When attached to a Rust "constructor" it will make the generated JavaScript bindings callable as new Foo().

For example, consider this exported Rust type and constructor annotation:

# #![allow(unused_variables)]
#fn main() {
#[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
    }
}
#}

This can be used in JavaScript as:

import { Foo } from './my_module';

const f = new Foo();
console.log(f.get_contents());

Starting from v0.2.48 there is a bug in wasm-bindgen which breaks inheritance of exported Rust structs from JavaScript side (see #3213). If you want to inherit from a Rust struct such as:

# #![allow(unused_variables)]
#fn main() {
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub struct Parent {
    msg: String,
}

#[wasm_bindgen]
impl Parent {
    #[wasm_bindgen(constructor)]
    fn new() -> Self {
        Parent {
            msg: String::from("Hello from Parent!"),
        }
    }
}
#}

You will need to reset the prototype of this back to the Child class prototype after calling the Parent's constructor via super.

import { Parent } from './my_module';

class Child extends Parent {
    constructor() {
        super();
        Object.setPrototypeOf(this, Child.prototype);
    }
}

This is no longer required as of v0.2.88.