wasm-bindgen/guide/src/design/export-customization.md

Customizing import behavior

The #[wasm_bindgen] macro supports a good amount of configuration for controlling precisely how exports are exported and what they generate in JS. This section is intended to hopefully be an exhaustive reference of the possibilities!

• readonly - when attached to a pub struct field this indicates that it's readonly from JS and a setter will not be generated.

  #[wasm_bindgen]
  pub struct Foo {
      pub first: u32,
      #[wasm_bindgen(readonly)]
      pub second: u32,
  }
  

  Here the first field will be both readable and writable from JS, but the second field will be a readonly field in JS where the setter isn't implemented and attempting to set it will throw an exception.

• constructor - when attached to a Rust "constructor" it will make the generated JS bindings callable as new Foo(), for example:

  #[wasm_bindgen]
  pub struct Foo {
      contents: u32,
  }
  
  #[wasm_bindgen]
  impl Foo {
      #[wasm_bindgen(constructor)]
      pub fn new() -> Foo {
          Foo { contents: 0 }
      }
  
      pub fn get_contents(&self) -> u32 {
          self.contents
      }
  }
  

  Here this can be used in JS as:

  import { Foo } from './my_module';
  
  const f = new Foo();
  console.log(f.get_contents());