js_namespace = blah

This attribute indicates that the JavaScript type is accessed through the given namespace. For example, the WebAssembly.Module APIs are all accessed through the WebAssembly namespace. js_namespace can be applied to any import (function or type) and whenever the generated JavaScript attempts to reference a name (like a class or function name) it'll be accessed through this namespace.

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
    
    type Foo;
    #[wasm_bindgen(constructor, js_namespace = Bar)]
    fn new() -> Foo;
}

log("hello, console!");
Foo::new();

This is an example of how to bind namespaced items in Rust. The log and Foo::new functions will be available in the Rust module and will be invoked as console.log and new Bar.Foo in JavaScript.

It is also possible to access the JavaScript object under the nested namespace. js_namespace also accepts the array of the string to specify the namespace.

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = ["window", "document"])]
    fn write(s: &str);
}

write("hello, document!");

This example shows how to bind window.document.write in Rust.

If all items in the extern "C" { … } block have the same js_namespace = …:

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = Math)]
    fn random() -> f64;
    #[wasm_bindgen(js_namespace = Math)]
    fn log(a: f64) -> f64;
    // ...
}

Then that macro argument can also be moved to the outer block:

#[wasm_bindgen(js_namespace = Math)]
extern "C" {
    #[wasm_bindgen]
    fn random() -> f64;
    #[wasm_bindgen]
    fn log(a: f64) -> f64;
    // ...
}

js_namespace may be set on the outer block or on an individual item, but not both at once. Nested paths belong in a single attribute:

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = ["window", "document"])]
    fn write(s: &str);
}

Writing js_namespace on both the block and an item inside it is a compile error; put the full path on whichever side is clearer.