Basic Rust Wasm App

This is an example of a simple way to build and run a rust + wasm app using only cargo and wasm-bindgen. When working this way there is one thing you have to remember: the wasm-bindgen used by your app and the wasm-bindgen-cli versions must be the same. So I'd suggest fixing your wasm-bindgen in the Cargo.toml:

wasm-bindgen = "=0.2.68"

then installing wasm-bindgen-cli a matching version with cargo:

cargo install --version 0.2.68 -- wasm-bindgen-cli

Details of the wasm-bindgen-cli are here or after installing just run wasm-bindgen -h.

Compiling to Wasm

First use cargo to compile to the wasm32-unknown-unknown target:

cargo build --target wasm32-unknown-unknown

This will generate a .wasm file at ./target/wasm32-unknown-unknown/debug/basic_wasm_app.wasm.

Then use wasm-bindgen-cli to create the required js glue code for loading wasm in web browser:

wasm-bindgen ./target/wasm32-unknown-unknown/debug/basic_wasm_app.wasm --target web --no-typescript --out-dir ./dist/js

Which generates the following two files in the dist/js folder:

• basic_wasm_app_bg.wasm - your app compiled to wasm
• basic_wasm_app.js - required js glue code to load a wasm module in a browser

Now all you need to do is use some very basic js / html to load and run the wasm:

<script type="module">
    import init from './js/basic_wasm_app.js';
    init();
</script>

Running

Now you just need to serve the dist/index.html file into a browser and check the console for "Hello world".

Open the file directly

If you want to just open the file url (no server) you will likely run into a CORS error. This is an easy fix with Firefox: browse to about:config and set privacy.file_unique_origin = false. Other browsers can be done... just search the internet.

Then just browse to file:///...your_local_path.../basic_wasm_app/dist/index.html.

Use a server

Spin up your favourite server and navigate to /dist/index.html. An easy rust based way to start is basic-http-server.

cargo install basic-http-server
basic-http-server

And go to http://127.0.0.1:4000/dist/index.html

Notes

Running WebAssembly

Loading WebAssembly in a browser is explained here. The wasm-bindgen-cli generates all this code for you and it exports a final async function init(input) {...} in the generated js glue code: in the script above all we did was import this function and then call it.

Why the app starts upon loading

As the fn main() is annotated with #[wasm_bindgen(start)] as soon as the wasm loads it runs. If you wanted to do it yourself:

lib.rs

#[wasm_bindgen]
pub fn my_manual_start() {
    web_sys::console::log_1(&"hello world".into());
}

index.html

<script type="module">
    import init, { my_manual_start } from './js/basic_wasm_app.js';
    async function run() {
        await init();
        my_manual_start();
    }
    run();
</script>