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.

fn main() {
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.

There's a few caveats to be aware of when using the start attribute:

  • The start function must take no arguments and must either return () or Result<(), JsValue>
  • Only one start function can be placed into a module, including its dependencies. If more than one is specified then wasm-bindgen will fail when the CLI is run. It's recommended that only applications use this attribute.
  • The start function will not be executed when testing.
  • Note that the start function is relatively new, so if you find any bugs with it, please feel free to report an issue!