# Compiling to webassembly

At this stage RustPython only has preliminary support for web assembly. The instructions here are intended for developers or those wishing to run a toy example.

## Setup

To get started, install [wasm-pack](https://rustwasm.github.io/wasm-pack/installer/) and `npm`. ([wasm-bindgen](https://rustwasm.github.io/wasm-bindgen/whirlwind-tour/basic-usage.html) should be installed by `wasm-pack`. if not, install it yourself)

<!-- Using `rustup` add the compile target `wasm32-unknown-emscripten`. To do so you will need to have [rustup](https://rustup.rs/) installed.

```bash
rustup target add wasm32-unknown-emscripten
```

Next, install `emsdk`:

```bash
curl https://s3.amazonaws.com/mozilla-games/emscripten/releases/emsdk-portable.tar.gz | tar -zxv
cd emsdk-portable/
./emsdk update
./emsdk install sdk-incoming-64bit
./emsdk activate sdk-incoming-64bit
``` -->

## Build

Move into the `wasm` directory. This directory contains a library crate for interop
with python to rust to js and back in `wasm/lib`, the demo website found at
https://rustpython.github.io/demo in `wasm/demo`, and an example of how to use
the crate as a library in one's own JS app in `wasm/example`.

```sh
cd wasm
```

Go to the demo directory. This is the best way of seeing the changes made to either
the library or the JS demo, as the `rustpython_wasm` module is set to the global
JS variable `rp` on the website.

```sh
cd demo
```

Now, start the webpack development server. It'll compile the crate and then
the demo app. This will likely take a long time, both the wasm-pack portion and
the webpack portion (from after it says "Your crate has been correctly compiled"),
so be patient.

```sh
npm run dev
```

You can now open the webpage on https://localhost:8080 and Python code in either
the text box or browser devtools with:

```js
rp.pyEval(
  `
print(js_vars['a'] * 9)
`,
  {
    vars: {
      a: 9
    }
  }
);
```

Alternatively, you can run `npm run build` to build the app once, without watching
for changes, or `npm run dist` to build the app in release mode, both for the
crate and webpack.