start

When attached to a function this attribute will configure the start section of the Wasm executable to be emitted, executing the tagged function as soon as the Wasm module is instantiated.

#![allow(unused)]
fn main() {
#[wasm_bindgen(start)]
fn start() {
    // executed automatically ...
}
}

The start section of the Wasm executable will be configured to execute the start function here as soon as it can. Note that due to various practical limitations today the start section of the executable may not literally point to start, but the start function here should be started up automatically when the wasm module is loaded.

Multiple start functions

Multiple #[wasm_bindgen(start)] functions can be specified across a module and its dependencies. They will be chained together and all executed during initialization. The execution order is arbitrary, so functions should not rely on other start functions having run before them.

#![allow(unused)]
fn main() {
#[wasm_bindgen(start)]
fn init_logging() {
    // ...
}

#[wasm_bindgen(start)]
fn init_state() {
    // ...
}
}

Private start functions

By default, start functions are also exported to JS as regular functions. To register a start function that runs at initialization but is not exported to JS, use the private attribute:

#![allow(unused)]
fn main() {
#[wasm_bindgen(start, private)]
fn my_init() {
    // runs at startup, but not callable from JS
}
}

Caveats

• The start function must take no arguments and must either return () or Result<(), JsValue>
• The start function will not be executed when testing.