules --out-dir .

Next up we need to write some HTML to interact with the wasm:

    <script src='./hello.js'></script>
        .then(() => wasm_bindgen.greet('World'));

and that's it! If you open up that web page in a browser (needs to be over HTTP) then you should see an alert for "Hello, World!".

The --no-modules output will not instantiate or compile the wasm module when included on a web page, instead it just parses and configures the JS bindings for the wasm-module-to-be. The page is configured with one exported global, in this case wasm_bindgen. The name of this global can be configured with the --no-modules-global option.

The global wasm_bindgen is a function that takes one argument: either the path to the wasm file to fetch or a WebAssembly.Module. When invoked wasm_bindgen will return a promise for when the wasm module is ready-to-go. After that all exported functionality on wasm_bindgen will be functional.

In the example above, after calling wasm_bindgen('./hello_bg.wasm') we wait for the wasm module to be fetched and compiled, and afterwards we're invoking our greet export.

Note that exports are available for binding before the wasm module has been instantiated, for example this would have also worked:

const { greet } = wasm_bindgen;

  .then(() => greet('World'));