export rust function to javascript using cwrap

.cargo|config

[target.wasm32-unknown-emscripten]
rustflags = [
    "-Clink-args=-s EXPORTED_FUNCTIONS=['_draw'] -s ASSERTIONS=1",
]

main.js

Module.addOnPostRun(function() {
  let draw = Module.cwrap("draw", null, ["string"]);
  draw("#canvas");
});

main.rs

#[no_mangle]
pub fn draw(sel: *mut c_char) {
    unsafe {
        match CString::from_raw(sel).into_string() {
            Ok(sel) => {
                draw_chart(&sel);
                // rust cannot free this pointer from js
                mem::forget(sel);
            },
            Err(_) => println!("Cannot find node")
        }
    }
}